Model A & B

Ford Garage

Camshaft to Crankshaft Positioning

The pic below shows the dimensions of the engine camshaft and crankshaft centerlines of the Model A and B engine block. The dimensions are from the machined casting drawing A-6015 dated September 30, 1929, from the Benson Ford Research Center.

When Ford originally machined the engine block, the bottom pan rail was the reference datum, and the crankshaft centerline was exactly located in that block plane.

The important point to note is that the centerline distance between the cam and crank must be held to 4.154 / 4.156 inches in order for the gears to mesh properly, having neither interference nor excessive clearance.

Additionally, the crankshaft, camshaft, and centerline of cylinders must all be set up parallel to each other. Many re-bored cylinders are off-position due to cumulative set-up error from multiple rebuilds. Crankshafts can also be slightly misaligned to their intended virtual centerline and position.

Additionally, the centerline of the cylinders are offset 0.125 inches to the left, away from the crankshaft centerline by design.

The second point to note is that the cross/car dimension between the cam and crank centerlines is 3.000 inches and is basic, without tolerance. The reason this is important is to maintain the correct camshaft position to the valve tappets, and more importantly, to the distributor drive gear bore in the block.

If this basic dimension of 3.000 inches is modified, it has a direct adverse effect on the relation and mesh between the distributor drive gear and the helical gear cut into the camshaft center journal. Again, the gears must mesh without interference or excessive clearance.


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 referenced to position and set up the remainder of the engine block machining operations, as shown below.

The two front and rear left hand main bearing bolt datum holes were each reamed to 0.505-inch gage pin size, as shown on the original Ford block drawing above. (Rear hole shown, bottom view, looking up. Front hole similarly noted.)


Further Considerations

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 such as used on the 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 camshaft laterally closer to the crankshaft drive gear (thereby changing the 3.000-inch basic dimension) will decrease the clearance and 'lash' between the cam and crank gears beyond the specified design tolerance. Expect binding, noise, reduced camshaft gear life, and fiber/metal debris in the oil.

It should also be noted that excessive (and egg-shaped) 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 method to set up the align boring fixture for the main bearings. A number of aftermarket align boring fixture designs used this approach including KR Wilson and LEMPCO. Tobin-Arp also offered similar fixtures for locating the main bearing boring bar to the block. Alternatively, Kwik-Way used fixtures which located off the block main bearing bolt holes, similar to the original Ford method.

Furthermore, the alignment of the crankshaft and camshaft must also be parallel and square relative to the alignment and squareness of each of the cylinder bore centerlines to each other. This important point is often often overlooked on engines that have multiple cylinder re-bores in their history! Set-up errors are usually cumulative!

Any excessive wear or slop in the boring or reaming when 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 inches. Check your camshaft and cam bore dimensions! Measure, measure, measure!


Summary

When properly rebuilding an engine, it is critical that the camshaft and crankshaft centerline positions must be correctly spaced at 4.154 / 4.156 inches to each other.

The cylinder bores must also be square and aligned to each other, and the crankshaft must be parallel and square to the cylinders.

In addition, it must be verified that the pan rail datum surface has not been previously moved, milled, or modified by some clueless tinkerer. Verify that the crankshaft and bearing centerlines are properly centered in the block pan rail plane.

The 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 links 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.

More related information on Ford Garage:

  1. Model A & B Camshaft to Crankshaft Gear Alignment
  2. Model A & B Camshaft Timing Gear Varieties
  3. Model A & B Kwik-Way LBM Main Bearing Boring Bar Fixtures
  4. Model A & B Tobin-Arp Main Bearing Boring Bar Fixtures
  5. Model A & B LEMPCO FSU Main Bearing Line Boring Machine
  6. Model A & B AER Insert Main Bearing Data & Instructions
  7. Model A & B Camshaft Gear Front Timing Cover Differences
  8. Model A & B Original 5-Z-1832 Cam Gear Wrench
  9. Model A & B Camshaft Thrust Plungers
  10. Model A & B Original Ford Camshaft Details

March 2006