Correct drill size for 1/8" npt tap

[paker]

Goal: To make a drain hole (1/8" npt) in a gear housing (1/2" thick aluminum casting)
Problem: Drilled 11/32" hole. First 3 threads were easy. Now working on the 4th thread. It is becoming progressively difficult. Even 1/4 turn of the tap is a struggle. Not having a proper tap holder setup is making it worse. About half of the hole depth is threaded.

Question 1: Did I choose the right size drill? I wonder if drill size should vary with thickness.
Question 2: Do I really need to tap the entire hole depth? Can I stop after 4-5 threads?
Question 3: Is there a tap extender? The gear housing is in a car and access is limited. I cannot use the handle of the tap holder. https://www.homedepot.com/p/Gyros-0...VEb7ACh2t-An_EAQYAyABEgKdRPD_BwE&gclsrc=aw.ds Instead I am using this https://www.lowes.com/pd/Hillman-2-in-3-Phillips-Shank-Screwdriver-Bit/1000244185. If I can extend the tap holder for 1", I can use the handle bar.

Any advice will be much appreciated.

PS: I have done some tapping/threading before. When threading became difficult, something bad used to happen. I am afraid of breaking the 1/8" npt tap in the hole.

 

[cvairwerks]

You using a straight thread tap or a tapered tap? If you are using a tapered tap, it's wrong hole size. If a straight tap, correct size.

 

[jumbojak]

I want to say that's a size Q, off the top of my head. Should probably look it up though.

 

[Bigblue&Goldie]

I believe letter size R is what you need. Some charts say Q.....so who knows? The threads are tapered on NPT, which probably explains your issues.

 

[LXCam]

R


As for tapping a NPT hole, you’ll be doing lots of reversing.

 

[paker]

It is a tapered pipe tap. R=0.339, 11/32=0.3438. So I actually used an oversized drill.

Is there a tap extender? Basically I need a tall tap holder.

 

[jumbojak]

What type of oil are you using?

 

[paker]

Gear and chain oil. I am lying on my back and the hole is straight up. That's why I am using a thick oil.

 

[like2wheel]

Is there a tap extender? Basically I need a tall tap holder.

I've often used a 12pt socket, extension & rachet
There's usually a size that fits well.

 

[LXCam]

Lisle makes a tap holder that works on some but not all 1/8” NPT taps (depends on the square drive size).

 

[Wrench97]

It may be a 1/4" square if that's the case a 1/4" socket extension will work.

 

[paker]

I've often used a 12pt socket, extension & rachet
There's usually a size that fits well.

I found a 12 point socket that fits the tap. How do you connect the socket to the wrench? Do you mean a regular socket wrench? Instead of tap wrench that has handle on both sides?

It may be a 1/4" square if that's the case a 1/4" socket extension will work.

5/16" socket fits the tap square. It is a 3/8" drive socket.

 

[Wrench97]

Try using a ratchet or breaker bar to turn it, just be careful to hold it straight.
I'd feel better about it if it was a 8 point socket.

 

[MattT]

Question 2: Do I really need to tap the entire hole depth? Can I stop after 4-5 threads?

Don't run the tap all the way in or you'll end up too loose. You need to gauge the hole by screwing your plug in finger tight until you've got decent thread engagement but still have turns left outside the hole to allow for tightening. That 4-5 turns of the tap mark would be a good point to start trying the plug.

Gear and chain oil. I am lying on my back and the hole is straight up. That's why I am using a thick oil.

That's probably making things worse. Use a light cutting oil if you have any. Or WD-40 works pretty good on aluminum.

 

[paker]

Thank you for answering all my questions. I am going back to the driveway to finish the project.

EDIT:
Thanks to your advice I was able to complete the project. This was my first npt tapping and I didn't know tapping depth is adjusted to fit the plug. This was also my first socket wrench threading. I kind of panicked when I found the T-handle tap wrench wouldn't fit. Tapping definitely got easier with transmission oil, the lightest oil I could find in the garage.

 

[joe49]

Expensive but the proper way is to use Reamers for Taper-Pipe Holes.

 

[mikegt4]

Try using a ratchet or breaker bar to turn it, just be careful to hold it straight.
I'd feel better about it if it was a 8 point socket.

8 pt. sockets are great for taps. I have several "free" sets from co-workers who purchased tool sets that included "worthless" 8 pt. sockets. They are also good for square head pipe plugs.

 

[The Cobbler]

as mentioned , an npt reamer in the right size is the proper thing for pipe taps due to the taper of the threads . but drills work too,a s you found out

 

[Willie Makeit]

Expensive but the proper way is to use Reamers for Taper-Pipe Holes.

winner winner chicken dinner

 

[Qrazy]

Don't run the tap all the way in or you'll end up too loose. You need to gauge the hole by screwing your plug in finger tight until you've got decent thread engagement but still have turns left outside the hole to allow for tightening. That 4-5 turns of the tap mark would be a good point to start trying the plug.

This. And 11/32 is the correct drill size. NPT can be trying at times, as you stated it gets a little tougher as you go deeper (it's because it is tapered).

 

[paker]

I am not a professional machinist but received a semi-formal machining training and did some machining. But I have never heard of or seen a tapered reamer/8-pt socket. At least the machine shop I used didn't have one. Thank you for the advice.

 

[gungatim]

this is one situation where I would have went straight for a sliding T-bar on an extension and 8pt. socket. you know, those things everybody says they have no use for and never use, lol...

OP I literally went through the same situation almost, was fixing a stripped 1/8" npt hole on a gas burner for my griddle. it leaked, so I pulled the brass gas valve but couldn't get a new one to seal. (original manifold was made from standard 1" black pipe)

I put the old one in, and mig welded around it to get more material to re-tap but of course had to re-tap it. HUGE pita! was afraid of breaking the tap and only got a little turn each time. I ended up using a short pipe tap with my sliding t-bar instead of the tap wrench which was way too tall. went really slow and got better grip and leverage that way.

 

[white6589]

Paker, Can I try and guess what you're tapping a plug for?


I'm guessing a Ford AWD PTU unit for maybe a Ford Edge, so you can do the fluid exchange.

 

[Robby321]

3/4 threads usually enough. It has a taper, so check with the plug.

 

[paker]

Paker, Can I try and guess what you're tapping a plug for?


I'm guessing a Ford AWD PTU unit for maybe a Ford Edge, so you can do the fluid exchange.

Hahaha, the infamy of Ford Edge is widely known! To amuse other members who advised me, let me explain. Ford Edge AWD has a gear box that transfers power to rear differential when needed. It is sealed for life and has a fill hole and a vent. Apparently Ford engineers underestimated heat input from a nearby catalytic converter and an exhaust pipe, charring gear oil to a gooey mass. Premature failure is well documented in owners' forum. Some owners drill and tap a drain hole for ease of maintenance. Thank you again for your help.

gungatim,
I didn't know a tool called sliding t-bar existed. I am going to buy one for my own Christmas gift!