Boring (drilling) and tapping
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Posting Guru
Joined: Oct 2001
Posts: 1,777
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From: Culpeper, Virginia
Boring (drilling) and tapping
I had to drill the caliper retaining bolt off my truck. It appears to be a 5/16" daimeter bolt. Problem is I gouged the edge of the hole really well drilling. I figure go up one size and tap the new hole with threads. But I'm confused about proper drill size. If I pull out the 3/8" tap, it is much smaller than a 3/8" diameter bit. Is there a rule of thumb here? Like drill one size smaller than the tap? I want the threads to be 3/8" if I'm thinking this through right. One size bigger than factory. I've only used it once, please advise. Thanks again, BLT
Logistics Pro
Joined: May 2003
Posts: 4,238
Likes: 21
From: Northern Illinois
Boring (drilling) and tapping
tap sizes:
To Tap This Size Screw Or Bolt: Use This Drill Bit: (Closest Fractional
Decimal Inches
1/4-20 NC #7 13/64" .2010
1/4-28 NF #3 7/32" .2130
1/4-40 NS #1 - .2280
5/16-18 NC Ltr.F 17/64" .2570
5/16-24 NF Ltr.L - .2720
5/16-32 NEF 9/32" 9/32" .2812
3/8-16 NC 5/16" 5/16" .3125
or go to this site for more info:http://www.korit.com/tapndrill.htm
To Tap This Size Screw Or Bolt: Use This Drill Bit: (Closest Fractional
Decimal Inches 1/4-20 NC #7 13/64" .2010
1/4-28 NF #3 7/32" .2130
1/4-40 NS #1 - .2280
5/16-18 NC Ltr.F 17/64" .2570
5/16-24 NF Ltr.L - .2720
5/16-32 NEF 9/32" 9/32" .2812
3/8-16 NC 5/16" 5/16" .3125
or go to this site for more info:http://www.korit.com/tapndrill.htm
Elder User
Joined: Jun 2002
Posts: 519
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From: CT
Boring (drilling) and tapping
If you are tapping steel or a cast material you may want to use a fine thread. Less material removal during the cutting of the thread and the tap has a larger root diameter less chance of shearing should your thread centerline not run parallel to the drilled tap hole centerline. A good thread cutting lubircant makes a lot of difference as well.
Senior User
Joined: Apr 2002
Posts: 467
Likes: 0
From: WEEDSPORT NY
Boring (drilling) and tapping
I've made the offer before and it still stands. If anybody wants or needs a tap/drill chart, decimal equivalent chart, or a tapping fluid sample, just send me your name & address. I am not trying to sell anything here. It's just that the different manufacturers give us so many samples that we can't hand them all out. I would rather share them with somebody that can use them than throw them away.
