Tap Drill Size Important? (29/64 vs. 27/64) - Pirate4x4.Com : 4x4 and Off-Road Forum

super_kev · 07-31-2007, 06:27 PM

I was in the process of drilling holes for installing some 1/2-20 bolts, and as I was drilling out the tap size (29/64"), I realized that I probably wanted to go with a coarse thread (1/2-13), which has a tap drill size of 27/64". Will 1/32" larger make a difference when threading for a coarse bolt? I could run with 1/2-20, but 1/2-13 would be nicer (less chance of stripping in a gritty environment).

rockcrawln · 07-31-2007, 06:51 PM

what kind of load and in which direction is it going to be on the bolt? If its not much then its probably fine.

Nitrofueled14 · 07-31-2007, 07:01 PM

Try each one on a test piece of scrap, couldn't hurt.

PTSchram · 08-01-2007, 09:39 AM

0.030 difference in a 1/2" hole isn't gonna make much difference, but it will be easier to run the tap in.

olchev4x4 · 08-01-2007, 12:16 PM

You get more thread engagement on a fine thread. They are stronger.
there is .015 differance in thread height between 1/2-13 and 1/2-20. And the hole size will be .031 too big. So if it was me i would stay with 1/2-20.
IMHO.

solarpower · 08-02-2007, 03:31 PM

Quote:

Originally Posted by olchev4x4

You get more thread engagement on a fine thread. They are stronger.
there is .015 differance in thread height between 1/2-13 and 1/2-20. And the hole size will be .031 too big. So if it was me i would stay with 1/2-20.
IMHO.

I agree.

Take into account what your using this for, if you are tapping into thinner material then you will get more threads in your material...I will try to use a fine thread when going into 3/8 and under thick steel...depending on application...

chances are that it needs to be a decent fastener if you are using 1/2 blot/threads so I would just stick with 1/2-20, if you have four or five other bolts holding it then you can probably be safe with going 1/32 oversize...

bit yes .015 per side of thread is alot actually.You will get more stuff in it if you now tap it 1/2-13

07-31-2007, 06:27 PM	#1 (permalink)
super_kev Registered User Join Date: May 2005 Member # 48230 Posts: 3	Tap Drill Size Important? (29/64 vs. 27/64) I was in the process of drilling holes for installing some 1/2-20 bolts, and as I was drilling out the tap size (29/64"), I realized that I probably wanted to go with a coarse thread (1/2-13), which has a tap drill size of 27/64". Will 1/32" larger make a difference when threading for a coarse bolt? I could run with 1/2-20, but 1/2-13 would be nicer (less chance of stripping in a gritty environment).

07-31-2007, 07:01 PM	#3 (permalink)
Nitrofueled14 Registered User Join Date: Mar 2006 Member # 69721 Location: Little Chute WI Posts: 199	Try each one on a test piece of scrap, couldn't hurt. __________________ 1978 F250 Custom - Fun truck 1996 Dakota - Daily driver

08-01-2007, 09:39 AM	#4 (permalink)
PTSchram Registered User Join Date: Jun 2002 Member # 12499 Location: In a Snap-On truck tryin' to make it pay Posts: 10,561	0.030 difference in a 1/2" hole isn't gonna make much difference, but it will be easier to run the tap in. __________________ Wrenchin' to riches! If you want peace, prepare for war.

08-01-2007, 12:16 PM	#5 (permalink)
olchev4x4 Registered User Join Date: Jul 2007 Member # 95408 Location: Coatesville PA Posts: 82	You get more thread engagement on a fine thread. They are stronger. there is .015 differance in thread height between 1/2-13 and 1/2-20. And the hole size will be .031 too big. So if it was me i would stay with 1/2-20. IMHO. __________________ Randy 1972 Blazer

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07-31-2007, 06:51 PM	#2 (permalink)
rockcrawln Registered User Join Date: May 2004 Member # 30586 Location: Redwood Valley, CA Posts: 1,168	what kind of load and in which direction is it going to be on the bolt? If its not much then its probably fine.