The MGA With An Attitude
HOW MANY CLICKS Equals 1 Degree? - IG-117

At 03:58 PM 11/19/2009 -0700, Bill Ham in Southaven, Mississippi wrote:
"How many clicks on the dist. micro timing knob equal 1 degree"?

Okay, I concede I have to post this, as it has been asked repeatedly. I figured it out once but didn't write it down, so now I had to do it again. This information comes from a standard Lucas distributor that I just happen to have in hand.

The thread on mine is #10-32-UNF, as I measure with a thread gauge. The thumb fits a little loose, so I can't swear it wasn't originally something more obscure. It might have been 3/16-32-BSF (.187" major diameter instead of .190", and 55-degree thread angle rather than 60). It may have been #2BA, 4.7-mm Dia x .81-mm Pitch (.185-D and 31.36 threads per inch). One of these alternate threads may have been chased with a #10-32 die for clean up. In any case it is the thread pitch that counts here, and I use 32 TPI for calculation.

Near as I can measure casually, the pin on the moving vacuum advance plate where the vacuum unit pull spring attaches is 1.00-inch from center or rotation of the plate (axis of the distributor shaft).

The knurled thumb knob on mine has 43 serrations around the perimeter, but this is not necessarily the same for all such parts.

The rest is a matter of calculation. If you were to rotate the vacuum advance plate through one complete turn the pin would move 2 x Pi inches. Divide that by 360 degrees to get pin motion for 1 degree.
2 x 3.1416 / 360 = 0.17453 inch
Multiply that by thread pitch to get number of turns of the nut for 1 degree.
0.17453 x 32 = 0.5585 turns
Multiply by clicks per turn to get clicks per degree.
0.5585 x 43 = 24.0 clicks (for 1 degree at distributor)
Divide by 2, because the distributor turns half as fast as the crankshaft.
24.0 / 2 = 12 clicks (for 1 degree at crankshaft)

This result may be different if you have different knurling on the thumb knob. For 25 serrations in one turn the result would be:
25 / 43 x 12 = 7 clicks

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