From time to time, and in different threads, reference is made to burnt valves. First "real" motor work I ever did, many moons ago, was a valve job on a 250 Pontiac OHC six.

However, I have no idea exactly what the cause of a burnt valve is, and, how can they be prevented?

If a valve (especially exhaust) does not seat correctly (due to valve guide wear) or fully (due to incorrect lash adjustment) it will overheat. Overheat warps it. Superheated exhaust gas leaks by, like an inefficient cutting torch. Eventually, a pie slice-shape is missing from the tulip of the valve=no compression=sucks.

 

From time to time, and in different threads, reference is made to burnt valves. First "real" motor work I ever did, many moons ago, was a valve job on a 250 Pontiac OHC six.

However, I have no idea exactly what the cause of a burnt valve is, and, how can they be prevented?

 

Holy crap!! I had a LeMans with that OHC 250 and had to do a vale job! That son of a gun was a runner tho, with the 2 speed powerg(s)lide tranny.

I was talking to a Subaru tech at an indipendent shop. (the guy I got the free Loyale from) He was explaining how you have to be carefull who does the timing belts on the 2.5. Apparently you need to hold the cams in position under tension. Otherwise when you undo the belt the cam can slip out of position and some of the valves can get a tiny nik in them. Before you know it... burnt up valves

Sorry, but I can't believe a tooth or two off will cause the piston to come anywhere close to the valves. Me thinks you'd have to be way off for that to happen.

 

Tiny

What would the hardened steel valves contact to put a nick in them? It would have to be very hard, maybe a lump of carbon build-up?

I don't know, and I don't know what the duration of these cam lobes are, but there must be at least 120 degrees of duration where the valves aren't open. That leaves a lot of room for error as far as the belt is concerned.