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 Research Center for US production.

It is true that some other camshaft designs were released worldwide in England, Germany, and the Soviet Union for small bore Model A and B engines, and for later Model B engine derivatives produced through at least the 1940s.

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

The Model A camshaft drawing A-6250-A1 was obsoleted on August 2, 1932, and was superseded by the production Model B camshaft B-6250 (along with the B-6500 push rod) for Model A Ford Service use.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 were no dimensional changes of the Model B lobe profiles or lobe timing of the camshaft throughout its Production and Service life, and no apparent evidence exists that it was superseded by a different camshaft part number or design level.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 quickly and significantly reduces the effective lift of the cam lobe (and valve). Well-used cams may perhaps 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.

Ford-Published Camshaft Specifications:

Camshaft Specification Model A
A-6250
Model AF
AF-6250
Model B
B-6250
British BE
BE-6250-B
Model BF
BF-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 0.338 inches
Intake Valve Design Clearance (gap) 0.015 0.015 0.015 inches
Intake Valve Lift 0.287 0.319 0.324 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 (gap) 0.015 0.022 0.017 inches
Exhaust Valve Lift 0.287 0.319 0.322 inches
Exhaust Valve Opens BBDC 51.5 56 74 crankshaft degrees
Exhaust Valve Closes ATDC 4.5 8 26 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 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 intake and exhaust lobe design lifts, but has equal actual intake and exhaust valve lift. The explanation lies in the different design tappet clearances (valve lash) between intake and exhaust.


Design Valve Push Rod Clearances:

Ford Design Tappet Clearance (inches)
Camshaft
Design
Intake Valve
Clearance (gap)
Exhaust Valve
Clearance (gap)
Model A
A-6250
0.015 0.015
Model B
B-6250
0.015 0.022
Model BF
BF-6250
0.015 0.017

It should also be noted that Ford designed the Model B push rod (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 a Model B Ford NOS B-6500 solid tappet on the bottom, with a smaller (1.117") base diameter Model A NOS A-6500 solid tappet on the top. In addition, the Model B tappet is longer than the Model A tappet.

Only use the 1.117 inches (or larger) base diameter tappets with any Model A or B camshaft.

The inferior ~1.000 inches (or less) undersize base diameter (jam-nut) tappets on the market should never be used with an original Model B camshaft.

The use of those too-small 1.000-inch tappets can result in severe damage to both themselves and the camshaft lobes.

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 Research Center:

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 related information on Ford Garage:

  1. For more Model A & B related information, use the Site Search box at the top or bottom of this page.
  2. Model A & B Bill Stipe Camshaft Specifications
  3. Model A & B Original and Reproduction Valve Tappet (Push Rod) Details
  4. Model A & B Valve Tappet Adjustment ~ Rule of Nine
  5. Model A & B Camshaft Timing Gear Varieties
  6. Model A & B Camshaft to Crankshaft Gear Alignment
  7. Model A & B Camshaft to Crankshaft Positioning

January 2004