Drill & Tap Size for GT40 Intake - Mustang Evolution

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Old 10-09-2014, 02:06 AM   #1
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Drill & Tap Size for GT40 Intake

Hi all,
I need to drill & Tap my Intake to make it a direct swap. I need to Tap a 3/8" NPT . I need to know what size drill bit to use.
I've looked at Drill Tap Size Charts.....But they don't make much sense to me.
Some people are trying to tell me a 9/16 " Drill bit...but that has to be wrong.
How can the drilled hole be larger than the Tap size...right ???
I'm thinking it needs to be a slightly smaller bit like a 5/16" or 1/4" bit to Tap a 3/8" NPT.
So what size Drill Bit do I use to Tap a 3/8" NPT ???
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Old 10-09-2014, 06:50 AM   #2
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When you buy the 3/8 NPT tap you'll see that the 9/16 drill bit is correct.
Tap…. 3/8-18 NPT
Drill Bit… a 37/64 drill bit almost impossible to find, so use a 9/16”drill bit
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Old 10-09-2014, 07:54 AM   #3
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5/16 drill for 3/8-16 and Q for 3/8-24. 9/16 is way to big

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Old 10-09-2014, 02:21 PM   #4
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Hmmm,..I think the guy gave us the wrong info Stock. I just checked the Thread Size on the fitting I got & it's 5/8"...not 3/8". ( Unless his fitting had a different size end )
Well that Stinks !!! My Tap & Die Set only goes up to 1/2" so I'll have to buy a Tap & Probably a Long Drill Bit. Looks pretty hard to get up in there to drill it right.
Another Question....Is there a way to remove the Chrome Vacuum Line from the Intake or do I need to cut it off & Drill it out like you would a Rivet ? The Line appears to be Pressed in...not Screwed in.
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Old 10-09-2014, 02:31 PM   #5
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Posting a couple pics of Motor since I added a few parts.
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Old 10-09-2014, 02:32 PM   #6
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Old 10-09-2014, 02:34 PM   #7
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Old 10-09-2014, 06:23 PM   #8
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Quote:
Originally Posted by Soda56k View Post
5/16 drill for 3/8-16 and Q for 3/8-24. 9/16 is way to big

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Not sure where you're getting your information but if you read the chart below just like so many others... I says for 3/8" pipe thread you should use 37/64" drill bit and like I stated above finding a 37/64" is hard to find; the next step up is 9/16" drill bit which is easy to find.
Read this if you still think that you should use a 5/16" or 3/8" drill bit.
Tap Chart NPT National Pipe Taper Threads

And if that doesn't work for you do your own search on which bit to use for a 3/8" pipe thread.
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Old 10-09-2014, 06:28 PM   #9
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Quote:
Originally Posted by Hellish Horse View Post
Hmmm,..I think the guy gave us the wrong info Stock. I just checked the Thread Size on the fitting I got & it's 5/8"...not 3/8". ( Unless his fitting had a different size end )
Well that Stinks !!! My Tap & Die Set only goes up to 1/2" so I'll have to buy a Tap & Probably a Long Drill Bit. Looks pretty hard to get up in there to drill it right.
Another Question....Is there a way to remove the Chrome Vacuum Line from the Intake or do I need to cut it off & Drill it out like you would a Rivet ? The Line appears to be Pressed in...not Screwed in.
Yeah he definitely passed out the wrong information on what bit to use.

What I've read about that line is with a twisting motion while using pliers you use heat on the line which makes the epoxy or what ever is holding the line it in place release and you can just pull the tube out.
Like I said it's what I've read.

Curiosity what fitting is 5/8" ? On a fox body all the sensor fittings are 3/8" except the heater tube and that's 1/2".

Be Proud......Great looking engine.
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Old 10-09-2014, 07:38 PM   #10
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That was my fault. I didn't see the NPT after the 3/8 when I read it. Thought he was just looking to tap a 3/8 hole

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