I need to tap a 3/8" female thread, any tips?

TerryBPP · 02-02-2006, 10:57 AM

I have tapped smaller holes, some where eaiser thatn others (this sounds way to dirty). But I need to tap a 3/8" NPT hole in a piece of 1/8" aluminum. I can only do it once. I have the right tap but I'll be doing it by hand. No lathe. Any tips?

lendaddy · 02-02-2006, 11:01 AM

Can you fit the tap in a cordless drill? I assume you're concerned about staying true to center line, right? The drill will do the turning while you just hold for centerline.

stevepaa · 02-02-2006, 11:03 AM

How will you get any thread bite in 1/8 Al?

BGCarrera32 · 02-02-2006, 11:06 AM

You're asking for trouble if the material is only 1/8" thick. Weld a bung to it to beef up the area.

id10t · 02-02-2006, 11:08 AM

Take her out for a nice meal and a few drinks first.... maybe even consider waiting until the second date.

TerryBPP · 02-02-2006, 11:08 AM

Haven't tried it but its a pretty good size tap. I doubt it will fit. Luckily the aluminum is on the softer side so if I have to turn be hand it wont be the hardest part.

The part is actually for my turbos oil return on project evil e30. It was very small and restrictive, causing the oil to back up into the turbo. So I have to tap the hole to use a 3/8" to 10AN fitting.

stevepaa · 02-02-2006, 11:21 AM

You should figure out a way of tapping, then welding a fitting.

legion · 02-02-2006, 11:22 AM

I've done exactly this several times.

I have built custom paintball guns, and the 3/8" NPT is the standard threading for air lines. Most paintball guns are made from billet aluminum, and the air lines are braided steel with stainless steel threaded ends.

I would clamp the piece down to a drill press and start with a small pilot hole. Move up to the full size (I can't remember the proper drill bit to use with the 3/8" NPT off the top of my head) drill bit.

I would tap by hand and not use a drill. My reasoning is that it is too easy to spin the drill too fast and strip out the hole in the aluminum. Most taps are self-centering and taper out as they cut the threads deeper and deeper.

TerryBPP · 02-02-2006, 11:30 AM

legion, you just gave me the courage (plus 3-4 jack & cokes) to do this when I get home.

M.D. Holloway · 02-02-2006, 11:33 AM

I find it funny that Terry of all people is asking us how to tap a female hole...(I really never got out of 7th grade)!

TerryBPP · 02-02-2006, 11:40 AM

Hey I'm still a virgin!

I can't even type that with a straight face.

targa911S · 02-02-2006, 11:43 AM

Quote:

Originally posted by legion
I would tap by hand and not use a drill. My reasoning is that it is too easy to spin the drill too fast and strip out the hole in the aluminum. Most taps are self-centering and taper out as they cut the threads deeper and deeper.

Totally agree.

notfarnow · 02-02-2006, 11:56 AM

Yep, by hand. Just did this in an aluminum veggie tank. I do 1/2 turn, then 1/4 back, then 1/2 turn, then 1/4 back. That's the way I was taught to use a tap.

widebody911 · widebody911's Garage

The smallest tap I have is 1.5" diameter. Like Jake said, a little in, then partially out. Lubrication helps, too.

Dixie · Dixie's Garage

USE A TAP HANDLE!!!

dad911 · 02-02-2006, 02:11 PM

Quote:

Originally posted by legion
I've done exactly this several times.

I have built custom paintball guns, and the 3/8" NPT is the standard threading for air lines. Most paintball guns are made from billet aluminum, and the air lines are braided steel with stainless steel threaded ends.

I would clamp the piece down to a drill press and start with a small pilot hole. Move up to the full size (I can't remember the proper drill bit to use with the 3/8" NPT off the top of my head) drill bit.

I would tap by hand and not use a drill. My reasoning is that it is too easy to spin the drill too fast and strip out the hole in the aluminum. Most taps are self-centering and taper out as they cut the threads deeper and deeper.

After drilling the hole, put the tap in the drill press chuck, DO NOT APPLY POWER!, and turn it by hand, using AlumTap lubricant. That will keep the tap in perfect alignment with the hole.

Repeat, no matter how tempting, don't turn on the motor!, turn it by hand. 1/4 to a half turn at a time, and backing out to remove chips often.

jim72911t · 02-02-2006, 03:39 PM

You'll need a 37/64" pilot drill, but you could probably get by with a 9/16." I don't like the idea of starting with a small drill bit and going up in size; I've seen too many holes ovalized using this method. Drills are designed to cut from center, not to enlarge a pre-existing hole.

I like the suggestion of putting the tap in a drill press and turning the spindle by hand; that'll surely keep everything square. Even better would be if you had access to a mill, then you could tap it using the mill's spindle motor in back gear.

I just did something similar with a 3/4 NPT tap (about an inch in diameter) into some 1/8th or 3/16th material (cant recall). I only got a couple of threads, but it turned out to be enough. (I was lucky enough to have a mill at my disposal, though)

As a fellow E30 owner, I've been following your project with great interest.

Jim

TerryBPP · 02-02-2006, 03:49 PM

Done! Friggin piece of cake. 9/16" bit (was a touch to small but worked) and then tapped. The billet aluminum was so soft I didn't even need to lube it. Hardest part was keeping the ackward shaped aluminum piece square in the vice.

Did it by hand using 2 cresent wrenches! My T-handle wasn't big enough. Sometimes ghetto works. But I definetly wouldn't have been able to do it if it wasnt aluminum.

Thanks for the help.

911boost · 911boost's Garage

37/64 is the correct size. I just tapped the driver side exhaust manifold for my EGT gauge on the wrx. Use lube and tap, back out clean threads, then tap some more. I was very careful as to not tap all the way through the manifold so that when I tightened the probe it sealed up nice.

PS, I did it by hand, and it turned out just fine.

Bill