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Honda Navi 125cc Big Bore Kit — Why Wild Boar Took a Different Approach

HONDA NAVI 125CC BIG BORE KIT A DIFFERENT APPROACH

The Yuminashi 53mm Piston was one of the early aftermarket solutions for the Honda Navi. The piston kit came with a 53mm Piston/Rings/Pin-Cir-clips/. The customer was required to bore the original navi cast iron cylinder and fit the unit. Before the release of the Wild Boar 125cc racing piston cylinder kit this was really the only option for Navi enthusiasts to increase bore size.

Soon after the release of the Wild Boar 125cc Big Bore kit, Yuminashi released their upgraded 54mm piston with Dry Lubrication Coating around the skirt/DLC coated piston pin and a 54mm alloy cylinder to complete the new generation piston. The Yuminashi 130cc kit also uses extra thick gaskets or gasket stacking- Head 0.58mm/Base 1mm.

Yuminashi Next Generation 130cc ( Product Listing )

  • ● Dry Film Lubricant Skirt Coating: The dark gray coating on the piston skirts is a specialized dry film lubricant. It provides critical cold start protection by preventing direct metal-to-metal contact between the aluminum piston skirt and the cylinder wall when oil pressure is low or absent during a fresh engine start. Additionally, the micro-structure of the coating holds microscopic amounts of engine oil. This lowers initial friction, prevents scuffing or galling during the critical engine break-in period, and helps maintain localized lubrication under extreme loads.
  • ● 12mm DLC Coated Piston Pin: We replaced the standard steel pin with a Diamond-Like Carbon (DLC) coated pin. This ultra-low friction barrier prevents galling against the connecting rod under high RPMs and extreme temperatures.
  • ● 0.58mm Copper Head Gasket: Copper provides exceptional lateral thermal conductivity out to the cooling fins, eliminating hot spots around the combustion chamber while ensuring a blowout-proof crush seal under increased dynamic compression.
  • ● 1mm Thicker Cylinder Base Gasket: This thicker gasket acts as a thermal break. It actively insulates the crankcase from the conductive heat of the cylinder block, keeping your engine oil cooler and preventing thermal runaway in the bottom end.

BASE/HEAD GASKET APPROACH

This is another fundamentally different approach Wild Boar took.

Wild Boar use the same gasket thickness as the Honda Navi base/head gaskets.

In fact, in an emergency you could use a set of standard Honda Navi gaskets as a replacement following all technical considerations and installation procedures.

Yuminashi-Base 1mm/Head 0.58mm

Why not use thicker gaskets ?

Why gasket stacking can be problematic

A gasket isn’t normally intended to act as a thermal-insulation spacer.

Adding a second gasket can:

  • Change compression ratio by increasing the combustion-chamber volume.
  • Increase piston-to-head/squish clearance if used at the head.
  • Change cam timing if increasing cylinder/base height.
  • Alter the cylinder’s relationship to the crankcase and head.
  • Create additional sealing interfaces, giving you another potential leak path.
  • Change the designed clamping/compression characteristics of the gasket.
  • On a small engine, even a few tenths of a millimeter can represent a meaningful geometric change.

  • Double Thickness Head/Base Gaskets can create a Frankenstein “make it fit” solution rather than an engineered solution.


Why CPR7EA-9 makes sense for the Wild Boar 125

Yuminashi state in the product listing-

Increased displacement and the raised YPS-12 piston dome drastically alter cylinder pressures and clearances. The stock heat range #7 projected spark plug is no longer sufficient or safe.

The Wild Boar configuration changes several things compared with the original Navi NGK MR7C-9N Spark Plug.

109 cc → approximately 125 cc

53 mm piston

Higher-compression piston crown

Different combustion-chamber geometry

Higher cylinder filling potential

Greater combustion loading

That doesn’t automatically require a different heat range, especially with our purpose designed kit.

But it makes spark location and combustion stability more important.

The CPR7EA-9’s projected configuration is a logical choice for Wild Boar because it is a standard NGK plug already specified across numerous 125-class Honda applications.

The Wild Boar 125cc kit includes an NGK CPR7EA-9 spark plug selected for the modified 125cc high-compression engine configuration.

NGK Heat Range 7 • 0.9 mm Gap • 10 mm Thread • 19 mm Reach • Projected Insulator • Resistor Type

Unlike the standard Honda Navi MR7C-9N, the Wild Boar kit uses a CPR-series projected-insulator plug while retaining NGK heat range 7.

Different engine configuration → different plug selection → same NGK heat range → projected-insulator CPR design.

