12 Bolt Fill Plug

[CJM72]

Anyone know the correct size filler plug for a 72 Blazer 12 Bolt differential? Mine has a round head with recessed square drive that is stripped out. Can't seem to find a replacement prior to extracting this one out.

On a side note, I may need to use a propane torch to get this cracked loose. I'm nervous about using the torch so close the gas tank! Any advice?

[CJM72]

Follow up question: The fill plug has a pie shaped plastic tag underneath the head of the plug. Anybody know what that is? It appears to be painted over by the PO. Were these only put on 12 bolts with a Posi? I haven't had the cover off to inspect and the glove box door (and SPID) has been swapped from another vehicle. I'll try to get a picture at some point.

[CJM72]

After a lot of internet searching, it appears to be GM part number 3989514. Here is a picture of it. Can't seem to find a source, but it appears to be 1/2 npt which is similar to car fill plugs of the era. Any one got one of these in good shape they'd like to sell?

[Blazerowner]

This?

https://www.ebay.com/i/263538070046?...SABEgIVA_D_BwE

[CJM72]

[QUOTE=Blazerowner;8795884]This?

Hey Blazerowner. Close, but I'm looking for the one that has the head on it to retain the posi fluid tag. Closest thing I can find in the aftermarket is the reproduction plugs for the 68-72 camaros, chevelles, etc.

Was hoping to just get a direct replacement, so I could extract it, top off the fluid, and replace. Rarely how it works on these old trucks!

[Mike C]

C3 Corvette used that same plug. Probably available through a Corvette vendor. I know my '74 rear center section has that same tag.

[Blazerowner]

Found this but not recessed.

https://www.ebay.com/p/657757508

[Mike C]

Here is an eBay seller who shows (2). Not sure if I want the tag enough to spend the $70 more than an Eaton magnetic plug costs though.

https://www.ebay.com/itm/133028931690

[Blazerowner]

It is pretty cool though. I’m tempted since mine is long gone. It’s like the 205 bolt for the shifter. I paid $100 for a nos one.

[DeadheadNM]

I believe the fitting for a posi axle is an “outie” like shown whereas the non-posi axles use an “innie” and these styles are not interchangeable as the threads differ. Here are example posi fittings and these have the shoulder to retain the posi tag.

[PDXk5]

Originally Posted by DeadheadNM

I believe the fitting for a posi axle is an “outie” like shown whereas the non-posi axles use an “innie” and these styles are not interchangeable as the threads differ. Here are example posi fittings and these have the shoulder to retain the posi tag.

Pete,
My rig has the original posi differential with original red tag and an innie square drive plug. This is a pic of the side of my differential before I had it rebuilt and a photo of my SPID. Not sure if mine is an oddity or there are others but I thought I would share.

[DeadheadNM]

Well that’s interesting Jeff thanks for posting. Wonder how to sort this out in terms of what may be differences per year or perhaps axle differences between models. I’ll check the parts catalog when possible and see if that has useful info.

The bottom right example in my photo was pulled from a pickup with posi.

Edit - I started a thread to get more data: http://67-72chevytrucks.com/vboard/s...10#post8797610

[simicrintz]

Couple pics of my '71 if it is helpful.

[DeadheadNM]

Thanks for posting - am wondering if there was a switch for 71-72 axles maybe. Have some additional posts on the thread over on the pickup side.

[Accelo]

All the Corvette vendors have them. They changed style in 1973, they stock those also.
This one is $12.