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MG Midget and Sprite Technical - 1500 gearbox drain plug

I've been stalking this board for years and found it a valuable source to keep our 79 1500 midget on the road,
Now though I'm stumped and actually need to ask a question as I can't find anything in the archives.

Our gearbox has been noisy for years, and as a stopgap I thought I'd change the engine oil - can't do any harm and might do some good.

I had expected to have battle with the filler plug, but I think I've found that without it being difficult to get to (about 2" up on the drivers side of the gearbox).

What I can't find is the drain, all the Haynes/MGOC and google pictures show it on the bottom of the gearbox, but there's nothing.

The only thing I can think is that the plug I think is the filler is the drain, but I can't really see that being the case

any help /advice much appreciated

t Kent

My 1977 1500 has a square plug right in the middle of the bottom of the gearbox.

I know Fords among others about that time got rid of gearbox drain plugs as the oil was supposed to last the lifetime of the car.

Maybe MG thought that too, late in the Midgets life, that is if yours is a very late one which being a 1979 model, it will be.
JB Anderson

Drain is on the bottom and the filler is on the side

mark 1500 on the road Preston Lancs

thanks - a picture tells a thousand words - so I've got a filler but no drain.

There is an hexagonal flat on the gearbox that looks as if that's where it would go - but it's never been drilled.

Be interested to hear if anyone else has had this problem and any solutions

t Kent

suggest you ask on a triumph forum they use more of these gearboxes so may have come accross this before
mark 1500 on the road Preston Lancs

I know that the very last 1500 Midgets had no drain plug on the diff, but I never knew about the gearbox.
Dave O'Neill 2

Just thinking out loud...

Could you not drill the casing, therefore draining the oil, then tap and plug it?
M Le Chevalier

just to close this out:-
There's no gearbox drain (but there is a drain on the diff).

I bought one of the syringe type oil remover things from Ebay for a fiver and managed to suck about 250ml of oil out through the filler.

Based on how much more I put back in (& BTW the syringe is great for that), I left about 200ml in the gearbox (so yes the gearbox was running about half full).

I'll run for a while then do the same again - if I can't do a full oil change, at least I can dilute out the old oil (I'm running about 25% old oil at the moment, one more change and it'll be down to 6%).

One outstanding thing, is there a list anywhere of Midget numbers - it would be interesting to see how late ours actually is
t Kent

Terry Horler's book 'Original Sprite & Midget' has a lot of useful information with regard to production dates, etc.
Dave O'Neill 2

my 1500 supposedly came from the last batch of 500 cars ever built (production date is 1978). BTW, it was originally painted black. It has a gearbox drain plug but no diff drain plug.
I'd follow Malcolm's advice. HTH
R W Bowers

There's a late 1500 for sale on ebay at the moment.

Here's a shot of the underside, showing the axle without drain plug.

Dave O'Neill 2

Resurrecting this thread, I want to drain the gearbox - itís getting crunchy especially when hot and I really donít know what the oil is like.

Was Malcolmís suggestion viable (iíd Be doing it freehand from axle stands), and if so could anyone suggest a combination of drill, tap and plug, My guess is I donít need to go to a full authentic plug, so maybe something about 1/4 inch?


Hmmm. If it we were me I think I'd get a good quality suction machine to take it out through the top-up aperture instead.

But if you are going to go the sump plug route, obviously make sure that you have a enough width to tap sufficient thread. And are you sure 1/4inch will be sufficient to get all the oil out? Of course would suggest you wash it out with oil after drilling, to get rid of any metal filings etc you will probably be creating.

I did use a clever bolt for another application recently. It came with a dedicated drill bit, and after drilling the hole, the bolt cut its own thread so it gave a perfect fit. Called Taptite I think.

Be interested to hear how you get on.
Graham V

Uh oh... What did I say?! :-)

I still don't see why it couldn't be done. Start small, say 3mm. Its cast iron so take it low and slow! Take it easy to keep the swarf small and so you don't suddenly break through and hit the gears (and all the oil will come out on your hand!). Keep pulling the drill out and cleaning away the swarf, at least gravity is on your side.

Drill it out in 1 mm steps, again to avoid creating big bits of swarf.

Standard plug is a BSP thread (can't remember what size), but you could tap it UNF with a copper washer. 5/16 would probably do which requires a 9/32 drill (7.2 mm)

Once done, pour some oil through the filler to wash out the bottom of the casing then plug it up, fill and go! :-D

You can drill straight free hand, can't you?
Malcolm Le Chevalier

Or 5.5mm for 1/4" tap is about right. Again, go slow with the tap, half turn fowards, 1/4 turn back to break up the swarf.
Malcolm Le Chevalier

Your filler almost looks like it has been filled in with something. Maybe just the picture.
Malcolm Le Chevalier

<<You can drill straight free hand, can't you?>>

Drill through the middle of a cotton reel to get the hole straight and square to the casing. Or make up a similar shaped drill guide first using a pillar drill. Drilling a guide hole through a small block of hardwood will do very well.

Or, it's been sheared off flush with the casing?

Back in the day my dad and I did this to several Ford boxes, and I've contemplated doing it to the midget but not done so yet.
We always used 1/4"BSPP solid plugs except once when we couldn't find one in time and used a 1/4" NPT which I wasn't completely happy with.
We would drain the box first as much as possible with a length of aquarium tube and a veterinary syringe through the filler. Takes a little while but saves a lot of mess.
Malcolm and Guy's advice on drilling and tapping is excellent, but I would add a note to use lots of grease on the tap. Not for lubricant, but it helps to retain swarf in the tap flutes and hopefully out of the gearcase.
There's a reason for the solid plug too, rather than a hollow one. We would carefully cross drill the hexes through each pair of flats and drill through a convenient rib on the gearcase casting (there's one such right astern of the flat on your box). This let us lockwire the plug in place, which race scrutineers seem to like.
Lastly,we gave up using copper washer seals - we found Dowty washers work better on BSPs.


Missed the edit window....
If and when I do my gearcase I'll probably use No.4 (1/4") SAE because SAE uses UNF thread so easier to find the correct tap and I like the sealing arrangement better. SAE uses an O ring retained by a metal ring (usually St/St).
It's a fiddly job but not difficult.

ok thanks everyone - that gives me the shopping list - first try and suck as much out as I can, i know roughly how much is in there, so if I can get most out, maybe i just stop there.

After that drill - stepwise, finish with #3 which looks like 5.4mm. Use Guys hardwood guide approach, and all the swarf precautions

tap with 1/4" unf and fit a solid plug.

Two questions how thick do you think the casing is at this point - from the photo the boss has to be 8mm ish plus is there casting behind that, and whats the clearance to the gears at this point - just so I don't get too long a plug

btw, I had another look at the boss - there's no hole or broken off stud - just a misleading scratch

This thread was discussed between 15/09/2015 and 07/05/2018

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