WILD BOAR vs YUMINASHI

We chose these 2 aftermarket units to compare side by side to show a different Wild Boar approach.

LOOK BEYOND THE BORE. LOOK AT THE PISTON.

When it comes to increasing the displacement of the Honda Navi, it is easy to focus on one number:

53mm.

But bore diameter is only one part of a performance piston design.

Piston skirt geometry, wrist-pin support, internal structure, compression height, crown design, ring lands, cylinder configuration, piston-to-cylinder clearance and installation geometry all contribute to the complete piston and cylinder package.

That is why Wild Boar took a different approach when developing its Honda Navi 125cc Alloy Racing Kit.

Instead of simply asking:

“What 53mm piston will fit the Navi?”

Wild Boar asked:

“What should a purpose-designed 53mm piston and cylinder package look like when it is developed specifically around the Honda Navi?”

WILD BOAR vs YUMINASHI

A CLOSER LOOK AT TWO DIFFERENT PISTON DESIGNS

When the two pistons are placed side-by-side, the differences are immediately visible.

Look closely at:

  • Piston skirt structure
  • Wrist-pin boss support
  • Internal reinforcing structure
  • Skirt openings and relief areas
  • Retained material around the lower skirt
  • Skirt length
  • Crown design
  • Overall piston geometry
  • Material distribution

This comparison is about what can actually be seen and measured.

It is not intended to claim that the Yuminashi design will automatically fail or that appearance alone proves one piston is stronger.

Instead, it gives Navi owners the opportunity to examine the physical differences visually and understand the different design approaches.

 

WILD BOAR

53MM PRECISION-CAST PERFORMANCE PISTON

Purpose Designed • Precision Cast • Developed Around the Honda Navi

YUMINASHI

53MM/54mm Piston

Alternative 53mm/54mm Bore-Up Approach

The two designs take visibly different approaches to piston structure and material distribution.

And that is where the comparison becomes interesting.

1. PISTON SKIRT STRUCTURE

One of the most noticeable differences is the way material is retained within the piston skirt.

The Yuminashi piston shown in our comparison has a more heavily relieved and open internal configuration.

The Wild Boar piston retains a more substantial visible skirt structure, including material around the wrist-pin region and lower skirt.

This is not simply about making a piston look bigger or heavier.

The skirt forms part of the piston-cylinder interface and contributes to the way the piston is supported and controlled within the cylinder.

Wild Boar therefore treated skirt geometry as an important part of the complete piston design.

DON’T JUST LOOK AT THE BORE.

Look at where the material is.

Look at where it has been relieved.

Look at the complete skirt.

2. WRIST-PIN BOSS STRUCTURE

The wrist-pin area is one of the critical load-transfer regions of a piston.

Combustion forces and reciprocating forces are transferred through the piston, wrist pin and connecting rod.

Look closely at the area surrounding the wrist-pin bosses.

The two piston designs visibly use different arrangements of surrounding material.

YUMINASHI

The comparison piston shows larger relieved/open areas surrounding the wrist-pin bosses.

WILD BOAR

The Wild Boar piston retains more visible surrounding structure around the wrist-pin bosses and connecting areas.

This visual difference does not, by itself, prove a difference in ultimate strength.

What it does show is that the two piston designs use different approaches to material distribution and internal structure.

For a performance piston, those design decisions deserve attention.

3. INTERNAL PISTON STRUCTURE

Turn the pistons over and another difference becomes apparent.

The internal webs, ribs, skirt openings and connecting structures are not identical.

The Wild Boar piston retains substantial visible structure connecting the wrist-pin region with the surrounding skirt.

The Yuminashi piston shown in the comparison uses a more heavily relieved/open configuration.

WHY DOES THIS MATTER?

A piston is much more than a circular piece of aluminum with a larger diameter.

The complete design includes:

  • Crown
  • Ring lands
  • Compression height
  • Wrist-pin bosses
  • Internal webs
  • Skirt profile
  • Skirt length
  • Material distribution
  • Piston mass
  • Cylinder relationship

Changing one area can influence the requirements of another.

That is why Wild Boar developed the piston and cylinder as a combined package.

4. PISTON SKIRT LENGTH & SUPPORT

The piston skirt plays an important role in controlling lateral piston movement within the cylinder.

As engine speed increases, the piston undergoes repeated acceleration and deceleration.

That makes piston mass, skirt geometry, piston profile and cylinder relationship increasingly important considerations in a high-RPM application.

