1/4" BSF and 4BA taps

[Hal O'Peridol]

Got mine via UPS today. What diameter bit should I use of the initial hole prior to tapping?

Thanx!

Information

Warning: This is a relatively older thread This discussion is older than 360 days. Some information contained in it may no longer be current.

[Edward Horton]

Tap Drill Sizes

[Bruce_in_Oz]

QUICK TABLE:

Size Dia/ Dia/ Pitch/ Pitch/ Core Core Depth/ Depth/
Inch mm TPI mm Dia/" Dia/mm Inch mm

4BA 0.142 3.6 38.5 0.656 0.111 2.807 0.016 0.396
1/4BSF 0.25 6.35 26 0.977 0.201 5.1 0.025 0.625

So, for 4BA (core diameter 0.111") you will need either a No.34 (0.111") or, for a little clearance, a No.33 (0.113").

For 1/4" BSF, A No.7 (0.201") or No.6 (0.204")

See Mr. Brownells "wish book" or check your local tool shop.

(Now lets see what happens to the formatting when I post this!!)

[Hal O'Peridol]

Thanks Bruce!

[Hornant]

So 1/4" BSF and No.4 BA. Sounds to me like you fitting a 'T' scope mount.

I am doing the same. I have the 1/4" BSF tap but where do you get hardened No.4 BA screws and a and tap for the job.