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MG Midget and Sprite Technical - Gearbox Drain/Filler Plug

I recently dismantled a 1275 gearbox and was surprised to find that both the drain and filler plugs were 3/4” AF.

I’ve only ever seen 11/16” filler plugs. It does appear to have been a Goldseal ‘box, so I don’t know if that would have any bearing on it.

Anyone else seen anything other than 11/16” ?

Dave O'Neill 2

they are still 1/2 bsp
b bithell

It's not uncommon Dave. A 1/2" BSPP (female) hose end fitting will normally be 11/16" hex, as will a 1/2" (#8) JIC or SAE (both of which are actually UNF), but many 1/2" BSPP male threaded fittings have 3/4" hexes for two reasons:
It saves having to have two spanners the same size to screw them together and on a face seal where it screws into a port rather than a hose end it gives more area for a Dowty seal to work on. An 11/16"hex plug is fine with a copper washer, but doesn't cover a Dowty properly.
As they're being used to plug a gearbox with negligible oil pressure it's a bit academic - the copper washer would be fine and a 11/16"hex likewise, but my guess is that someone damaged or more likely lost the original plugs and just bought a couple from the local tractor dealer. Which is what I would do in preference to buying it from M*** or someone.
Greybeard

And of course you can get the BSP plugs with hex sockets to suit an Allen key and IIRC some come with a square sockets which require the spanners with tapered square bits.
David Billington

So you do David. I'd forgotten about them. Mainly I suspect because my experience of hydraulic fittings standards is almost entirely from medium to high pressure industrial hydraulics and the Allen key type of plugs are fairly rare there.
I imagine the reason is that a male hex requiring a spanner is much easier to tighten correctly. Plus Allen key holes are easier for the ham-fisted to f*** up than male hexes, as I've found to my cost far too often. (Thank Providence for Stillsons 😉).
But I think Allen key plugs would be perfect for a Spridget box. Very neat.
Greybeard

Grey

No washers here, as they are tapered plugs.

I just thought it a little odd that they were both the ‘wrong’ size.

I’ve just pulled a plug out of another gearbox to compare. The original (on the right) has a much deeper recess for catching all the bits of debris that are usually found in there.

Dave O'Neill 2

Dave,
the ones MGOC sold were (are?) tapered with Allen key hex sockets as I had to be corrected from thinking the Allen key size was in mm as I thought I'd been told and I think it might have been you that told me they were actually 3/8" (be in the Archive somewhere).
Nigel Atkins

Nigel

Do you mean axle or gearbox?

There is a socket (Allen) screw/plug for the axle, although IIRC it takes a 10mm key. I bought some from Roversport/Unipart 20+ years ago.
Dave O'Neill 2

It seems that I don’t remember correctly. I was convinced that I had a 10mm Allen key in the top of my toolbox, but it turns out it is a 3/8”.

Dave O'Neill 2

What about one of these for the drain plug--
Are the 1075 bungs 1/2" or 3/8" bsp
Haven't one here to measure
What is the o.d. of the thread-----?

https://www.flowfitonline.com/mintor-accessories/magnetic-plugs/hydraulic-magnetic-drain-plug-12-bsp-tscm3g
William Revit

Tapered... there's a surprise. Okay, why not?
Excuse the waffle assuming parallel thread then. Off on a tangent...
Plus I misremembered the sizes anyway.
Greybeard

Dave,
probably me spreading the contasion, I was sure I was told 10mm and bought a socket with 10mm Allen and then had to file it down to make it fit to learn here that it was actually 3/8" - but it might have been for the axle if it was tapered threads, I should have checked the Archives before posting but I didn't feel lucky today.
Nigel Atkins

I jump on this as William posted magnet plugs and they are some experienced people around here :)
=> Is it useful, luxurious or placebo to use magnet for classic cars (engine, gearbox, diff) ? :)
CH Hamon

Cedric

I used to sell magnetic sump plugs for the engine. I may have a few left. daveo138 yahoo co uk
Dave O'Neill 2

Nothing wrong with having them I think but gearbox and rear axle seem more appropriate to me.

The engine has a filter, the engine oil and filter are changed by most owners regularly, whereas the gearbox and rear axle do not have filters and many owners very (very, very) rarely change the oil.
Nigel Atkins

This thread was discussed between 18/07/2019 and 22/07/2019

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