Wild Boar therefore placed particular attention on:

SKIRT PROFILE

SKIRT LENGTH

SKIRT CONTACT AREA

PISTON STABILITY

MATERIAL DISTRIBUTION

The objective is not simply to make the skirt as long as possible.

The objective is to create a purpose-designed skirt profile appropriate to the piston, cylinder and intended application.

5. 53MM vs 54MM

DON’T MIX THE TWO YUMINASHI PRODUCTS

This distinction is important.

Yuminashi currently lists a 53mm / 122.60cc light-bore piston separately from its 54mm / 130cc Performance Series aluminum big-bore kit.

These should not be presented as though they are the same piston or the same product configuration.

YUMINASHI 53MM

The current 53mm listing describes a 3.00mm oversize piston for boring the stock 50mm Navi cylinder to 53mm.

Yuminashi specifies:

  • 53mm piston
  • 53mm piston ring set
  • 12 × 33.8mm wrist pin
  • 53mm copper head gasket
  • Recommended 0.05mm piston-to-cylinder wall clearance

Yuminashi also states that the stock cylinder is bored to 53mm and describes the resulting cylinder-wall thickness as 1.45mm.

WILD BOAR 125CC ALLOY RACING KIT

Wild Boar takes a different route:

Purpose-designed 53mm piston

Purpose-designed alloy racing cylinder

Purpose-designed piston skirt geometry

Complete installation package

The two approaches are therefore fundamentally different.

6. ADAPT THE CYLINDER — OR BUILD THE PACKAGE AROUND IT?

There is an important distinction between these two approaches.

The Yuminashi 53mm product is designed around boring the original Navi cylinder from 50mm to 53mm.

Wild Boar’s approach is to supply a purpose-designed alloy racing cylinder matched to its 53mm piston.

Neither statement requires us to claim that one approach automatically fails.

The difference is simply:

YUMINASHI

Modify the original cylinder to accept the larger piston.

WILD BOAR

Use a purpose-designed alloy cylinder and piston combination.

For customers considering a performance conversion, that distinction is worth understanding.

7. INSTALLATION GEOMETRY MATTERS

Increasing piston diameter is only one part of a big-bore conversion.

The relationship between:

  • Piston compression height
  • Cylinder height
  • Deck height
  • Base gasket configuration
  • Head gasket thickness
  • Squish clearance
  • Piston-to-valve clearance
  • Piston-to-cylinder clearance

all forms part of the installation geometry.

Wild Boar’s 53mm Alloy Racing Kit was developed as a complete piston-and-cylinder package, with the installation components selected around that configuration.

BUILD THE PACKAGE AROUND THE ENGINE.

Not simply around the bore diameter.

8. PISTON COATINGS — TWO DIFFERENT APPROACHES

Some performance pistons use additional surface treatments as part of their design.

Yuminashi’s later 54mm / 130cc Performance Series specifies a dry lubricant piston-skirt coating and a DLC-coated piston pin as part of that configuration.

That is a documented feature of that product.

It is important, however, to distinguish this later 54mm Performance Series configuration from the current Yuminashi 53mm light-bore piston listing.

Wild Boar takes a different approach.

The Wild Boar 53mm piston is designed around:

  • Piston geometry
  • Skirt profile
  • Skirt contact area
  • Material distribution
  • Piston-to-cylinder relationship
  • Wrist-pin support
  • Lubrication requirements

The philosophy is straightforward:

DESIGN THE PISTON FIRST.

THEN DETERMINE WHICH SURFACE TREATMENTS ARE ACTUALLY REQUIRED.

A coating can be a useful engineering feature.

But it should be considered as part of the overall piston design—not viewed in isolation from the underlying piston geometry.

9. DLC PISTON PIN — WHAT IS ACTUALLY BEING COMPARED?

Yuminashi’s later 54mm Performance Series uses a DLC-coated 12mm piston pin.

That is a feature of the product configuration and can be compared factually.

Wild Boar uses a different piston-pin specification and design philosophy.

The important point for customers is not simply:

“DLC vs non-DLC.”

The real engineering questions are:

  • What is the pin substrate?
  • What is the pin hardness?
  • What is the surface finish?
  • What is the coating specification?
  • How thick is the coating?
  • How is the coating applied?
  • What is the piston-pin loading?
  • How does the pin interact with the piston boss?
  • How does it perform at operating temperature and RPM?

A coating specification alone does not tell the entire story.

10. WHY HIGH RPM CHANGES THE DISCUSSION

The Honda Navi is a small-displacement engine.

At elevated RPM, the piston repeatedly accelerates and decelerates at high frequency.

