[wolf k]

^ Yes, in a GM, the axial thrust from the distributor gear is upward, the bottom of the distributor housing absorbs the gear thrust. Therefore, in a GM it is the depth of the distributor into the block that creates the correct gear mesh. Because GM distributors sit on top of the intake manifold, an adjustable collar is a good idea to compensate for decking of the block, milling of the heads, etc.

In our Fords, the distributor height is not effected by any block decking or cylinder head milling.