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Garage Math
Submitted by Larry of C&L Enterprise

To determine the major diameter of a “number’ screw
(6-32, 10-32, etc):
The number times (X) .013 + .060
A #10 screw=10 X.013=. 30+.60=.190 OD
A #6 screw=6 X .013=.078+.060=.138 OD

This works for wood screws, sheet metal screws and machine screws.

Most American counter sink screws are 82 - some are 10000 (Mostly military). All metric counter sink screws are 90.

Tap Drill Sizes
Tap Drill Size = Major Diameter-1/Pitch
Example: 1/4-20 THD
.250-1/20 /.250-.050=.200 DIA
Example:1/4-28 THD
.250-1/28 / .250-.036=.214 DIA
Example:10mm X 1.5 THD
10mm-1.5mm=8.5 DIA

The most commonly broken tap is a 6-32. It has the coarsest thread to major diameter ratio of any common used thread.

For a Class 2 thread (industry stand) the tap drill size is .104 to .114. That is a 10% difference in hole size!!

You WILL break a tap at .104 DIA by drilling a .111 hole. You get a 70% thread (standard) and will not break many taps.


In the old days, determining the height of a tire was easy. A 7.00-15 was 7.0+7.0+15=29” tall

The new metric is harder.
Size(mm)/25.4 X aspect ratio X2+rim (in inches)
Example: 235/75/15
235/25.4=9.251in. X .75=6.94X2=13.878+15=28.878 DIA

Example: 205/60/16
205/25.4=8.070X.60=4.842X2=9.685+16=25.685 DIA


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