That means piston design deserves closer examination when increasing displacement and performance.

The comparison therefore isn’t simply:

53MM

or

53MM vs 54MM.

The better questions are:

How is the skirt structured?

Where has material been retained?

Where has material been relieved?

How are the wrist-pin bosses supported?

What is the piston skirt profile?

What is the piston-to-cylinder relationship?

How was the cylinder designed around the piston?

What manufacturing process produced the piston?

These are the details that distinguish a complete piston design from a simple bore-size comparison.

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Why Did My Honda Navi Big Bore Engine Seize Or Blow

Why Did My Honda Navi Big Bore Engine Seize Or Blow

There are a few common causes for an engine seizing when you install an aftermarket big bore kit in any motorcycle

How Inferior or Poorly Applied “Dry Lubricant” Coatings Can Cause Engine Problems

Dry lubricant coatings are often applied to piston skirts to reduce friction, improve scuff resistance and provide additional protection during the critical period between piston and cylinder-wall surfaces. However, the quality of the coating and the way it is applied are extremely important—particularly on small-displacement, high-revving engines where piston clearances, temperatures and component loads can be tightly controlled.

An inferior, excessively thick, poorly bonded or incorrectly applied dry lubricant coating can potentially create problems rather than prevent them.

Potential Problems Caused by an Inferior or Poorly Applied Coating

1. Coating Flaking or Peeling
Poor adhesion can cause sections of the coating to lift, flake or peel from the piston skirt. The resulting debris can circulate through the engine and potentially contribute to accelerated wear.

2. Increased Piston-to-Cylinder Clearance Problems
If the coating is applied inconsistently or excessively thick, the effective piston dimensions can change. This can reduce the intended running clearance and potentially contribute to piston drag or seizure when the piston and cylinder reach operating temperature.

3. Piston Scuffing and Galling
A coating that wears away prematurely may leave the underlying piston material exposed. Under high temperature and load, this can increase the possibility of skirt scuffing, galling and transfer of aluminium to the cylinder wall.

4. Accelerated Cylinder-Wall Wear
A poorly finished or contaminated coating can act as an abrasive surface rather than a protective one. Coating particles or detached material can contribute to wear of the cylinder wall.

5. Increased Friction
The purpose of a dry lubricant is to reduce friction. If the coating has excessive thickness, an unsuitable surface finish or poor lubrication characteristics, it can have the opposite effect and increase friction between the piston skirt and cylinder.

6. Localised Hot Spots
An inconsistent coating thickness can alter the contact between the piston skirt and cylinder wall. Uneven contact may concentrate load into smaller areas, potentially increasing local temperatures and wear.

7. Piston Sticking or Seizure Risk
On a small-displacement engine operating at high RPM, even relatively small changes in piston-to-cylinder clearance can become significant as components expand with temperature. An excessively thick or poorly controlled coating can contribute to insufficient running clearance.

8. Coating Breakdown at High Temperature
Every coating system has temperature and application limitations. If the coating is unsuitable for the piston application or operating environment, it can degrade, lose its lubricating properties or break down prematurely.

9. Contamination of the Engine
Poorly prepared surfaces, inadequate curing or coating breakdown can introduce particles into the engine’s oiling system and combustion environment. These particles can potentially contribute to wear of other components.

10. Uneven Piston Movement
An inconsistent coating around the skirt can produce differences in friction and clearance from one area to another. This may affect the way the piston moves within the cylinder and increase localised loading.

11. Increased Piston Rock or Skirt Loading
If the coating changes the intended contact pattern or wears unevenly, piston-to-cylinder contact can become concentrated in particular areas. This can increase skirt loading and potentially contribute to noise and wear.

12. Premature Coating Wear
A coating that is poorly matched to the piston material, cylinder finish, operating temperature or intended application may wear away quickly, providing little long-term benefit.

Why This Can Be Particularly Important on Small-Displacement Engines

Small-displacement motorcycle engines can operate at very high RPM while using relatively small pistons and tight piston-to-cylinder clearances. This means there can be relatively little margin for errors in piston dimensions, surface preparation, coating thickness or coating adhesion.

A quality dry lubricant coating should therefore be regarded as part of a controlled piston-engineering process, rather than simply a layer of material sprayed onto the piston skirt.

The coating needs to have appropriate adhesion, thickness, surface finish, temperature resistance and compatibility with the piston and cylinder system. Correct piston-to-cylinder clearance must also be established with the coating taken into account.

The Bottom Line

A properly engineered dry lubricant coating can provide useful protection against friction and skirt wear. However, a poor-quality coating or incorrectly applied coating can potentially become another source of engine wear.

