Before i drill too big a hole.....

I need to drill and tap for 3/8 npt. I have the tap (that was $16!). Internet says i need a 9/16 hole.

I have a 37/64 drill bit. SLIGHTLYbigger than 9/16. 

Im trying to not spend money if i dont have to.

Should i buy a bit, or will 37/64 be ok? 

How do you have a 37/64 drill bit and not a 9/16?

Thats a good question. I have wound up with a bunch of strange sizes over the years, mostly through yardsales and such. But my index only goes to 1/2, and generally that has all the sizes i need.

37/64 I believe is the size to drill out rotors for arp wheel studs with the .625” knurl...

In reply to Dusterbd13 :

I won’t give you permission to use the wrong size, just explain that a tap is designed for certain diameters.  The answer is yes you can use it but the threads will be undersized so they won’t hold as well. If you need the full strength then don’t do it, if it’s the 12th hole of 24 you might get away with it.  

Unless it’s a pipe tap in which case the only issue is the fitting will be deeper. 

For NPT it shouldn't be an issue, I just checked my chart at work and it actually recommends 37/64.

I usually drill slightly undersize if my option is too big or too little.

For NPT, I have a neat-o set of tapered reamers I got from Harbor Freight in a little wooden box.  Drill first, then ream, then tap.  Makes tapping a whole lot easier.  (To hold them, I use a 12 point socket in a square-drive adaptor in a drill)

Cool. Its npt for a water temp fitting in an aluminium block. 

Thanks y'all.  Ill post pictures of it in a little while. 

The outside diameter of the thread is the major diameter, the bottom of the thread is the minor diameter.  The minor diameter is the major minus the pitch diameter.  P = 1/tpi.

3/8 - 16 minor diameter = .375 - .0625 or .3125  (5/16").

Dan

In reply to 914Driver :

You are absolutely correct--- On machine threads.

NPT is measured by the I.D. of the pipe, not the threads. The tap drill for a 3/8-18 NPT is 37/64" (.578")

Dan's formula works on all machine threads, standard and metric, and is very handy to know if you don't have a chart or internet available.

According to this chart:  http://www.carbidedepot.com/formulas-tap-pipe.htm

9/16 is the right size if you have one of the reamers that Knurled has.  If you don't, then you should use 37/64.

Not my best work, but if it don't leak i dont care.

Sorry,  my eyes saw pipe thread, my brain went right to my old world.

dculberson said:

How do you have a 37/64 drill bit and not a 9/16?

I have 37/64" drill bits because it's the correct size for a 3/8 NPT and we ship them with 3/8 NPT taps. I'm pretty sure I don't have a 9/16 unless there's one on a step drill somewhere.

In reply to 914Driver :

BUT- You are one of the few people I have met in over 30 years of machining that knows the formula for determining tap drill sizes!

NPTs are somewhat forgiving of a slightly larger hole because they are tapered. So if your hole is slightly oversize you will tap farther in and your fitting will thread farther in. That can be a problem sometimes, like when you are putting a fitting in perpendicular to a fluid path and your fitting goes in so far that it blocks that path partially. But if it's a tiny difference, the taper makes up for it by just threading in further.

SaltyDog said:

In reply to 914Driver :

BUT- You are one of the few people I have met in over 30 years of machining that knows the formula for determining tap drill sizes!

Want to hear about 10-32,  5-40s etc.?

 10 (or whatever number) X .013 + .060 = major diameter.  Yes I do metric to english in my head.

Thanks Salty.

In reply to 914Driver :

Finally!!

Someone else who "Can Math", as the kids say today!