got to be real bad lean !!! what type plugs and ignition ?

Thanks for everyones advice. I have changed all the plugs, and just took it to the local motor guru to get it checked out. Like others here he suggested that timing could be a problem, and that a compression test would be a good idea. He believes that although anything is possible, there shouldn't be any major damage. I'll post the results when I hear back.

Spark plugs that are not tightened properly do not transfer heat to the heads correctly and can overheat. Proper torque on spark plugs is more important than some folks realize.

I have seen spark plugs loosen in a small block circle track motor and snowmobile motors that were detonating,I personally tightened them correctly. I haven't seen bbc plugs loosen but what would make it any different. I think you are too lean and it is detonating,you probably just can't ear it,Smitty