On a small-displacement, high-RPM engine, problems such as coating failure, excessive thickness, poor adhesion, uneven application and premature breakdown can contribute to:

Increased friction • Piston scuffing • Skirt wear • Cylinder wear • Coating debris • Hot spots • Piston noise • Piston sticking • Increased seizure risk • Premature component wear

For this reason, the quality of the piston, coating system, surface preparation, application process, curing process and final piston-to-cylinder clearance all matter—not simply whether a piston has been advertised as having a “dry lubricant” coating.

“A failed or poorly bonded dry lubricant coating can shed debris into the engine oil and, when combined with increased friction, piston wear and combustion blow-by, contribute to accelerated oil contamination, darkening and degradation.”

DLC-coated Piston Pins

DLC-coated piston pins can sound impressive — but coating a piston pin does not automatically make it a better or more reliable component.

On a small-displacement performance engine, the piston pin operates under extreme cyclic loading and depends on precise clearance, correct surface finish, proper lubrication and coating adhesion.

A poorly engineered or incorrectly applied DLC coating can potentially cause:

  • Excessive coating thickness — reducing critical running clearance.
  • Pin binding or seizure — when clearance becomes inadequate.
  • Coating delamination — DLC can crack or separate under repeated loading if adhesion is poor.
  • Surface damage — incorrect finish can compromise lubrication.
  • Coating debris — failed material can contaminate the engine’s lubrication system.
  • Piston-boss and small-end wear — incorrect dimensions or surface characteristics can accelerate wear.
  • Additional friction and heat — defeating the purpose of a low-friction coating.
  • Piston-pin noise — incorrect clearance can contribute to abnormal mechanical movement.

Performance Engineering Is More Than a Coating

A DLC coating is only as good as the engineering behind it.

For a high-performance small-displacement engine, pin material, hardness, dimensional accuracy, surface finish, coating thickness, adhesion, lubrication and running clearance all work together.

The coating should serve the design — not become the selling point.

 

 

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Thicker Base / Head Gaskets On A Small Displacement Engine

Thicker Base / Head Gaskets On A Small Displacement Engine

On a small-displacement motorcycle engine, using a base gasket and/or head gasket that is roughly twice the normal thickness can be intentional, but it changes several important engine dimensions. The reason depends heavily on whether the thicker gasket is being used at the base, head, or both.

1. Increase piston-to-head clearance

A thicker head gasket increases the distance between the piston crown and cylinder head.

This can be useful when:

  • A big-bore piston has a different crown shape.
  • The piston protrudes too far above the cylinder.
  • There is insufficient piston-to-head clearance.
  • Valve-to-piston clearance needs additional room.
  • The engine has been modified with a different piston, cylinder, camshaft or head.

On a small engine, even a few tenths of a millimetre can make a noticeable difference.

2. Reduce compression ratio

A thicker head gasket increases combustion-chamber volume.

For example, if an engine originally has a very thin gasket and you install one twice as thick, the additional volume can reduce the compression ratio.

That might be deliberately done when:

  • A big-bore kit has increased compression excessively.
  • A different piston has a higher crown.
  • The engine is experiencing detonation/knock.
  • The builder wants to run lower-octane fuel or reduce thermal loading.

However, reducing compression isn’t automatically an advantage. It can reduce torque and combustion efficiency.

3. Increase cylinder height with a thicker base gasket

A thicker base gasket moves the cylinder upward relative to the crankcase.

That can alter:

  • Piston deck height
  • Piston-to-head clearance
  • Compression ratio
  • Squish clearance
  • Port timing
  • Cam timing indirectly, depending on the engine design
  • Piston skirt position relative to the cylinder

This is particularly important on a small single-cylinder engine.

4. Alter port timing

On a small motorcycle engine, moving the cylinder upward can change the relationship between the piston and the ports.

With a thicker base gasket, the piston travels farther away from the crankcase at TDC and BDC relative to the cylinder.

On a 2-stroke, this can dramatically change port timing.

On a 4-stroke, it is less about ports and more about piston position, compression, squish and the overall geometry.

5. Compensate for machining or component differences

Sometimes a thicker gasket is simply being used to accommodate dimensional differences between components.

For example:

Cylinder → gasket → crankcase

If the cylinder, crankcase, piston or head has been machined differently from the original specification, gasket thickness can be used to establish the required final dimensions.

This is especially relevant with aftermarket big-bore kits.

6. Prevent mechanical interference

This is one of the more important reasons.

