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Found this using Google...

TAPER PIPE SIZES - NPT
----------------------------TAP DRILLSIZE SIZE----------------------------1/8-27 R1/4-18 7/163/8-18 37/641/2-14 23/323/4-14 59/641"-11-1/2 1-5/321-1/4-11-1/2 1-1/21-1/2-11-1/2 1-47/642"-11-1/2 2-7/322-1/2-8 2-5/83"-8 3-1/43-1/2-8 3-3/44"-8 4-1/4

Sorry -this format sucks... see below!
 

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Premium Member
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So i bought a set of NPT taps and for some reason the taps dont give me the drill bit size to use. Can sombody list the drill bit sizes to use for the following NPT tap's

Try these
1/8" R
1/4" 7/16
3/8" 37/64
1/2" 23/32
3/4" 59/64
1" 1 5/32
1 1/4" 1 1/2
 

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deckboat owner
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480 Posts
For bigger diameters you are going to want to use the proper tapered reamer also. While it may start OK, you'll be opening the top up ALOT by the time you get deep enough. GET the reamers too!
 

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"The" masheenist
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5,444 Posts
For bigger diameters you are going to want to use the proper tapered reamer also. While it may start OK, you'll be opening the top up ALOT by the time you get deep enough. GET the reamers too!
AWWWWWWWW....

Don't be scared.

If you are pipe tapping aluminum or steel by hand, you will be fine.

Brian
 

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8,169 Posts
Gages....$$$

For bigger diameters you are going to want to use the proper tapered reamer also. While it may start OK, you'll be opening the top up ALOT by the time you get deep enough. GET the reamers too!

Also, don't forget the "thread standards". You wouldn't want the threads too deep, or too shallow. "GO" "NO GO" gages are only a few hundred dollars.:)hand

Ray
 

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"The" masheenist
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5,444 Posts
Also, don't forget the "thread standards". You wouldn't want the threads too deep, or too shallow. "GO" "NO GO" gages are only a few hundred dollars.:)hand

Ray
Come on Ray, please tell me the hand wave means you are just messing around....

You guys are unreal.

haha

Telling the guys he needs tapered reamers to pipe tap 2 aluminum holes a year and now GO/NO-GO GAUGES?

No wonder people are scared to ask questions.

Brian
 

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"The" masheenist
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5,444 Posts
Doesn't your transom plate already have a pipe tapped hole for the strain relief?

Guess i need to get a 59/64 bit, i need to tap my transom plate for the strain relief connector i got from grainer.

 

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"The" masheenist
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5,444 Posts
Please tell me that isn't galvanized steel...the waffle finish sure looks like it.

You will be fine drilling and tapping that hole. Keep it well oiled, turn the tap about a quarter turn at a time, back it up slightly to break the ship and start again, you will be fine.

before the setback It HAD holes :D, The new plate i made doesnt have them yet!

 

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"The" masheenist
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Ok,,,thank you. I was worried.

Don't listen to the naysayers on the the pipe deal. Just follow what I said and you will be fine.

Just keep an eye on the depth. It doesn't take more than a half turn to go from too shallow to too deep.

Brian

It's aluminum, was just from the scrap pile outside... It's alittle better looking now that i sanded it. It's getting powdercoated white.

check out http://www.performanceboats.com//showthread.php?t=37639 for more on it
 

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"The" masheenist
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Ya, realized that my first time using a NPT tap, i ran the tap all the way down. Lets just say the fitting bottomed out. :|err
Close to 3/4 of the way down, you need to be keepin and eye on the depth.
 

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deckboat owner
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480 Posts
When I posted my reamer suggestion, no mention was made of material, or actual size to be tapped. Recommended drill sizes for threads from 1/4" to 1 1/4" were requested. SO, if the hole was for an 1 1/4" NPT in tool steel, would ya still consider me a "naysayer" when I offer advice of using a tapered reamer? or add expensive go/no-go gages? I was just trying to help, and offer a warning if it was a big deep hole into some rotten material (nobody wants that).:p
 
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