Besly Cutting Tools, Inc., 520 Blackhawk Blvd #135, South Beloit, IL 61080 (815) 389-2231    Close

Besly Drill Selector
for Modern Application Taps
Fractional Sizes*
UNC/UNF Threads
ASME B1.1-1989
Tap Size and TPI
Drill Size Min. Hole Size
(for nickel and
titanium alloys)
1/4-20 5 .2055
1/4-28 7/32 .2188
5/16-18 6.70mm .2638
5/16-24 7mm .2756
3/8-16 8.10mm .3189
3/8-24 8.55mm .3366
7/16-14 9.50mm .3750
7/16-20 10mm .3937
1/2-13 11mm .4331
1/2-20 29/64 .4531
9/16-12 31/64 ..4844
9/16-18 13mm .5118
5/8-11 13.80mm .5433
5/8-18 14.60mm .5748
11/16-11 15.60mm .6142
11/16-16 ** .6325
3/4-10 16.75mm .6594
3/4-16 17.60mm .6929
7/8-9 49/64 .7656
7/8-14 13/16 .8125
1-8 ** .8880
1-12 23.50mm .9252
1-14 UNS2B 15/16 .9375
* Principle factors which influence the minor diameter tolerance are ease of tapping, standard drill sizes, and height of engagement. When tapping difficult-to-machine materials, the height of engagement may be shallower and still develop stripping strength greater than the external thread breaking strength. In these instances, the minor diameter may be increased to the maximum tolerance to reduce the possibility of tapping difficulties.

Refer to the following publications for specific information on limiting dimensions for internal screw threads:
 ●  ASME B1.1-1989, UNIFIED INCH SCREW THREADS.
 ●  FED-STD-H28, FEDERAL STANDARD SCREW-THREAD STANDARDS FOR FEDERAL SERVICES.
 ●  DRILLED HOLES FOR TAPPING BY THE UNITED STATES CUTTING TOOL INSTITUTE


** Recommend using either a special drill or reaming the hole to Min. Hole Size.
 

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