Car in bits again...
AO Silver Member
Join Date: Feb 2007
Location: Hampshire, UK
Hi Tommo - over-advanced ignition is a common cause, or sometimes poor fuel (octane too low), which essentially results in the same thing; as Philip says, the fuel-air mixture ignites before the piston reaches TDC, so the pressure actually tries to force the engine backwards. The resulting shockwave through the engine causes violent knocking which can damage crank journals as well as pistons. The exploding fuel mix also creates a massive heating to the top of the piston, which can melt the alloy.
EFI engines use accoustic sensors (microphones, basically) to monitor for this knocking phenomenon, and retard the ignition until it just disappears. With carbs you don't get this luxury of course.
If a valve is bent then either the clearances are all wrong, or the cam timing is out, which might also be the cause of the detonation. You could replace the bent valve and re-time the cams, if the piston crown is OK then you might get away with just doing that. Hopefully the bottom end survived, the short crank and large diameter journals means they are pretty robust.
Good luck with the repair work!
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