How to replace a piston

[Pappy]

First, we would like to thank Eric Lester from Elite Performance (http://www.eliteatv.com/) for allowing us into his shop to get the skinny on how one goes about replacing a piston and ring set on one of the newer 4 stroke engines.

Eric gave us a heads up that he had a Kawasaki 450R in the shop due to a damaged piston and cylinder and we would be free to come hang out and try to capture all the steps in this process.

Erics customer was riding when his engine started loosing power. Immediatley the rider knew there was a major issue with his engine and shut it down as soon as possible. Knowing the damage was internal, his KFX was taken to Elite Performance for a diagnosis and subsequent repair.

The engine did not need to be removed for this procedure, however Eric wanted us to have full access to all angles if needed to do our thing!

[Pappy]

The first and foremost thing everyone should have for their ATV is a good shop manual. It will walk you step by step through almost everything and can save you the cost of it the very first time you repair your machine yourself versus letting the dealer have a crack at it

[Pappy]

A few others things will make any job go a little bit faster and should help prevent some issues....

1) Get the correct manual for your machine

2) Get the tools you need BEFORE you start

3) Make sure you have all the parts you need BEFORE you start if possible. With a piston needing replaced, until you open the engine up you wont really know what will need to be replaced, however have as many of tehe parts you need on hand for most jobs.

4) A clean work area and hopefully a clean engine/part to work on.

There are a few tools for this job that you may not have and should either buy or rent and have a working knowledge of what they do and why they are used.

Beyond the typical sockets and wrenches, you will need:

Torque Wrench
Micrometers (Optional)
Bore Gauges
Feeler Gauges

[Pappy]

With this engine being on the bench, a spare set of hands will be needed at a few points to help steady the engine. A helper always comes in handy even on an "In" frame install.

The first step in the actual work will be to remove the slotted inspection covers. This will allow you to bring the engine to TDC (Top Dead Center)

[Pappy]

Once the covers are removed, you can turn the crank to align the timing marks found at the upper inspection hole.

[Pappy]

A bit better view of what Eric is looking at when he is bringing the engine to TDC

[Pappy]

With the engine at TDC, the Valve cover can be removed

[Pappy]

With the valve cover removed, you can now see and access the valve train, including the dual cams on this engine.

[Pappy]

The cam Caps are next on the list and they should be loosened in a pattern so that they come loose evenly as to not put uneven pressure on the caps. The manual will show you the small details, just take your time.

[Pappy]

Using a pick or very thin flatblade, the cam caps can be removed, but they need to be loosened first and again, they should be taken up evenly as to not cause damage to them.

[Pappy]

Here the caps are shown along with the 2 small oil lines that connect the two caps. The oil ines come free easily, so even when doing a valve adjustment, keep them in mind.

[Pappy]

The cams will then need to be cleared of the timing chain. Have someone hold the chain to prevent it from falling down into the bottom end or use something to keep it up and out of the way.

[Pappy]

Here the cams are removed and you can see one keeper that looks like a half moon.

[Pappy]

Once the cams are removed, the manual states that the three 6mm bolts should be loosened prior to loosening the head bolts to prevent any damage.

[Pappy]

With the 3 bolts removed, now you can proceed to removing the head bolts

[Pappy]

The headbolts on this engine differ in length. The manual will instruct you as to what length bolts installs in which position when you begin reassembly

[Pappy]

With the cylinder removed, you will want to use some shop towels or rags to tuck down into the bottom end to prevent debris, bolts etc from falling into the now exposed engine.

[Pappy]

The piston in this engine has lost a ring that has damaged the piston and scored the cylinder wall

[Pappy]

Eric uses a bore gauge to determine the condition of the rod end and also checks for freeplay. This engine would only need a new piston and rings along with a bored cylinder to begin reassembly.

Your manual will provide all the needed specs for inspecting and all of the proper torque specs for getting things buttoned up.

[Pappy]

The Cometic gasket kit used includes a nw oring for the water temperature sensor and is replaced

[Pappy]

The old gasket is removed and the surface area cleaned so that a fresh and clean area is ready to accept the new gasket

[Pappy]

The freshly bored and honed cylinder are now ready for reassembly

[Pappy]

With everything inspected and replaced, it was time to put in a ring to test the end gap/ring gap.

