Muncie drain plug threads

[Donald H.]

This is a 69 Muncie. I believe the fill or drain plugs are NPT thread. Some previous owner use a standard hex head bolt in the drain hole. I would like to replace with a correct plug.

From what little i know, NPT threads are cut at a different angle than a regular bolt. Did using a regular bolt mess the thread up so that a correct plug can no longer be used?

Thanks,

Don

[Domenic T.]

Re: Muncie drain plug threads

Don,
Sure wont do good, but you will have to determine that when you remove the hex bolt. If the hex grabbed low in the hole you might get lucky.
Lots of guys drilled & tapped the case and if that was the case it might be smaller than the 1/2 NPT.

Dom

[Tom P.]

Re: Muncie drain plug threads

On stock factory GM Muncie cases (NOT aftermarket cases), the fill and drain plug holes have 1/2in NPT threads (that is a tapered thread).
ONLY M22 cases got threaded drain plug holes, and ALLLLLLLLL 70-later cases (REGARDLESS of ratio) got drain plug holes.
Beginning with the very first 63 Muncie case, all cases had the boss for a drain plug hole (why they did not put drain plug holes in them, I'll never understand!!!). So, without exception, I always drill and tap a case for a drain plug, then REMOVE the donut magnet that is glued to the bottom of the case and install a magnetic drain plug.

IT MAY BE possible for you to run a 1/2in pipe tap in the drain hole to clean up the threads, then run the tap in a little deeper to assure that the threads are clean. When installing the drain plug, it may be a good idea to put plenty of thread sealant on the threads of the plug.

Here is the drain plug boss before drilling.



I first drill a pilot hole.



Then I use an 11/16 drill bit for final hole size.



Tap the threads (use plenty of thread cutting lube).






Magnetic plug installed.



This is the magnetic donut which I remove from all Muncie cases that have it.

[Donald H.]

Re: Muncie drain plug threads

Thanks, Dom and Tom

I was mistaken, not sure why I thought it was a 69, but it is not a 69 transmission. I was just able to see the main case casting and it is 3851325, so it's a 64 or 65 case. Here's a picture of the drain. I'm pretty sure the bolt is 1/2" as it measures 3/4" across the head and that matched other 1/2" bolts I have.

I think I'll order a correct drain plug and try it.

Thanks again,

Don

[Richard G.]

Re: Muncie drain plug threads

A 1/2 NPT is 14 tpi. and tap drill size is 23/32 or .718.

The closest bolt size to 1/2Pipe is 3/4".

A 3/4 bolt has a 10-tpi NC or 16-tpi NF and tap drill size is 21/32 or 11/16.
So, none of the standard bolt sizes will even come close to interchanging.

Aluminum can be forgiving. Both the tap drill sizes for the 3/4 bolts are smaller than the tap drill for the 1/2 pipe.
Maybe you will get lucky and will be able to restore enough of the threads to return to the stock 1/2" Pipe Plug.

It is also possible the plug is a specialty replacement oil plug. If you have a thread gauge I would check it when I removed it so I could make the best decision for converting back to the pipe plug. If you are not interested in head markings you can get 1/2 pipe plugs at any plumbing supply store.

[Tom P.]

Re: Muncie drain plug threads

Several years ago, I bought 100 magnetic drain plugs, nearly identical to what was once available from Chevy Parts dept, in case anyone needs one.

[Paul D.]

Re: Muncie drain plug threads

You have mail.

[Wayne M.]

Re: Muncie drain plug threads

Here's the original magnetic plugs from two M22's I have. On the left is the early style, from VIN 19N6307xx ( a '69 [Norwood] Camaro ?). The other is an over-the-counter P1D19C (mid April '71 ?).

[Tom P.]

Re: Muncie drain plug threads

I also have accumulated a few like the ones on the left. But I don't have any with the LISLE logo.

[Randy R.]

Re: Muncie drain plug threads

Use a thread sealant that sets up. Most sealant is a lubricant, not a sealant. Permatex makes one with Teflon. You may want to try some Teflon tape also. You might get lucky and find a plug that is not threaded as deep.

Good luck.