Thread - Pilot Drill Charts

[Jacques Reed]

Hi Lynn,

Just checked my Machinery's Handbook and see what you mean. The attached shows a 5/16 UNF tap requiring a Letter I drill: 0.272"

Not the kind of thing you would get at the local hardware store.

Number and letter size drills derived from American and British wire gauge sizes and are still sometimes used in American industry.

17/64" might be too small and risk tap breakage. 9/32" would not leave a lot of thread.

A 7.0 mm drill might work. Converting 7.0 mm to inches is 0.2756" which is only 0.0036" larger, about the thickness of printer paper, and might suffice.

I inherited the attached partial set of number drills. Sets like that or letter drill sets may turn up at swap meets.

Rubbing a wood screw on a block of soap also aids screwing it in. Oops! off again.

Cheers,