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The Model A camshaft drawing A-6250-A1 was obsoleted on August 2, 1932, and was superseded by the production Model B camshaft B-6250 (along with the B-6500 push rod) for Model A Ford Service use.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lobe profiles or lobe timing of the camshaft throughout it's Production and Service life, and no evidence exists it was superseded by a different camshaft part number or design level.
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 tip 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
|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||0.338||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||30||crankshaft degrees|
|Intake Valve Closes ABDC||48.5||56||78||crankshaft degrees|
|Exhaust Lobe Design Lift||0.302||0.341||0.338||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||74||crankshaft degrees|
|Exhaust Valve Closes ATDC||4.5||8||26||crankshaft degrees|
|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 lobe was redesigned for a larger tappet (push rod) clearance to allow for more exhaust valve thermal expansion, and more dwell time on the seat for cooling, compared to the cooler intake valve.
This change 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: