Countersunk head bolts imperial with slot

Countersunk bolts with slotted heads naturally differ from their metric counterparts in terms of diameters and pitches. Another difference is the countersink angle: imperial bolts normally have a countersink angle of 82°. Exceptions are the countersunk screws with 1/4"-24 thread used by Harley-Davidson on the engine and transmission, have a countersink angle of only 60°. So please do not use the "metric" 90° countersink from the tool shop around the corner.

A practical tip: if you just want to countersink or repair a damaged countersink in a nice old knuckle or panhead cover, you can use a centering drill like those used on lathes. They only have two cutting edges, but the right 60° angle. If you work slowly, the two cutting edges are completely sufficient.

As with all slotted bolts, it is important to use suitablescrewdrivers or bits. The tip of the blade must be as wide and as thick as the slot. Anything that is too thin and too narrow will spoil the slot. Then not only is the bolt head spoiled, at some point the tool no longer grips and the bolt has to be drilled out. And nobody wants this!