Originally posted by kthomas:
Some factory service manuals, like that for a 240Z, give the appropriate guide dimensions. Otherwise you compare it to a known stock part.

Known stock? The only part that I can think of would be a new one from the dealer. In some cases assembled heads are not available (for example, mazda only sells a bare casting).

Beyond that, these things all have tolerances. So the fact that mine is further in (or out) doesn't indicate cheating.

If its a 1/2 inch difference, then sure, thats obvious.