66 Muncie M21 Drain Plug Leak

[Joseph S.]

Have stripped threads causing an annoying leak on the 1/2" NPT drain plug (aluminum case) on the Muncie M21 in my '66 roadster.

The bad threads prevent me from tightening the plug to specifications. Have tried thread sealers with little success.

Proper fix would be a thread insert, however kits from Time-Sert (my choice), McMaster-Carr or Helicoil run from about $320- $375 for what I anticipate will be a single use. I could disable the drain plug, and try to JP weld the plug into case because I can still get in excess of 10 ft-lbs of torque on the plug but certainly not the best repair.

Has anyone else had experience with another product, insert kit, or fix? thx Joe

[Joe L.]

Re: 66 Muncie M21 Drain Plug Leak

Have stripped threads causing an annoying leak on the 1/2" NPT drain plug (aluminum case) on the Muncie M21 in my '66 roadster.

The bad threads prevent me from tightening the plug to specifications. Have tried thread sealers with little success.

Proper fix would be a thread insert, however kits from Time-Sert (my choice), McMaster-Carr or Helicoil run from about $320- $375 for what I anticipate will be a single use. I could disable the drain plug, and try to JP weld the plug into case because I can still get in excess of 10 ft-lbs of torque on the plug but certainly not the best repair.

Has anyone else had experience with another product, insert kit, or fix? thx Joe

Joseph-------


Except for M-22 (of which only 15 were installed for 1966), Muncies did not have a drain plug until the 1970 model year. So, if your car has one then it's been added or you have a replacement main case. What is the casting number of your transmission main case? It should be 3885010.

[Tom E.]

https://www.ezlok.com/e-z-lok-solid-wall-threaded-inserts-for-metal

Tom

[Frank D.]

Re: 66 Muncie M21 Drain Plug Leak

If you only have one plug then its a fill plug and you do NOT want to JB-Weld it in....I have my plug frozen in place on my 63 Muncie and just hope I don't have to add fluid for a LONG time. You could try wrapping the plug with the yellow solvent-resistant Teflon tape or the kit mentioned above.

[Joseph S.]

Re: 66 Muncie M21 Drain Plug Leak

Joe- The best I can determine it is an original 3885010 case. Somewhere along the way, the lower cast boss on the right hand side of the case was drilled, tapped and a drain plug added. The original fill plug is fine, it's the 'custom' drain plug which looks to have been added later where the issue is. thx Joe

[Joseph S.]

Re: 66 Muncie M21 Drain Plug Leak

Tom- I have looked at EZ Lock, and in fact used them successfully before. However I don't see that EZ Lock offers a 1/2" NPT internal threaded insert. 1/2" NPT is listed as .84"x14, I don't see any insert listed with that ID, maybe I'm missing something. Joe

[Tom E.]

Re: 66 Muncie M21 Drain Plug Leak

Joe,
I apologize, I did not check to see if they came in NPT threads and just assumed they did. As you probably already know, the NPT spec calls for tapered threads so a 1/4-14 SAE insert will not work.
Tom

[Ray K.]

Re: 66 Muncie M21 Drain Plug Leak

Joe,
Have you indeed verified that it is 1/2" NPT thread? Could be that someone used a bolt thread to tap the hole.

Ray

[Joseph S.]

Re: 66 Muncie M21 Drain Plug Leak

Good point Ray, never checked account the 1/2" pipe plug threaded in so easily. However, checked yesterday and it is in fact 1/2' NPT pipe thread-

thx Joe