Originally Posted by SafeChav
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.
A dodgy guide bent your valves
No, not quite, A valve moves in a guide, if the guide is slightly out of shape / cracked / bent, the valve can catch on an imperfection in the guide when revved which can cause it to momentarily slow, allowing the piston to catch it up with the inevitable result.
It is not that common with CVH engines, but I have had it happen personally, and know of @ least one other with the same problem.
There is the same problem with Ford 8v TwinCam engines. If a valve gets bent, it almost always disturbs a guide, which will mean that any replacement valves seem to end up suffering damage further down the line.
If you think about the symptoms being described, it does make sense.
A new valve is fitted in a used cylinder head. When started, it is fine. As soon as it is revved, the valve bends. The only way a valve can bend is if it is smacked by something to alter its angle of travel, in this case, a piston. The only way a piston can hit a valve when revving is if the valve is suddenly moving slower than the piston, hence the getting caught up on a guide.