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40 to 45. 40 is common.
The suggested spark plug gap for a 1973 Harley Davidson is 0.040. It is the suggested gap for most of the spark plugs of that era.
.040"
40
The spark plug gap should be .040"
the gap for a Harley plug on a 88c.i. is 30
1.00 mm
.040
35
.040
.040".