A modified small-displacement engine may have a piston that comes closer to the head or valves than the original design.

A thicker gasket can provide additional clearance.

For example:

Normal setup

HEAD
↓
thin gasket
↓
PISTON

Increased-clearance setup

HEAD
↓
thicker gasket
↓
PISTON

That additional distance can prevent contact under high RPM when components expand and move dynamically.

But there’s an important downside

If you’re looking at a small-displacement engine where both the base gasket and head gasket are approximately double the normal thickness, I would want to know why before assuming it’s an improvement.

Using substantially thicker gaskets can create a chain reaction:

Thicker base gasket
→ cylinder moves upward
→ piston position changes
→ squish changes
→ compression changes
→ piston skirt/cylinder relationship changes

And:

Thicker head gasket
→ head moves farther away
→ combustion volume increases
→ compression decreases
→ squish clearance increases

If both are thicker, the combined effect can be significant.

Particularly relevant to piston-skirt comparison

Important Note – short piston skirts on small-displacement pistons, there’s another potential reason to pay attention to the base gasket thickness.

If a thicker base gasket raises the cylinder, it changes the amount of piston skirt remaining inside the cylinder at different points of the stroke.

Depending on the piston/cylinder geometry, this can affect piston stability and skirt guidance, particularly near BDC.

This is not Ideal and in fact using double thickness gasket ( Gasket Stacking ) to compensate has a cascading effect.

Wild Boar is the leading Manufacturer for custom pistons.

Our kits do NOT require extra thick base or head gaskets to accomplish the correct piston cylinder geometry in fact in an emergency you could use a Standard Honda Navi Gasket Set with our kit.

For more useful Information on this topic click the following links below-

Piston Skirt Length On Small Displacement Pistons

Potential Issues With DLC Diamond Coatings On Piston Pins

 

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Piston Skirt Length On Small Displacement Pistons

Piston Skirt Length On Small Displacement Pistons

Particularly important for a small-displacement/high-RPM engine

A short skirt isn’t automatically a design fault. A manufacturer can deliberately use a shorter skirt to reduce reciprocating mass and friction. The important question is whether the skirt geometry, piston profile, clearance and cylinder/bore design provide adequate stability under the engine’s actual RPM and load conditions.

Our Wild Boar piston skirt length is optimized to give maximum performance,longevity, reliability under high rpm’s.

Potential problems with a piston skirt that is too short

  1. Reduced piston stability
    • The skirt helps keep the piston aligned within the cylinder.
    • A shorter skirt provides less guiding surface.
    • This can allow greater piston rocking or piston slap, particularly near TDC and BDC.
  2. Higher side loading
    • Combustion pressure pushes the piston against the cylinder wall.
    • With less skirt length supporting the piston, the load can be concentrated over a smaller contact area.
    • This can increase local cylinder-wall and skirt loading.
  3. Increased skirt and cylinder-wall wear
    • Greater rocking and localized contact can produce uneven wear patterns.
    • In a small-displacement/high-RPM engine, repeated loading cycles can make this more significant.
  4. Greater sensitivity to piston-to-cylinder clearance
    • A short skirt can make the piston more sensitive to the amount of clearance specified.
    • Too much clearance can increase rocking and noise; too little can increase the risk of scuffing when the piston expands.
  5. Potential for increased piston noise
    • More piston movement within the bore can produce characteristic piston slap or tapping sounds, particularly when the engine is cold.
  6. Greater loading on the piston pin and small end
    • Piston rocking changes the way forces are transmitted through the piston pin.
    • This makes the pin, pin bore and connecting-rod small end part of the overall system rather than independent components.
  7. Potentially uneven skirt loading
    • Instead of distributing contact forces along a suitable skirt area, a shorter skirt can concentrate forces into smaller regions.
    • Surface finish, skirt profile, coating and lubrication therefore become particularly important.

To Concise : – “A shorter piston skirt can reduce the available guiding surface and potentially increase piston rocking and localized skirt/cylinder-wall loading. Whether this becomes a problem depends on the piston geometry, skirt profile, piston-to-cylinder clearance, operating RPM/load and lubrication. Controlled dimensional and durability testing is required to establish the actual effect.”

Our Piston Skirt Length gives the best performance under high rpm’s with reliability and longevity our utmost priority.

The important engineering point is that the difference isn’t simply cosmetic:

Feature Wild Boar piston Piston Skirt Removal Comparison
Skirt length Longer Considerably shorter
Guiding surface Greater Reduced
Distance below pin area Greater Shorter
Potential piston-rocking control More available skirt area Less available skirt area
Reciprocating mass Potentially higher Potentially lower
Frictional skirt area Potentially higher Potentially lower

Why the shorter skirt matters

The skirt is one of the surfaces that guides the piston in the cylinder. Reducing its length can reduce the amount of piston-to-cylinder guidance available.

