tapping for 6-32

The 6-32 tap I got says to use a #36 drill for the pilot hole. The closest fractional size is 7/64... will this be ok, or will it be too big? this is for the small screws to hold the dynon heated pitot tube in the mast.

IMHO if you're going to work on airplanes, get yourself the proper numbered drill bits.

You can buy individual numbered bits at your local Ace Hardware (or the Canadian equivalent) for about $2. That's what I did for those random sizes needed for things like taps.

Test it!

Do a test hole in some scrap and see how it does. Less than .003 difference in diameter. (.002875)

Phil - head out to Canadian Tire and purchase a drill index with number-size drills. I purchased one from them a few years ago that has both number and fractional drills in the same box - very handy.

That's a fairly expensive part to deviate from best practices...

A few bucks for the right bit, versus a few hundred dollars for pitot tube when the screws don't hold it securely.

I know what I'd do.

Tap

I have fractional, some letter, and number drill bits.
The 7/64 will work just fine. .003 difference, AN bolts vary almost that much.
Recent post here MANY recommendations to use #27 drill for #6 screws. WRONG-proper size is #28.
I was a machinist in a previous life.

jrs14855 said:

I have fractional, some letter, and number drill bits.
The 7/64 will work just fine. .003 difference, AN bolts vary almost that much.
Recent post here MANY recommendations to use #27 drill for #6 screws. WRONG-proper size is #28.
I was a machinist in a previous life.

Click to expand...

On aircraft, #27 is the proper size unthreaded hole for a #6 screw.
If you use #28 and are dimpling the hole with the dimple dies commonly available you are probably stretching the hole with the die pilot and increasing the risk of cracking.

FYI

prkaye said:

The 6-32 tap I got says to use a #36 drill for the pilot hole. The closest fractional size is 7/64... will this be ok, or will it be too big? this is for the small screws to hold the dynon heated pitot tube in the mast.

Click to expand...

Here is a good set. Pan American Brand, Avery shipped these bits with my kit and I have been very happy with their durability.

Hard to beat the value.

To the question: a 6-32 with #36 hole will yield 78% thread engagement, a 7/64 will yield 71% engagement. 9% reduction in holding. The fastener application/loading would need to be considered to determine significance. Now you have more for decision making.

OTOH What material and length of thread engagement is this? You could make a hole in an identical scrap piece and see what the torque at failure is for that engagement. If it is close to 50 in-lb, I would use it, but you can make up your own mind.