There are a few ways to achieve this and Eric prefers to gently use the piston to push in the top ring into the cylinder

[Pappy]

A Feeler gauge is used to check the distance between the ends f the ring and if it is in spec, reassembly can continue. Your manual will have this detailed and usually the piston will include the specs for the particuliar application.

[Pappy]

The top ring will usually have a number or symbol stamped into it denoting the side that will be positioned towards the top of the piston.

[Pappy]

Just to show how the rings should look when installed, they should go end to end with the proper amount of freeplay or gap between the ends

[Pappy]

This would be the wrong way as this picture of the expander ring shows them overlaping ends

[Pappy]

Once the rings are installed in their proper position and the gaps either adjusted or at the correct measurements, you will want to make sure that the rings do NOT line up with each other. Having all the end gaps perfectly aligned would cause some issues as the engine would not seal the combustion chamber correctly and damage would occur.

[Pappy]

And not every piston will come stamped with a symbol to tell you which way it should face in the cylinder. On this piston, the valve reliefs show us how it should be installed to align with the intake and exhaust valves, others may have a symbol showing the proper direction of install. This can be complicated if you dont take your time and think things out especially when the engine is out of the frame and the head isnt positioned correctly when you make your determination on which way to install the piston. A carefull look at what your working with should steer you in the right direction and a double read of the manual.

[Pappy]

The new Wrist pin is coated in assembly lube and readied for installation

[Pappy]

One circlip is installed in the piston to act as a stop for the wrist pin

[Pappy]

The cylinder should be wiped down prior to reinstalling the piston. The instruction manual for the ATV or from the piston manufacture should tell you what oil to use. Eric prefers using 2 stroke oil as it burns away clean will provide the needed lubrication during assembly.

[Pappy]

Carefully the new piston and rings are slid into the cylinder, taking care to make sure that each ring goes easily into the bore. Take your time here, no sense in rushing and not being careful.

[Pappy]

A helper helps hold the cylinder and then the timing chain as its fed through the cylinder.

[Pappy]

The piston is guided onto the rod and the wrist pin is installed. Ive seen this done both ways, with the piston installed then the cylinder installed. You have to go with what work for you especially if its an in frame project as room can be at a premium when you have the frame in your way.

[Pappy]

Now the second circlip is installed to lock in the wrist pin

[Pappy]

Making sure that everying is in place, a gentle tap around the cylinder with a rubber mallet will help seat it

[Pappy]

The new piston is now installed and the top end is ready for the final stages of assembly

[Pappy]

In every step of this process, parts should be cleaned and inspected for wear and damage.

[Pappy]

The timing chain guide is reinstalled by inserting it into the slots inside the machined recesses.

[Pappy]

TDC is once again checked before reinstalling the cams

[Pappy]

The head bolts are coated with loc-tite

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The headbolts get torqued to spec

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Then the three 6mm bolts are torqued to spec as well.

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The cams are reinstalled and aligned by the marks stamped into them.

[Pappy]

After inspection, the cam caps are installed and the bolts will be torqued to spec in the pattern as laid out in the manual

(If you cant tell, I want you reading that manual!)

[Pappy]

The cam chain tensioner is reinstalled

[Pappy]

Grease is applied to all rubber seals as per the manual

[Pappy]

The valve cover is reinstalled and we are all but ready to reinstall the engine.

The valves will have to be checked and adjusted as well, but thats another "How To" for another day!

There are also some copper washers that should be replaced on the M10 head bolts. Pick them up before you start assembly to save some last minute ordering.

Sealant should also be applied to the valve cover gasket and torque specs followed to a T

Thats about it in a nutshell. It is not terrible complicated and as long as you have a Manual and the tools to complete the job, you shouldnt have any problems making this chore a fairly easy one. ofcourse what we didnt show you was the time and work it took to remove the engine or reinstall it, but that to isnt all that hard.

A big thanks to Eric from Elite who made this possible and a hats off to him for taking his time to walk us through this.

[Ryan]

Great write up and details!

[400exrider26400]

This helped alot Thanks!