Home Forums Hunting & Shooting Gunsmithing Bit size for 1/4-36 tap?

What size drill bit do I need to use for a 1/4-36 tap, someone suggested 5.6mm, and I just want to make sure I don't screw this up.

Thanks,

Don

Try a #8 or #9

So start with the #8 and try it, then if it's not right go witht the #9?

I'm just putting in a quick release bolt retention plunger on a muzzleloader.

http://www.stanford.edu/~jwodin/holes.html

Great chart, but it gives drill sizes for 20, 28, and 32. Does it matter that these will be 36?

Forgive my ignorance, I'm new to this.

Here is a really good formula for special thread tap sizes.


TAP DRILLS FOR SPECIAL THREAD SIZES
To determine tap drill size for special threads, use the following formula:

TAP DRILL SIZE = DIA - (.974 / PITCH)

EXAMPLE: 3/8-32
.375 - (.974 / 32) = .345
USE DRILL SIZE CLOSEST TO .345

In your case that would be.

.250-(.974/36)=.223

.250-.027=.223

A 5.70mm drill is .2244"

A #1 is .228"

A #2 is .221"

I'd go for the 5.7mm if you could find it but either of the others would likely work fine.

IMO a # 8 or #9 are way too small.

The minor diameter of a 1/4-36 UNS internal thread is .220" to .226"

#8 is .199"

#9 is .196"

Thanks very much nsaqam!!!

You beat me to it! I keep that formula tacked up over my workbench.

A simpler formula for finding tap drill size is:

Major diameter minus the pitch of one thread.

In the case of 1/4-36 the major diameter is .250, to find the pitch of one thread divide 1.000 by 36 which equals .030. Subtract .030 from .250 and your answer is .220, a 7/32 drill bit is .218 which is close enough and is a commonly available drill size. The fact that a 7/32 (.218) is .002 smaller will make no difference, more than likely it will produce a hole closer to .220 anyway. It never hurts to go a few thousands bigger as this reduces the power it takes to tap the hole, tap breakage is also reduced. Never try to tap a hole without some sort of oil, 3 in 1 or motor oil is fine for the occassional use. Special tapping fluid can be purchased from Brownell's and other sources if you are tapping many holes. The benefit is it helps produce better quality threads and prolongs tap life.

You're right, I went the wrong way. 1/4-28 takes a #3, for 36 TPI it would be a #2 or#1

Pick up a machinists handbook. Every formula and chart you could possibly need is in it.