Originally Posted by iansoutham

Throw the cylinder head in the bin.

I had exactly the same problem with an XR3i MFi cylinder head once. Kept bending valves every time it was revved. Turned out to be a dodgy guide, even though they looked fine and lapped in okay.

Changed the cylinder head for another one, was fine ever since. Still got the engine infact, even though the shell died from metal moth.