[AT] Drill size for bolt tap question

[John Hall]

This is what I use all the time at work--be sure to use cut taps, not 
form taps!!!!!!!!!!!!!

http://www.osgtool.com/Technical.asp?tid=1&id=1

  For most threads 67-75% is about average. A couple notes--drill bits 
almost always drill oversize--especially being used in a hand drill. 
Stainless steel and tool steels can be hard to tap so use a lower 
percentage. Some thread sizes calculate to the point there is no "good" 
drill size--sometimes metric/english drill is substituted, or the hole 
is milled or bored as needed. Having said all that, a 1"-8 hole with a 
15/16 drilled hole will produce only 39% of thread--absolutely horrible. 
No amount of Loctite will remedy that. You will need some sort of epoxy 
to take up the slack.

There are some threaded inserts that use a larger than normal drill 
size--I think McMaster Carr carries t
hem.
John Hall

On 4/13/2017 2:23 PM, Al Jones wrote:
> I've got some wallered out holes on a Farmall Super A torque tube that I
> want to fix.  They need to be 5/8" coarse thread, but somebody stripped 'em
> out, then did some sort of funky drilling, and now my holes are 15/16" in
> diameter.  I'd like to fix it myself but the longer I look at it the more I
> fear a machine shop visit is in order.
>
> My notion is to try to tap the holes for a threaded insert.  If I drill the
> holes out any larger for a larger tap, it's going to eat away the square
> boss on the side of the torque tube, so I want to avoid that. Now, the
> chart I am looking at says you need a 7/8" hole for a 1" tap. That means
> the existing holes are 1/16" too big.  Can I still tap it with the 1" tap,
> Loctite the daylights out of the insert, and get it to hold? These holes
> are on the left side, and there weren't many implements that used these
> holes, plus it's going to be mainly a show/plow day tractor.  But I do want
> them functional and they have to "look good."
>
> I have a good torque tube, but this is an early (probably built the first
> or second day of 1950 production) white demonstrator tractor, so I'm trying
> to keep the castings original.
>
> thanks!
> Al
>
>
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