That can potentially increase:

  • Piston rocking
  • Piston slap/noise
  • Localized skirt loading
  • Uneven skirt wear
  • Cylinder-wall loading
  • Sensitivity to piston-to-cylinder clearance
  • Loading transmitted through the piston pin and small end

One particularly important observation

The piston-pin position matters enormously here.

A useful comparison isn’t simply:

“This skirt is X mm shorter.”

It is:

How much effective skirt remains below and around the piston-pin centreline, and how much of that skirt remains inside the cylinder at TDC and BDC?

That’s because the piston is effectively being guided around the pin axis. A shorter skirt gives the piston less leverage against rocking.

 

Related Useful Links –

Double Thickness Head / Base Gaskets On A Small Displacement Engine

 

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Potential Issues With DLC Coatings On Piston Pins

Potential Issues With DLC Coatings On Piston Pins

1. Coating de lamination
If the DLC doesn’t adhere properly to the underlying steel, sections can crack or peel away. Once the coating is damaged, the exposed substrate can wear considerably faster.

2. Incorrect coating thickness
The piston pin is a precision component. An excessive or poorly controlled coating thickness can affect the pin’s effective dimensions and the clearance between the pin, piston and connecting rod.

3. Poor surface preparation
DLC generally requires carefully prepared substrate surfaces. Contamination, inappropriate surface roughness or inadequate pretreatment can reduce adhesion and coating life.

4. Brittleness / cracking
DLC is extremely hard, but some DLC formulations can also be relatively brittle. Under severe cyclic loading, an inappropriate coating/substrate combination can develop cracks.

5. Lubrication compatibility
DLC can have excellent friction characteristics, but its performance depends on the particular DLC formulation and lubricant. The wrong oil/additive combination or inadequate lubrication can still produce wear.

6. Counterface wear
The DLC pin doesn’t operate in isolation. The piston-pin bore and connecting-rod small end are equally important. A very hard pin surface paired with an inappropriate or poorly finished mating surface can shift the wear problem to the other component.

7. Coating defects
Pinholes, inclusions, uneven deposition or inconsistent coating thickness can create localized areas of accelerated wear.

8. Coating-process variation
Two parts both described as “DLC coated” aren’t necessarily equivalent. DLC encompasses a family of carbon-based coatings with different compositions and properties. Deposition process, hardness, thickness, adhesion and surface finish matter.

Particularly important for a piston pin

The piston pin has to survive millions of load reversals while maintaining the correct clearance.

So the engineering question isn’t simply:

“Does it have DLC?”

It’s:

“What DLC coating was used, how was it applied, and was the complete piston-pin assembly engineered around it?”

For a performance 125–130cc engine, you need to know:

  • Base pin material
  • Pin hardness/heat treatment
  • DLC type
  • DLC thickness
  • Coating hardness
  • Adhesion specification
  • Surface roughness
  • Pin diameter tolerance
  • Piston-pin clearance
  • Connecting-rod small-end specification
  • Recommended lubrication

“DLC performance is coating- and lubricant-dependent, so compatibility should be demonstrated rather than assumed.”

4. DLC introduces a coating-to-mating-surface issue

The piston pin doesn’t work by itself.

It interfaces with:

DLC pin → piston pin bore / connecting-rod small end → lubrication

The hardness and surface characteristics of DLC can change the behavior of the mating surface.

Some very hard DLC systems can have greater “attack ability” toward softer mating materials; this is why specialized DLC formulations have been developed for applications involving soft-metal counter faces.

One important point: A properly specified and applied DLC coating can reduce wear. Problems are more likely when the coating process, dimensions, mating surfaces or lubrication are poorly controlled.

A DLC coating can provide genuine friction and wear benefits, but those benefits depend on the specific coating formulation, substrate, deposition process, adhesion, mating materials and lubricant compatibility. Therefore, “DLC coated” alone does not establish superior piston-pin durability.

“The performance of a piston pin depends on its substrate material, heat treatment, geometry, surface hardness, fatigue strength and manufacturing tolerances. DLC is a surface treatment and does not by itself establish the structural strength of the underlying pin.”

Wild Boar Uses 9310 High Performance for our piston pin.

