The pic below shows the dimensions of the engine camshaft and crankshaft centerlines of the Model A and B engine block. The dimensions are copied exactly from the machined casting drawing A-6015 dated September 30, 1929 from the Ford archives.
When Ford originally machined the engine block, the bottom pan rail was the reference datum, and the crankshaft centerline was exactly located in that plane.
In addition, the rear and front crankshaft main bearing bolt holes on the left hand side (away from camshaft) were the master datum holes originally used to set up and position the remainder of the engine block machining operations.
Those two datum holes were originally reamed to 0.505 inch gage pin size, as specified on the original Ford drawing.
Based on the posted experience of some people with their rebuilt engines, it is clear that some rebuilders really do not understand this centerline relationship or its importance.
Increased position tolerance (variation) between the camshaft and crankshaft position can probably be accepted in an engine which uses a timing chain, but there is no room for sloppy machine work or flakey improvement theories in a direct gear drive design like on a Model A.
In particular, one infamous "rebuilder" of ahooga.com fame used to claim that he 'improved' the engine by moving and crowding the camshaft closer to the distributor drive gear. Actually, this is physically impossible to do without destroying the design relationship of the camshaft to the crankshaft and/or the valve tappets.
It should be noted that moving the cam laterally closer to the drive gear (changing the 3.000 inch basic dimension) will increase the clearance and lash between the cam and crank gears beyond the design tolerance.
It should also be noted that excessive wear of the camshaft bore in the block can also affect the align boring of the main bearings if the rebuilder uses a false cam and link to set up the align boring fixture for the main bearings. Measure, measure, measure!
Furthermore, the alignment of the crankshaft and camshaft must also be parallel and square relative to all the alignment and squareness of the cylinder bore centerlines to each other.
Any excessive wear or slop in the boring or reaming set-up from a false cam or cam bearing bore can affect the running clearance between the camshaft and crankshaft, as well as the crankshaft to the cylinder bores. The original cam bearing bores in the block are 1.5615 / 1.5625 inches. The original camshaft journal size is 1.559 / 1.560 inch. Check your camshaft and cam bore dimensions!
This positional relationship of crankshaft, camshaft, and cylinder bores is the basis for a soundly rebuilt and smooth-running engine. Errors are cumulative and compound. Don't tinker with the design!
Visit the link in the list below for information about tooling used to properly locate the boring bar during engine rebuilding, in order to maintain proper crankshaft and camshaft timing gear engagement and clearances.