Reply to "dyno of 351"

If I may comment.

The dwell period occurs from about 30° BTDC to 30° ATDC. There is very little piston motion during the dwell period, the pistons are as high in their bores as they can be during the dwell period. Unlike the reciprocating motion of the pistons, the cam rotates at a consistent speed, thus the valves are in motion during the dwell period even though the pistons' motions have ceased. Since the valves are in motion during the dwell period, the clearance between the valves and the pistons is in a continual state of change during the dwell period.

The exhaust valve clearance should be checked as the piston finishes rising in the bore from about 30° BTDC to about 10° BTDC. The intake valve clearance should be checked as the piston prepares to descend into the bore from about 10° ATDC to about 30° ATDC. That's all the clearance that ever needs to be checked, from the mildest street engine to the hottest race engine.

Piston to valve contact normally occurs when the "closing" exhaust valves float, because the pistons chase the exhaust valves as they close, so this clearance is the more critical of the two to check ... and this is why we normally expect more clearance (0.100 inch) at the exhaust valve. The intake valve clearance is not so critical because opening intake valves don't float, and because the pistons are "fleeing from" from the opening intake valves.

Doug I would have commented about hijacking the thread but you've been right in the middle of the comments being made regarding piston to valve clearance. So long as you were commenting I assumed the hijacking met with your approval.
.