Model A & B

Ford Garage

Camshaft Specifications

Contrary to some popular beliefs, there is no evidence of multiple different Model A or Model B camshaft profiles or timing designs on record at the Benson Ford Archives.

There is a camshaft and valve push rod (tappet/lifter) design for the Model A, and a somewhat different design for the Model B. The stories of multiple different Model B camshaft grinds is not supported by the Ford drawing and release records.

The Model A camshaft drawing A-6250-A1 was obsoleted on August 2, 1932, and was superseded by the production Model B camshaft B-6250 (along with the B-6500 push rod) for Model A Ford Service use.

The Model B camshaft B-6250 had one cam profile design for the intake lobes, and a different cam profile for the exhaust lobes, making a single complete camshaft design.

There are no dimensional changes of the Model B lobe profiles or lobe timing of the camshaft throughout it's Production and Service life, and no evidence exists it was superseded by a different camshaft part number or design level.

There is a possibility that Ford-Köln in Germany redesigned the Model B camshaft in the 1940-50s for use in the G28T engine built in Germany for the Ruhr truck and industrial uses. If true, that camshaft should have a BG- or G28TG- prefix number, but I can find no record of such a number in the Archives in Dearborn, or in the the late 1930's Ford-Köln factory parts books. Records indicate that Ford-Köln used the B-6250 camshaft design exclusively at least through 1936.

Compared to a Model A lobe profile, the Model B camshaft design has a more pointed lobe design (smaller nose radius) and are known for rapid and considerable wear on the end of the lobe. This wear reduces the effective lift of the cam lobe (and valve) quickly and significantly, and perhaps used cams may be mistaken as a 'different' lobe design compared to unused cams.

Accelerated wear on the nose of the lobe reduces the overall lift of the lobe and valve, and diminishes the rate of valve seat opening of the valve. Net result: Loss of torque and power.

Camshaft Specification Model A
A-6250
Model B
B-6250
Units
Valve Open Duration 236 244 crankshaft degrees
Lobe Separation Angle 112 114 camshaft degrees
Valve Overlap Angle 12 16 crankshaft degrees
Req'd Push Rod Base Diameter 1.117 1.187 inches
Push Rod Original Length 2.486 2.518 inches
Intake Lobe Design Lift 0.302 0.334 inches
Intake Valve Design Clearance 0.015 0.015 inches
Intake Valve Lift 0.287 0.319 inches
Intake Valve Opens BTDC 7.5 8 crankshaft degrees
Intake Valve Closes ABDC 48.5 56 crankshaft degrees
Exhaust Lobe Design Lift 0.302 0.341 inches
Exhaust Valve Design Clearance 0.015 0.022 inches
Exhaust Valve Lift 0.287 0.319 inches
Exhaust Valve Opens BBDC 51.5 56 crankshaft degrees
Exhaust Valve Closes ATDC 4.5 8 crankshaft degrees
Piston Position
BTDC Before Top Dead Center
ABDC After Bottom Dead Center
BBDC Before Bottom Dead Center
ATDC After Top Dead Center

From the table above, it can readily be seen that the primary differences between the Model A and Model B camshaft are that the Model B intakes and exhausts have higher lobe and valve lifts, and longer valve open duration compared to a Model A.

In order to combat burned exhaust valves and seats experienced on the Model A, the Model B exhaust was redesigned for a larger tappet (push rod) clearance to allow for more exhaust valve thermal expansion, compared to the cooler intake valve. This in turn drove a different ramp design on the exhaust valve cam lobe profile in order to smoothly engage the valve and lift it with minimal noise and lobe wear. The Model B intake valve lobe was also redesigned for higher lift, but with smaller tappet clearance than the exhaust.

The distinction between cam lobe lift and valve lift is also important.
Note that the Model B design has different cam lobe lifts, but equal valve lift comparing intake and exhaust profiles. The difference and reason is due to the different design tappet clearances (valve lash). It should also be noted that Ford designed the Model B tappet base diameter size and length to match the Model B cam lobe profiles. Ford also issued Service Bulletin warnings not to mix and match Model A and B camshafts and tappets.

Pictured above is a comparison of the larger (1.187") base diameter of an NOS B-6500 Model B Ford solid tappet on bottom, with a smaller (1.117") base diameter NOS A-6500 Model A solid tappet on top. In addition, the Model B tappet is longer than the Model A tappet.

The actual lift seen by the Model B intake and exhaust valves themselves was 0.319 inches, significantly larger than the 0.287 inch lift of the Model A valves. This higher valve lift and longer open duration provided by the Model B cam allows for better filling, evacuating, and breathing of the cylinders, and results in increased torque and power.

Based on the geometric lobe profiles of the cam, a Model A (or B) cam cannot be "reground" to achieve both its original lift and duration from the available metal.

A cam re-grinder could restore either original lift or duration, one or the other or some compromise, but not both.

The pics below are excerpts from the original Model A camshaft drawing A-6250-A1 for illustration purposes.

The Model A and B information cited above was compiled from the following original Ford detail drawings at the Benson Ford Archives:

A-6250-A1 dated 1-3-30
A-6250-A1 dated 8-2-32
B-6250 dated 11-11-31
B-6250 dated 1-24-33

More Camshaft web pages on Ford Garage:

  1. Model A & B Bill Stipe Camshaft Specifications
  2. Model A & B Original and Reproduction Valve Tappet (Push Rod) Details
  3. Model A & B Camshaft Timing Gear Varieties
  4. Model A & B Camshaft to Crankshaft Positioning

January 2004