For a quick rule of thumb use a match book cover(approx. .020").
.30
.030 for both 2 and 4 cycle
.25000
A piece of a cigarette box should do it .
The older model mercury outboards, with maker points, should have a 0.020" gap setting.
The maker points in a 1967 model, 110 Mercury outboard, are gapped to .020 in.
The model 4BR75A, 4 hp Johnson outboard is a 1975 year model. The point setting on this engine would be .020in.
The ignition points on the 1976 model, 25 hp Johnson, are set to .020 in.
The gap on the 351 Cleveland changes timing. Adjust the point gap until it is at 28-32 and begin timing then.
0.019 point gap
The point gap for a 429 v8 is : .017 The point gap for a 429Pl v8 is: .020
.035 spark plug gap, .02 point gap.