Point gap and dwell Is an either/or situation. Changing one will alter the other.
Definition time :
"Dwell" means "The number of degrees of distributor rotation the points are closed"- - - - -literally speaking, the amount of time they "dwell together". A 6 cylinder engine has a separation between the high points of the distributor cam of 60 degrees- - - - -60 X 6 = 360 degrees of distributor rotation, and since the distributor turns at camshaft speed, it takes two turns of the crankshaft to make the distributor turn once. All six spark plugs fire during one rotation of the distributor. An 8 cylinder engine has distributor lobes 45 degrees apart, and a 4 cylinder distributor cam has a 90 degree separation. Typically, a 66% "points closed" interval is ideal for getting maximum coil magnetic saturation, which is where the 40 degree dwell figure comes from.
There's a disadvantage to the 66% dwell time, however- - - -the point gap tends to be very narrow, and the 100+ volts that gets induced in the primary coil winding as the spark plug fires, and the short time available for the condenser to discharge before the points close again can cause burned points, especially with the much more efficient operation of a 12 volt coil over the former 6 volt system. In the interest of increased point life, GM shortened the dwell time (and increased the point gap) when they switched to a 12 volt electrical system.
With about another 3 paragraphs, I could explain how a dwell meter is actually nothing but a voltmeter with a specially calibrated scale, but that's a discussion for another day. If your points are worn, replace them. It's impossible to get an accurate gap setting with anything but perfectly smooth points. Use a dwell meter, adjust the distributor to the specifications Mike provided, and forget about the point gap!
Jerry
