If I remember right, it's the amount of time the points stay closed. How long they stay closed for the coil to zap the plugs. The longer the dwell the longer the spark. Too long a dwell then it uses up to much juice and there won't be enough time to recharge for the next zap. Too little then the spark won't be hot enough.
attach a dwell meter and set the points for 30 degrees.
My advice would be to set the points at .020" with a feeler gage to get the engine running, and then use a dwell meter to set the dwell after the engine is running.
When set properly, the gap would probably be about .018". The correct way to set the points would be to hook up a dwell meter and adjust the points for 30 degrees dwell.
30 degrees dwell
Use a dwell meter to set the points at 30 degrees dwell.
If you set the points at 30 degrees you will be fine.
i always had a meter with dwell on it you adj your points base and keep the gap the same Addition to above: Haven't messed with a points ignition in quite some time, but still have a Sun distributor machine and remember how to do it, if it's a dual point distributor the easiest way is to block one of the sets of points with a matchbook cover or thin piece of cardboard in between the points and set the dwell on the other set, then put the matchbook cover between the point set you just set the dwell on and set the dwell on the other set, then remove the matchbook cover and your done. If you're doing this on the car (as opposed to doing it on a distributor machine) you should then re-set or at least check the timing.
28 degres dwell
Use a dwell meter to adjust the points to 30 degrees dwell.
use a dwell meter to set points to 30 degrees
It depends on the year. For quite a while the points could be accessed through a little hole on the side of the distributor cap. The points were installed "close enough" to get the engine started then you would use a "dwell tach" to set the points through the little hole. But for the record, newer GM engines don't use points. The "dwell tach" the previous poster is referring to is a combination tachometer and dwell meter that can be purchased at most auto parts stores. The points should be set for 30 degrees dwell. Most GM cars stopped using points and went to HEI around 1974.
There is a window in the distributor cap. Just inside it there is an adjustment screw for the points. You will need a dwell meter to adjust them properly, the dwell should be set between 28 and 32 degrees.