Dwell isn't magic, and it isn't difficult to understand. The distributor has 360 degrees of rotation for the 4 cylinders of the model A engine. That leaves 90 degrees of rotation for each cylinder. If you look at the distributors cam, you can see that it is going to keep the points closed about 50% of the time, therefore, the dwell is going to be somewhere in the 45 degree range (half of 90 degrees). So just estimating, the dwell should be something between 35-55 degrees.. What I suggest is to set the points at the center of their range (0.020"), measure the dwell, and that would be what I would use in the future. (I run a trouble free Honda electronic distributor, so I can't actually check it for you.)
Note1: I would expect the model B cam to provide a bit more dwell due to it's profile, but...
Note2: If you watch Paul shinn's video on distributor rebuilds, he sets the points at 0.018". I do not buy that setting. The normal wear of the fanalic cam rubbing block on the movable point arm causes the points to naturally close up over time. It just seems wrong to start out with the points set to the minimum setting. I suggest 0.020" as a better setting.
Note3: I built the dial indicator points checking tool seen in the restorer magazine a few months back, and took it and a bunch of distributors to our club meeting. Using feeler gauges, the club members were asked to set the points to 0.020". Every single one of them set the points significantly wider than 0.020", some as wide as 0.030". It seems to be human nature the adjust them wide, or at least a common mistake. A dwell meter would verify correct settings. (A dial indicator can also verify correct adjustment.) I now have very little confidence that the average model A owner will set points at the correct gap.
Note1: I would expect the model B cam to provide a bit more dwell due to it's profile, but...
Note2: If you watch Paul shinn's video on distributor rebuilds, he sets the points at 0.018". I do not buy that setting. The normal wear of the fanalic cam rubbing block on the movable point arm causes the points to naturally close up over time. It just seems wrong to start out with the points set to the minimum setting. I suggest 0.020" as a better setting.
Note3: I built the dial indicator points checking tool seen in the restorer magazine a few months back, and took it and a bunch of distributors to our club meeting. Using feeler gauges, the club members were asked to set the points to 0.020". Every single one of them set the points significantly wider than 0.020", some as wide as 0.030". It seems to be human nature the adjust them wide, or at least a common mistake. A dwell meter would verify correct settings. (A dial indicator can also verify correct adjustment.) I now have very little confidence that the average model A owner will set points at the correct gap.
