8 3/4 Diff Drain Plug Help

[66 Sat]

Hi, I want to install a drain plug in my 8 3/4 diff for easier servicing. I read an article in Hot Rod magazine on how to do it but am confused about the diameters of the drill bit & plug they refer to.
In the article it states to drill a hole using a 21/64" drill bit and then tap the hole with a 1/8" tap and insert a 1/8" plug. Now I grew up mostly metric so didn't think anything was wrong until I went to the hardware store to buy the stuff. A 21/64" bit is way bigger than the 1/8" tap. Should the article read 3/8" tap? I've never tapped a hole before but assume the tap is slightly bigger than the hole to be able to cut the threads.
I realise this may seem obvious to some people, but I would rather ask a dumb question than stuff this up.

The link to the article is here https://www.hotrod.com/articles/mopp-0912-installing-a-drain-plug-in-the-differential-housing/

Cheers

 

[dadsbee]

21/64 is the correct drill for a 1/8 - 27 TPI NPT pipe plug. A "Q" is actually truly correct, but 21/64 is close enough.

 

[Big Bad Dad]

1/8 pipe is NOT the same as a 1/8 regular drill. 1/8 NPT, (National pipe thread), is bigger. You need to look for the proper size bit and tap. I did not read the article, but I would question if the steel in an 8-3/4 housing is thick enough to hold a pipe plug. I did something similar about 35 years ago, but had a brass fitting brazed on the bottom of an 8-3/4 housing and put the plug in it.

edit.. I believe you are confusing 1/8 pipe thread with 1/8" bolt thread. They are two different animals. The pipe thread is tapered, which helps plug a hole when tightened. The bolt thread is straight, and will leak.

 

[MOPOWER871]

I like the idea of a drain plug,would suggest welding a bung in place of tapping the case. if you drill the case then remove the center section so you can remove and metal fragments that will be left over from the drilling and tapping process that may have gotten inside. then add a magnetic drain plug. would also consider safety wire the plug to one of the centers studs. anything worth doing is worth doing right the first time.

 

[beanhead]

MOPOWER is right, weld in a threaded bung first...the metal of the housing isn't really thick enough to have enough threads to get a reliable seal. I'm sure someone will be along shortly to say I'm full of it, they just threaded the housing and it's worked for 30 years blah blah but I wouldn't trust it...leaks suck! The last sentance in the above post is gold..but should you choose the article's method, they call for '1/8" pipe tap' which is closer to a half an inch, as bigbaddad stated..

 

[66 Sat]

Ok thanks for clearing it up guys, I'll see about getting a bung welded in instead.

 

[Bad Sport]

Ok thanks for clearing it up guys, I'll see about getting a bung welded in instead.

Just make sure it is to the side of the ring gear and not directly under it, if any part of it should protrude through the case enough to make contact.

When I did mine I just drilled and tapped. It's been fine so far................fingers crossed.

 

[Cranky]

I like the bung hole idea better too.....wait, that didn't come out quite right.

 

[19polara64]

I just tapped mine in my 64. Works fine. Because it’s such a small diameter there is enough meat imho. Keep in mind it takes a while to drain for sure. If you want a big plug to drain quickly then definitely weld in a big bung. Also a magnetic plug is a plus!

 

[Dipstick]

I've been drilling/installing a 1/8" NPT plug or years, the housing is plenty thick enough. I install it off-center to avoid the welded seam at the bottom center and use a socket-head plug so little protrudes.