Material Typical role High-RPM suitability Key characteristic
4130/4140 Standard performance Good Strong, economical
5115 Performance/aftermarket Good Case-hardened, good wear
9310 High-performance Excellent Tough core + hard case
H13 tool steel Premium racing Excellent High core hardness/strength
300M Extreme racing Extreme Very high strength/fatigue capability
M2 / specialty tool steels Top-level competition Extreme Very high hardness/strength

 

Wild Boar 9310  DLC piN COATING
High-tensile pin material High-tensile substrate + DLC
No coating dependency Coating adhesion matters
Steel heat treatment important Steel + coating properties important
Surface finish important Surface finish + DLC important
Lubrication important Lubrication + DLC/oil compatibility important
Dimensional accuracy important Dimensions + coating thickness important
N.o DLC wear-through mechanism DLC wear/degradation must be considered
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Dry Lubricant Coatings On Low Displacement Pistons

Dry Lubricant Coatings On Low Displacement Pistons:

An inferior or poorly applied dry-film lubricant coating

An inferior or poorly applied dry-film lubricant coating on a small-displacement motorcycle piston can cause real problems, particularly on a high-RPM, high-specific-output engine. The important point is that the coating is a secondary friction/scuffing protection system—it does not replace engine oil.

What can happen if the coating is poor?

A properly engineered piston-skirt coating is intended to reduce friction, scuffing and adhesive wear between the piston skirt and cylinder wall. Some coatings also provide a degree of protection during moments when the normal oil film is marginal.

If the coating is poorly bonded, too thin, incorrectly formulated, too soft, or applied to an improperly prepared piston surface, several things can happen:

  • Coating wears through prematurely
  • Piston-to-cylinder friction increases
  • Aluminum transfer onto the cylinder wall can occur
  • Skirt scuffing develops
  • Local hot spots can develop
  • Piston clearance can effectively change as the skirt wears
  • In severe cases, piston seizure can occur.

Piston/cylinder scuffing is associated with rapidly increasing friction and temperature, and can progress to seizure.

Why it can matter even more on a small 110–125cc engine Like your Beloved Navi

A small piston doesn’t automatically mean a low-stress piston.

For example, a small motorcycle engine may have:

High RPM + relatively high cylinder pressure + lightweight/short piston skirt + high piston acceleration

That can produce significant skirt loading despite the piston being physically small.

The coating becomes particularly useful during conditions where the oil film is less effective, such as cold starts, high load, high temperature or transient operating conditions. Research has specifically found piston-skirt coatings can reduce friction and wear during cold-start conditions when formation of a good oil film is more difficult,this is not needed for our Wild Boar Big Bore kit as it does the job of normal lubrication naturally and without piston skirt lubricant coatings.

The quality of the coating matters

A good dry-film system isn’t simply “black paint on the piston.”

The substrate preparation, coating chemistry, thickness, curing/bonding process and surface texture all matter. For example, one established dry-film coating system specifies piston-skirt coating thickness around 12–37 microns, while also requiring appropriate surface preparation for adhesion.

There are also different generations of piston coatings. Some modern formulations are specifically engineered for greater wear resistance and oil retention rather than merely providing a low-friction surface.

A poor-quality or poorly bonded skirt coating removes an important margin of protection and can accelerate skirt wear, friction and scuffing—especially when the engine is operated at high RPM and load.

And once the coating begins to fail, the underlying piston/cylinder interface becomes increasingly dependent on the conventional oil film. Insufficient or disrupted lubrication is a well-established cause of piston-skirt seizure and scuffing.

Wild Boar Pistons are Precision Cast and are made for higher RPM without the need for any Dry Lubricants.

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Honda Navi Big Bore Piston Cylinder Kit

First Honda Navi Big Bore Alloy Kit.

As a lot of us long time Australian Navi enthusiast know, the only “Big Bore” kit available on the market was an existing 53mm piston on the market machined down then thicker gaskets used to make the piston work, then a hollowed out original cast iron cylinder was used to make the “Big Bore” kit complete.

Wild Boar Racing Cylinders is the World’s First Navi Alloy Racing Cylinder Kit designed and made in Australia to be a stand alone upgrade.

No expensive cam upgrades or the rest of the ongoing expensive mods, just a great foundation to build upon that will hugely increase performance,that has been trialed and tested here in Australia.

Our Wild Boar Honda Navi 125cc Big Bore Piston Cylinder Kit is the first true light weight high compression piston and cylinder kit on the market.

The original Navi cylinder is a heavy cast iron cylinder designed more for economy then performance, our WILD BOAR kit reduces weight while giving a huge increase in power and torque even for the stock standard Navi.

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