drill for pipe threads
Senior User
Joined: Jun 2003
Posts: 188
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From: Wesley Chapel, FLA
drill for pipe threads
The charts call for No. 7 drill bit for a 1/4-20. The shank on a No. 7 measures .199". Call it a sloppy 3/16 (3/16 = .1875"). The 1/8"NPT calls for an "R" letter drill. An 11/32" would work (might be harder to start), as would anything up to a 3/8", depending how deep your fitting can be. I don't have an "R" drill to measure to tell you what is closest to it. I do know on letter and number sized drills there may be no actual fraction equivalent, but you can get real close if you have a bit index in 64th's. For the popular sizes like 1/4 - 20 I buy a real good bit that is exactly the one called for, and save it just for tapping holes of that size (keep it right with the tap). The size they call for does work the best. They are usually available at industrial supply houses. By the way, don't use a 29/64" for the 1/4". 29/64" is a 64th over 7/16"! I'm sure that was a typo, or a cranial cramp that made Mr. Suds say that.
Senior User
Joined: Dec 2002
Posts: 251
Likes: 0
From: Phoenix USA
drill for pipe threads
inliner,
1/8 NPT uses #S
1/4 NPT uses 29/64
for anybody who needs more check out:
http://www.smithfast.com/pipetap.htm
Have fun.
1/8 NPT uses #S
1/4 NPT uses 29/64
for anybody who needs more check out:
http://www.smithfast.com/pipetap.htm
Have fun.
Elder User
Joined: Apr 2003
Posts: 524
Likes: 0
From: L.Bucks,Pa.
drill for pipe threads
As I said,I didn't have a chart handy.But those sizes are correct,
dependent on straight or taper thread.BTW, correct size for a 1/4-20 is #7(.201).The diff. between an "R" and 11/32 is only a few thousandths of an inch.Won't make a difference and may be easier to thread.Check this site and print it.I know,he asked NPT.
http://www.korit.com/tapndrill.htm
dependent on straight or taper thread.BTW, correct size for a 1/4-20 is #7(.201).The diff. between an "R" and 11/32 is only a few thousandths of an inch.Won't make a difference and may be easier to thread.Check this site and print it.I know,he asked NPT.
http://www.korit.com/tapndrill.htm
Last edited by Sudsrat; Aug 21, 2003 at 03:31 PM.
