55.2-59 12 bolt rear end filler plug

[Iceman_70]

My 56 stepside came with a 12 bolt rear end. I’ve rebuild it some time ago, and today I wanted to finally clean it, put the gasket and fill it up. Problem is that I can’t find back the filler plug... I even don’t remember if I lost it, misplaced it or if there even was a plug when I bought the truck...

I measured the hole (3/4 inch) and google told me I need a 1/2 NPT plug. To verify I took the 3/8" NPT to 1/2" NPT adapter that came with my water temp sensor, but great was my surprise that it didn’t fit. The threads in the filler hole look finer then those on the adapter... Or there different 1/2 NPT sizes ?
I keep on having difficulties with these inch measurements... Can someone please clarify what I need, or how I can make sure I buy/order the right part. Don’t have a lot of specialized shops in Belgium, so I mainly order these parts in the US.

[dsraven]

not sure but maybe something in here that tells you. otherwise try a machine shop or grab a thread pitch gauge and see the threads per inch. npt (national pipe threads) are tapered so they are a little easier to tell what you have, a machine plug would have parallel threads in comparison. by that I mean NPT threads are tapered so at the beginning of the thread the plug is actually smaller diameter than further up the plug. the further up the plug you go the fatter the plug becomes.

https://www.hoseassemblytips.com/what-is-npt/

https://en.wikipedia.org/wiki/Thread_pitch_gauge

original plug, if in rear cover and not the front side of the cast center section, likely looked like this, with a flange on the head of the bolt and possibly gasket under the flange.

https://www.heartbeatcitycamaro.com/...g-Original-GM/

[mr48chev]

I'm thinking the ones with pipe threads were the ones that had the plug in the case close to the pinion shaft.

The one in the cover should be similar to this one. https://www.classicparts.com/1934-72...ctinfo/93-223/

When most guys think "12 bolt rear end" they are thinking 12 bolt passenger car rear end as I suspect Dsraven did and I did at first.

[Iceman_70]

It's indeed a TRUCK 12-Bolt ...
I checked my NC/NF tap box I have, and seems the 3/4NF16 tap fits the opening. I gently tried to screw it in/out by hand, making sure not to harm the threads...


Comparing this to the 1/2NPT adapter I have. I found that 1/2NPT has 14 TPI, while I guess my tap has 16 TPI :

[burnin oil]

Yes that tap is 16 threads per inch.

[mr48chev]

That in not a pipe tap though. The correct pipe tap looks like this. https://www.homedepot.com/p/Drill-Am...T1-2/304632105

The tap you have there is a 3/4 National fine tap. It cuts straight threads where the pipe tap cuts slightly tapered threads.

[mick53]

Pipe is measured on inside diameter so a 1/2 inch pipe will have an outside diameter of about 3/4 of an inch. It can get a little confusing when you try to figure out what plug you need. So a 1/2" pipe plug will measure just under 3/4"

[Iceman_70]

Thx for the replies !

Objective for me is to find a plug for my rear end, and I really don't know what I need. Is it pipe thread, or something else ... I don't know ... I was just using the tools I have to find as much info as I can.

[dsraven]

so, if your tap screwed in fully with no unusual interference, simply get a 3/4-16 bolt with a flanged head and use a plastic sealing washer under the flange if you are unable to find a proper plug anywhere. barring that gasket idea, a simple home made oil resistant rubber washer would work or even a copper ring. try a big truck parts store, like a truck stop if you have them, and see if they have a washer to fit. they may even have a plug with that thread, who knows. some will even have a magnet.

https://www.dormanproducts.com/p-21570-090-008.aspx

https://www.summitracing.com/tx/sear...size/3-4-16-in

https://www.napaonline.com/en/p/NDP7041015

https://shop.advanceautoparts.com/p/...226/17190131-P

[mr48chev]

OOPs I didn't see the "Belgium" part. Until now and your folks

What Dsraven is suggesting is something like this https://www.amazon.com/Dorman-090-07.../dp/B000DEM6BO

This link is for a plug for hydraulic fittings. If you scroll down to the dash 8 size it has 3/4 16 threads and seals with an O ring. You should be able to find that at a hydraulic shop that sells hydraulic hoses, fittings and cylinders.

The 6409-08 should be the correct size. If you printed the page a hydraulic shop should be able to figure it out. https://www.hydraulicsdirect.com/ORB...lug-p/6409.htm

Don't be embarrassed to ask first before doing something. One of us can usually explain how to do the task correctly or direct you to a set of instructions that tell you how to do it correctly.

[Iceman_70]

yeah being oversees makes the things a little more complex ... If I go to a local junkyard, I only find European or Asian cars

In the meantime I found a hydraulic shop that can measure the filler hole, and will help me find a plug ...

Thx for all the help !

[Wrenchbender Ret]

The only one that will fit right is like the one Mr48 showed. It was used for many ears in all sizes of GM trucks.
George