I have a 3/8 NPT to -8AN adapter to put in my oil pan. I went to Home depot and bought a drill and tap kit that came with a 3/8 tap which said on the back to use a 5/16" drill bit. Not thinking, I went out, just finished drilling it, and THEN looked at it and said to myself "my my that's a small hole..." sure enough, looks like 3/8 NPT means a 3/8 Inner Diameter and I just drilled and tapped a 3/8 diameter hole!

Sooo... what is the CORRECT size tap and drill bit size for a 3/8 NPT hole??
Also, where to find such a drill/tap? Because the 3/8 one I got which is way too small for the 3/8 NPT was one of the largest I was able to find - the kit I bought only goes up to 7/16...

Help please...
-Ryan

 

9/16" or 37/64" IIRC.


make sure you got the right tap.... if it's like 3/8-16 or 3/8-24 it's not the right thing.

 

yep 9/16 or 37/64

and its 3/8-18 npt i believe

 

it should simple be a 3/8 NPT tap.

-16 or -24 would signify NC or NF threading, not NPT.

 

Local industrial place says it's supposed to be a 37/64" drill bit and a 3/8-18 pipe thread tap (which is apparently very different in size from my 3/8 bolt thread tap)...

Can anyone verify this? Does that sound right?

edit: posted this before I saw a few more replies. sounds like I've got the rights sizes now. Are you saying I can use either a 37/64 or a 9/16 bit?

-Ryan

 

yeah sounds right.

 

Its either slightly bigger or slightly smaller then the tap, either of those bits will work.

 

am i confused then?

http://www.fastenal.com/web/products...ex?sku=0326935

 

no you're right -18 is correct. -16 and -24 isn't

 

Usually, the proper drill size is written right on the tap.

C

 

I used a 9/16" bit, seemed to work perfectly, especially since you have to drill at a slight angle because I know I couldn't fit my drill right on with my A/C in the way.

 

I used a 9/16" drill as well, worked just fine.

 

Perhaps if your profile included the city you live in, somebody local would chime in who could let you borrow one. Otherwise, most Nothern Tool stores have those sizes on the shelves.