Topic: How do I know/test the piston to valve clearance?

[Tretnine]

I was given a pair of old trail bikes (an '89 and an '86 XR100R). The '89 runs alright (needs to be rejetted for altitude, I live close to Boulder CO). The '86 was in boxes. The guy I got it from (who in turn got it from another guy) said the motor was in the process of being rebuilt. After inspecting the parts, I found that the valves are missing and the piston is damaged. The piston is dented as if the two ran into each other.  I also found an extra camshaft from Hot Cams. My guess is that the "hot cam" caused the problem.

Of course, I'm tempted to put the "hot cam" into the running bike, but I don't want to tear it to pieces, too. The cam is supposed to be an easy install and it, basically, is just supposed to work. If I put the cam in, how do I know that there will be ample clearance between the piston and the valves? How do you test this?

[Jerry Rxman Griffin aka MuthaF'er]

I know of 2 ways. Use modeling clay on top of the piston and rotate the engine through TDC then measure the indention in the clay or put a dial indicator on top of the spring retainer and when it's at TDC then manually push the valve down until it makes contact and see what the indicator says.

[Tretnine]

If I understand your first method, you'd put everything together for a dry fit and rotate through TDC. For the second method, one of the valves should be fully open at TDC, with your dial indicator in place, you push the valve in as far as it will go (till it hits the piston) then you look at the difference. Correct?

Also how goes? I live up in Boulder county. I'm also trying to dial this bike's carburetor in. How many jet sizes do you usually need to go down from stock to make your carbs perform up here around 5k feet?

And thanks for the response.