Ring Gap

Here we are again working on the KFX. I know this is the third post on this one issue so far, but don't let that scare you. In actual time, it's only been a few hours worth of work. If you'll remember, this was the KFX that we improvised a thread repair on. It had a brief ten minutes of run time before it died. Since then we've adjusted the timing and checked the valve clearance, and we also ran a compression test; neither of which got us any closer to starting. At the end of our post about the compression test, we concluded that we thought either the rings or the valve walls were worn out. So today, it's off with it's head (and jug). So the first new thing I had to do was drain the antifreeze. Draining Antifreeze After that I removed the upper motor mount. Removing Upper Motor Mount Then I removed the valve cover and the carburetor... Carb Removed ...And I took off the exhaust head pipe. (In the photo below, it's still attached. But trust me, we removed it.) Exhaust Head Pipe Now I had to remove the cams. I had done this in a previous post when showing how to set the timing. You can read about removing the cam shafts here (on this KFX) or here (on our Grizzly). Cams Removed Once the cams were out of the way, I could get to the head bolts. [alert-announce] Side Note: I have never had an issue with this, but they say that the head can warp if you do not break all the bolts loose quickly. I recommend not to stop once you start loosening them. There are only six on the KFX so this is not too much to ask but two of them are on the outside of the head. Make sure that you don't forget to look for them. [/alert-announce] After I got the head bolts out I had to take out the bolt holding the timing chain up. (We usually tie the timing chain off so it doesn't fall all the way down, in case you're wondering.) Timing Chain Bolt Now I was ready for the head to come off. Removing Head I removed the final two bolts holding the cylinder (which is also commonly referred to as the jug). Last Two Bolts All that was left was to take the cylinder off the engine. Jug Removed We had a sneaky suspicion that we would need rings earlier, so we went ahead and ordered some since we knew we would be tearing down anyway and didn't want to wait for them. New Piston Rings What I did not expect was that this had an aftermarket piston that did not take stock rings. So what we thought would be a huge time saver, ended up being us trying to return rings that didn't fit. More joys of being a mechanic and working on used stuff. (I do love the challenge.) But to verify that the low compression was caused by the rings, I am going to do a ring gap test with both the old rings and the new ones to see if the cylinder walls have too much wear. For this test you will need a feelers gauge, the cylinder cleaned, and the rings to test. I took the ring and carefully put it in the cylinder about an inch from the top. Ring in Cylinder Then I took the feelers gauge to see what the ring gap was. Ring Gap The photo above is of the used ring and the biggest gauge was .030 and it was not big enough. The stock gap is no more than .014 so this got me worried that my cylinder was not stock. When I checked the new rings, I got .017. Ok, that is a lot better but still not perfect. It turns out that I do have some wear in the cylinder. So now what? Well, I could just slap in a stock piston with my new rings and send it out the door but that would not make me a very good mechanic or a very good friend. So I am going to send the cylinder out to be stripped and re-plated. Yes, the KFX 400 engine has a plated cylinder (Nickel Silicon Carbide composite) and can be stripped and re-finished back to stock. This is a super cool feature because you can go back to stock sizes and not have to worry about if it is bored out and finding non-stock parts. So until next time...
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