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MG MGB Technical - Rear Axle Drain Plug Thread

Can anybody confirm the thread size for the drain plug on a Tube Axle please? I want to chase out the female thread.

I think that it is 1/2" BSP Taper.

BTW, I bought new fill and drain plugs from MGOC and they come with hexagon 'holes' for an allen key rather than that daft square taper arrangement on the originals.

Thanks,

Neil
Neil22

Hi Neil,

I can confirm that the thread is BSP. Not sure as to the size, but that is easy to determine, by measuring the OD of the plug. The thread size is about 1/4" less than the OD.

FWIW, pipe sizes are based on the ID of the pipe, hence the OD of the threads is larger.
Tube sizes are the OD.

Herb
Herb Adler

"from MGOC and they come with hexagon 'holes' for an allen key"

Ha! After I spent some time welding the head end of a 3/4" bolt into the square end of mine! The thread is the same as the side-fill gearbox level/fill plug, but at a different point along the taper. The square-hole axle plug goes into the gearbox (although may go in too far) but the gearbox plug is too big for the axle.

Can you chase a tapered thread without machine tools?
Paul Hunt

Neil-
It's a 1/2” BSP square drive drain plug. This is a rather quaint plumbing item. Of course, the British like to do things their own way, so these are not compatible with any American threading system, plus their plug has a square hole in it for the wrench (spanner). The tool necessary to remove this charmingly quaint drain plug from the bottom of the differential housing is a 7/16” extension for a square pipe plug. Do not bother trying to use a hexagonal Allen wrench. You will just end up with a ruined Allen wrench trying to get the plug out. It is well worth purchasing the correct tool for this job as it makes life so much easier. MAC Tools makes them. I know because that is where I got mine. Sometimes you can get a cheap one from a plumbing supply house. Once you have gotten the drain plug out you have the option of swapping it for an American-made stainless steel ˝” BSP plug from a hardware store / plumber’s shop. They never rust in place, and to remove them all you need is a simple 3/8” Allen wrench.
Stephen Strange

"they come with hexagon 'holes' for an allen key rather than that daft square taper arrangement on the originals."

Paul Hunt

They take a 3/3" A/F hexagon key. I don't suppose that I have changed the oil in my rear axle for ten years, and the 'old' plug I took out is identical so they must have been around for a while.

The male thread (on the plug) is definitely 1/2" BSPT (tapered). The female thread (in the axle and the cover) appears to be 1/2" BSP (straight).

I'll send you one of my old ones in the post if you want to look at it Paul. Let me know if you are interested and I will e-mail you via your website.

FWIW, quaint or otherwise, I would imagine BSP to be in more widespread usage than NPT...

Neil

Neil22




"Of course, the British like to do things their own way, so these are not compatible with any American threading system"

Actually it's the other way around, the Yanks went off at a tangent to the established British standards.

Herb
Herb Adler

Neil of course meant 3/8"

you can get a drain plug key with 3/8" male end that's from a 19mm-3/4" base so that you can use a 19mm or AF 3/4" spanner (wrench) on it to help loosen the drain plug if fitted tight and then just use bar that goes through a hole in the key to tighten on refit of the plug
Nigel Atkins

Of course yes it is the plugs that are tapered and the holes straight, so able to be chased.

I would like to see one of the hex plugs Neil, thanks.
Paul Hunt

Neil, If you just want to clean crud/old sealant out of the thread, make 2 or 3 "cuts" with a 3 square file across the threads of your old plug and wind it in with some lubricant, will clean the thread up nicely. If the thread is damaged you will need to use a tap.
Allan Reeling

This thread was discussed between 08/08/2013 and 17/08/2013

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