Boy - do the P&G valve gapper comments bring back memories of the good ole days of racing! Being in the auto repair business back then, I used the P&G almost daily. (I was one of those guys who always removed the distributer to install points and verify operation on a distributer machine too.)

Of course your explanation of rocker / valve gap changes with thermal expansion are right on. I would add that in order to minimize valve train wear (especially cam lobe / follower) in our model engines, the gap should be maintained at the minimum. There is precious little, if any, lash ramp ground into the cams. I've found that the YS engines require a tappet adjustment after the first 5 tanks or so to account for breakin wear, then every 50 flights or so.

Earl