FWIW:
Just one way to look at Model A timing vs. Model B timing with mixed A & B distributors & mixed A & B timing covers is:
1. TDC on both A's & B's occur when no. 1 piston is at the top & at the center of crankshaft rotation between the no. 1 piston moving up & down on the "compression" stroke. One can accurately test compression stroke by removing the no. 1 spark plug. The compression stroke occurs when valves are closed; hence, with one's thumb over the no. 1 spark plug hole, one can feel air trying to escape. On the following exhaust stroke, valves are open & no air pressure will be felt.
2. The A & B crank shaft gears as well as the A & B timing gears are interchangeable; hence, both have the same number of teeth with a ratio of two to one whereby the larger timing gear makes two revolutions for each one crankshaft revolution.
3. After installation, the A & B timing gears, (which operate the up & downward movement of valves & the A & B distributor shaft rotation), they are alike in that both the timing gears & the distributor shafts are rotating twice as fast as the crankshaft. This engine movement thus far was not designed to be adjustable after proper installation & remains fixed for both A & B.
4. Next is the change in the later new B timing cover, with a different location of the timing pin, to advance the B's ignition timing for the new B centrifugal advance distributor. With a new B timing pin cover, if one measures the new B timing pin location, the new B timing pin will be located 9-1/2 degrees above that of an A timing pin. When measured from the timing gear's center, the radius measurement to the timing gear indentation is 2-1/2"; i.e., the B timing pin is located .415" or about 7/16" upwards of the A timing pin location. Remember the crank is turning CW & the timing gear is turning CCW when one faces the front of the radiator.
5. What this means is that on an A engine or on a B engine with an A timing cover, the timing pin slips in at TDC; however, with an A or with a B engine, with a new B timing cover, the pin slips in BTDC whereby the no. 1 piston will not have yet reached the top.
6. Just one way to avoid ignition timing confusion if one uses both an A distributor & a B distributor from time to time, and/or if one does not know if one has an A or B timing cover, just spend about $30.00 to buy the neat & sturdy A, removable, timing cover device advertised on e-bay. After mounting the device on either an A engine or a B engine, accurately determine TDC for no. 1 piston on the "compression" stroke & provide a vee mark on the crankshaft pulley adjacent to zero (0) degrees on this timing device. Paint bottom of vee white.
7. When using an A distributor, time same when crankshaft pulley vee mark is adjacent to zero on the timing device.
8. When using a B distributor, time same when the crankshaft pulley vee mark is located adjacent to 9-1/2 degrees BTDC mark on the timing device. Because the crankshaft pulley's radius is similar to the radius of the timing gear's pin indentation, the crankshaft's vee mark when at 9-1/2 degree BTDC will occur approximately at 7/16" on the passenger's side of the timing device's zero mark.
9. For timing either an A & a B distributor, when the crankshaft's vee mark is positioned on the corresponding timing device marks specified for A & B in above paragraphs 7. or 8., the points should just begin to be opened by the CCW rotating distributor cam.
10. One can perform a final timing check with a timing light if so desired.
Hope this helps.
After 2 years I finally found my extra "A" timing cover.
