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How to Rebuild the Top End on a KFX

When we last left off with the KFX, we had just checked the ring gap and determined that the rings on the piston were wore out. Just as a reminder, this is a project that we worked on for a friend, it never belonged to us. We got with the owner to discuss a few options. We knew that the rings and the piston were wore, and would definitely need replaced. We also knew that the cylinder was a little wore as well. In situations like this, we strongly feel that if you’re going to rebuild the top end, you might as well do it right. The decision was made to send the cylinder off for re-plating and to install a new top end kit, which includes the piston, the rings, and the gaskets.

Cylinder Re-plating

You may or not be aware that most newer cylinders have a Nickel-Silicon Carbide coating (NSC). What this means is that when the cylinder is wore down or gouged, it is possible to re-plate it and bring it back to stock. Even deep gouges can often be welded and then re-plated.

For example, check out the cylinder below. While it's not the KFX cylinder (it's actually from a KTM dirt bike), it's a great example of the kind of repairs that can be made. The photo on the left shows a gouge in the cylinder. The photo on the right shows the cylinder after being re-plated. While the gouge was significant, it was still shallow enough that the cylinder didn't even need to be welded. The re-plating was enough to repair the damage with minimal cost.

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Because we’ve worked with Millennium Technologies in the past (they plated the KTM cylinder above for us), and because we’ve had such good luck with their services, we sent the KFX cylinder for them to be re-plated.


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While the photo above isn't a TRUE before and after, you can still see the difference very clearly. Both cylinders in the photo above are from a 2005 KFX 400. The cylinder on the left is an overbore and the cylinder on the right is the stock cylinder that we sent to Millennium Technology to be re-plated.

You can see that when the cylinder came back, it looked brand new. Another bonus for working with Millennium Technologies is their lifetime guarantee on the plating. When other brands have had issues with the plating peeling, knowing that Millennium Technology stands behind their workmanship gives us peace of mind.

Top End Rebuild Kit

For the piston, rings, and gasket, we opted to purchase a Namura Top End Rebuild Kit. More often than not, purchasing a kit provides significant savings over purchasing the pieces individually.

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Another benefit to purchasing a kit is that you can be sure you have everything you need. The kit we purchased came with a new piston, rings, gaskets, circlip, and piston pin.

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The installation of the cylinder and top end kit was very easy. (Especially since we’ve tore this engine down more than once.) Because we have done this so many times, we’ll be fairly brief here while still providing links to more detailed descriptions.

Since the cylinder was already removed so that it could be re-plated, we started off with everything tore down. The very first thing we did was dig out our handy dandy manual from Clymer and found this easy to read illustration...

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Following the guide in our manual, we slid each of the new rings onto the piston. There are three rings; a compression ring, and two oil rings. When you install the rings onto the piston, you want to make sure that they are in the right order, and that the gaps in the rings are staggered. The gaps are important for ensuring the proper amount of oil gets into the cylinder with each stroke and that proper compression is maintained.

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After the piston rings were installed, we installed the circlip only on one side.

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Now we put some engine assembly lube on the piston pin.

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Also inside the rod.

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Then we placed the piston onto the rod. While holding the piston in place, we slid the pin through the rod.

With the piston in place on the rod, you want to make sure you install the second circlip to hold the piston pin in place.

After the piston is installed, you'll want to make sure you install the bottom cylinder gasket. Otherwise, you may have to take everything back apart and then put it back together again. (Ask us all about that next time you see us!)

top end Then it's time for some engine assembly lube in the cylinder bore. top end

The real fun begins when it’s time to place the cylinder down over the piston and the rings. Before you start, make sure you pull the timing chain through the cylinder, and mind the timing chain guides. We recommend tying the chain off with some wire, just so it doesn’t fall back down while you’re working.

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This is one of those tasks that feels like it takes more than two hands. Unfortunately, if you try to use more than two hands, you’ll just end up stepping on each other and cursing. Rest assured, it can be done with two hands. We won’t guarantee it can be done without cursing.

The easiest way we have found is to start the cylinder over the piston and push in one ring at a time until the cylinder bore is covering that ring. You will have to work the ring around but each one will compress and go in - eventually. Patience is key here.

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Once the cylinder was seated, we placed the head gasket on top of the cylinder.

About this time, we realized we had yet another obstacle in the way of finishing this project. Can you spot it?

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Yup, that would be a missing link in the timing chain. Sigh. So ultimately we ended up taking everything back apart, replacing the timing chain, and repeating the entire process. Don't worry, we won't recap that here.

But for the sake of finishing what we started, the next step would be to replace the head with the head bolts. You'll need to refer to your manual for the torque specs and method. At this point, the top end kit should be completely installed.

The final steps are to replace the cam shafts and adjust the timing. Because of the way the cam shafts are marked, it is an incredibly easy process to properly time this unit. For specific details on the timing, check out our post Timing is Everything.

This feels like the perfect opportunity to throw in those words we can’t say enough of: every ATV is different. For specific timing instructions on your ATV, refer to the service manual.

So what was the worst cylinder you have taken apart? Have you ever got everything back together and then found a part? Do you have a better way of doing this?

Let us know in the comments!

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