Duane,
The compressor mounts to the -C- Datum Surface.

Geoff,
I see the logic behind your proposal and can agree with the basic Idea, however, as I understand the rules of GD&T according to ASME Y14.5M 1994, A datum may be a feature of size, or a pattern of features. 4.5.7.2 lays forth rules for using a Single Axis of Two Coaxial Features (i.e. using "A-B" in a FCF). Is it legal to make the centerline a Datum? With respect to the "fight for alignment" between A-B and C you raise a good and valid point, what if we put a Profile of a Surface tolerance on Datum C with respect to A-B of say .010". That would give us some "wiggle" room, yes?

Keith
Manager, Quality/Lead CMM Programer
Devonshire Manufacturing Inc