valves hitting piston

I have a bit of an off topic question but I'm posting here because this site if quite active. My buddy has rebuilt his KTM and now the exhaust valves are tapping the piston. The valves are original and they have not been lapped. After the rebuild the engine runs but clacks heavily.

The valves were properly adjusted and the timing chain is not stretched and the cam timing is fine. My buddy wonders if it is possible that he simply overtightened the head bolts, which are the same bolts for the cylinder....he wonders if he has squashed the head gasket enough to allow for this new condition of valves hitting piston! I don't think this is possible but all other things seem correct. Is there that little space between the valve closed a the the piston at TDC?

The bike wasn't doing this before.

Also compression is now only 90 psi.

Just wondering as I'm quite buffaloed now!

I'm no 4 stroke guy, but if the valves would hit the piston, the valves would probably break instantly and the motor would be trash?

it would definitely destroy the valves....

i would recheck the cam timing, my money is on that not having been done correctly....

Was it a Wiseco?

Did you follow the directions with the piston, or the KTM manual?

I put a Wiseco piston in an XR650R backwards once, the valve reliefs didn't line up with the valves and the piston hit them. Luckily no damage.

I was following the directions out of the Honda manual, not the Wiseco directions. The arrow on the Wiseco piston points the opposite way of the OEM piston.


90 pounds compression means there is a problem, it should be much higher than that. Either the piston/rings are not sealing properly or the valves are not sealing properly.

Time to open that motor up again

I have seen valves hit piston after a new top end when the piston is installed backwards. Other than that if you made no other changes figure on cam timing being off. Like I said though if you are sure about your cam timing then your piston is possibly installed backwards.

Another thing. I would never even think about reusing a head gasket. The only exception to this would be on my Old 1991 RM125. It used a pair of O rings for gaskets.

Valves dont always instantly break if they come into light contact with the piston. I would not run it till you find out what is going on though. They will cause damage. If you stop now and repair the contact problem, you may not have any other damage yet.

Is your buddy the guy who is posting a similar question in the KTM forum on here?

if your positive cam timing is correct, then you put the piston in backward. if it has valve reliefs the exhaust side reliefs are most likely closer together than the intake side because the exhaust valves are smaller. pull the head and take a look and hope you didn't bend a valve yet.

If the piston is in correctly, then cam timing has to be wrong, make sure your looking at the right mark on flywheel, maybe it's lined up with "F" instead of "T". Start looking till you find it, should be easy to see if you take your time and think it through.

Does your Katoom have auto-decompression. If it does, then the 90 psi compression test doesn't mean anything. If the motor has been running and hitting valves, you will be very lucky to have escaped without bending something.

The clanking noise - is the tensioner and guide installed correctly?

Is your buddy the guy who is posting a similar question in the KTM forum on here?

If recent post then 'yes' likely....he called me and now I can't stop wondering about it!

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