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MG Midget and Sprite Technical - Gearbox filler plug size

Hi all,

Although I've been trying hard, I can't release gearbox filler plug (standard 4-speed, midget Mk.III), mainly due to the fact, that I do not have the right size of the spanner. The "mm" sizes do not fit - 17mm is too small, 19mm too large, the only 18mm I have in the garage is traditional spanner and with this one you can't access the plug (gearbox is in the car, it need to be done with the ratchet). I have also tried 5/6 inch = too large.

I obviously need new set of tools... What is the right size of the spanner to release the gearbox filler plug, please?

Thanks,
Stepan
Stepan Marek

Stephan,
plugs tend to get changed over the years and can be various sizes and fits - sounds like yours might be 11/16" I'm not sure if that's original or not but I think that's the same size as the engine block drain plug so it could well be

have you not got a rubber grommet under the removable transmission carpet which you can remove and use a socket on it

if you can I'd suggest siphoning out some of the gearbox oil to see what condition it's in and if you don't know the last time the oil was changed I'd consider changing the oil

ETA: this is an old British car almost all nuts are imperial
Nigel Atkins

Hi Nigel,
It is probably due to my poor technical English, but I am trying to release as you described - with rubber grommet removed, via hole in the transmission tunnel. I also realize it is imperial nut, but I have to find out what size it is to buy the right set of tools (for future use).
I use socket , but don't have the right size and any of those adjustable tools don't work here due to complicated access.

And the reason for this all is exactly as you say - check the oil level and quality, as gearbox is getting to be noisy...

Stepan
Stepan Marek

Stepan

It is 11/16", as Nigel said.

It is the same size as the standard wheelnuts, although modern replacements tend to be 17mm. Do you have a wheelbrace?
Dave O'Neill2

wheelbrace, good thinking there

if you're getting imperial tools a full set of sockets and spanners is good or the following (A/F) socket/spanner sizes will cover most nuts 3/4", 11/16", 5/8", 9/16", 1/2", 7/16", 3/8", 5/16"

if you don't know the last time the oil was changed I'd consider changing the oil

to get as much old oil and muck out of the gearbox have the oil as warm as possible before draining and leave to drain as long as possible

if you do the oil change then you know for sure it has been done and when, the old oil will give you a clue to former maintenance by comparing it with the new oil and a clue to the condition of the gearbox if the old oil has metal bits in it

unless the oil was very low or very bad you'll not notice much, if any, major difference with this oil change but it will help maintain and ease the gearbox by having new oil of the correct type at the correct level


Nigel Atkins

Dave,

yes I do have a wheelbrace, but only usual one = 17mm (for new wheelnuts). I tried 17mm ratchet and it didn't work...
Stepan Marek

you not got the original Midget wheelbrace then (with the original car jack)

could you get an adjustable spanner or pliers (not ideal but needs must) on it from underneath
Nigel Atkins

Hi Stepan, I had to get the car up and go underneath. Adjustable with long piece of tube. It undid, I drained and refilled it. Best thing I ever did for noise control. Reckon it had never been done before.

The filler and drain plugs were a bit rough when I had finished so dressed with a file.
Dave Squire (1500)

Stepan, it is 11/16". Try to find a 6-point socket, that will fit better than the usual 12-point ones. Also due to the alignment of the filler plug and the access hole in the tunnel it is best to use a 3/8" drive extension. If you use 1/2" ratchet and extension it only fits on at a slight angle due to the thickness of the extension bar.
David Smith

if the 3/8" drive extension is also a wobble bar that can help too
Nigel Atkins

Thanks guys,I will buy the right socket and will give it a try during the weekend... One of the reasons I am doing this is that I found out, that last time (2 years ago)the oil has not been changed as per my instructions, but only topped up and not with 20W50, but with some synthetic transmission oil for modern cars. Something like half a liter.
Hopefully there will be no damage (but noise I hear from there from 1st to 3rd gear is not very nice. On the other hand, this gearbox was always noisy and works smoothly, perhaps I'm only paranoid).

Stepan Marek

what's done is done but I wouldn't worry too much some owners chose to use transmission oil in the gearbox and are happy with it

as I've put do a through drain by getting the oil as hot as possible and leaving to drain as long as possible to get as much old oil and muck out as possible, leave the fill plug off whilst draining to get a good flow from the start

half a litre was quite a lot of top up required as the gearbox only holds 1.3 litres (info from Driver's Handbook)

checking for topping up is part of the 6 month/6k-mile and 12 month/12k-miles services - obviously Sod's Law will apply and every time you check they wont need topping up and when you don't check they will need topping up

refill with about ¾ of stated quantity of oil then top up very slowly until just below the aperture, leave excess to run out (can take ages if you overfill)

they probably put in something like 75w/90 transmission oil before but if you look at this chart you'll see that it's within the 20w/50 (SAE) engine oil range

if it was fully synthetic then it might have been the GL-5 which isn't so good for gearboxes but some are GL-4 and the semi-synthetic tend to be GL-4

if you're really worried use this oil change as a flush and change this oil again in say 100-200 miles of reasonable gearbox use, once you've got the plugs out this time you should find they're easy next time

NOTE: check the drain plug is the same type and size and if not that you have a tool that will fit it

personally I like to clean around the plugs and then give them a spray with a penetrating/releasing fluid such as PlusGas (ordinary WD40 is not as good) and let it soak in as long as possible then try to slightly tighten, to break any muck/rust(?) seal, before undoing, let the chemical do the hard work

if you can get that wobble bar it might help too


Nigel Atkins

This thread was discussed between 24/06/2013 and 25/06/2013

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