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MG Midget and Sprite Technical - 1500 Gearbox Woes

Hi guys,

Overhauled my gearbox last year with a kit from a well known triumph parts supplier. New bearings, seals, synchro rings and gaskets, plus a new clutch of course.

It's a spit overdrive box, but for the purposes of this tale the overdrive part is not relevant.

Refitted it, together with a rebuilt engine last June. It was fine on the trip to Classic Le Mans in July, but after about 1500 miles disaster occured. One night it made a horrible clunk as I down shifted into 3rd and lost drive!

Discovered it still had drive in top gear tho, albeit sounding evil.

Ended up swapping for a spare used unit I had under the bench.

Finally got round to stripping it last weekend only to find a badly worn layshaft, damaged first motion input gear and mauled laygear.

Not sure what happened, but wondering if the case hardening was defective on the new layshaft!

Thoughts anyone?

SR Smith 1

First motion shaft
SR Smith 1

If that's a new layshaft, I'd be talking to the supplier, as that is not hard!
Dave O'Neill 2

Hi Dave,

That was my first reaction after stripping it.

Friend of mine is a matallurgist at Delphi systems, thinking of asking him if he can test it before tackling the supplier of the rebuild kit.

The needle rollers that came out of it looked find by the way

SR Smith 1

And yes, it was a new layshaft Dave. Came in the kit.
SR Smith 1

I sent round an e_mail t'other day re a 1500 box for 20. Might give up some good internals.
Alan Anstead

Just read it Alan, I better have that, if only for the laygear. The first motion shaft on the spit is different, fine spline clutch.
SR Smith 1

Wow....I dont think you need to have it tested. By the looks of it you know it was too soft to handle the job.

Definatly call the parts supplie and have a talk with them and see if you can negotiate some type of resonable terms. You have to at least try that avenue.

Sorry to say but most times Ive seen a situation like this usually the answer is unsatisfactually handled.

Goodluck and I wish you the best but if you find a decent spare box to canibolize you may be better off.

Just some banter from someone thats been there before you.
Steven Devine

When you get a mo' pop over and pick it up.
Its out of a Midget.1500
Alan Anstead

Could be offside here but I thought the 1500 box and the Spittie box were very different.

I had it in mind the Spittie shared the single rail Triumph box with the GT6 and others that would accept the J type O/D and the stock 1500 box (like mine) was basically a modified Marina unit that wouldn't take the O/D. Which is why I've been keeping my eye open for a Spittie/GT6 box with overdrive. Just in case.

How much of it is transferrable I wonder.

But I hope you get the bits you need Steve.

Hello Greybeard,

The 1500 Midget took the Spitfire engine/gearbox unit complete, the only difference was the exhaust manifold and the thermostat housing, which were unique to the MG application.

I can assure you that the 1500 Midget box is for all intents and purposes interchangeable with the single rail Spitfire 1500 and also the Dolomite 1300/1500 box. the are slight differences in the clutch release parts, but if you swop over the Midget pushrod and slave cylinder it's fine.

It's this compatibility that makes it quite straightforward to install a triumph overdrive transmission. Indeed my car has been converted since 1992.

All the ones I've seen use a clutch plate with fine splines as well, but just use the triumph clutch plate with the Midget cover.

The GT6 and the Marina share lots of the internal parts, including the main casing but obviously the bellhousing and input shaft are different.
SR Smith 1

Here's mine going in last year.

Not the best pic, but if look closely you can see the overdrive.

SR Smith 1

"All the ones I've seen use a clutch plate with fine splines as well, but just use the triumph clutch plate with the Midget cover"

Long time ago now, when I did mine. I forget why, but the O/D gearbox I used had a longer input shaft which I I swapped out and fitted the MG gearbox input shaft so retaining the original clutch cover and driven plate.
Guy W

Possibly an 1850 Dolomite or GT6 one Guy?

The 1300/1500 triumph bellhousings I thought were all the same?
SR Smith 1

I'm not sure what it was from. I bought it from someone who intended to fit it in a Midget, but never got round to it. Cost 40 in mid 90's. I drove from Cumbria to Milton Keynes and back one evening after work to collect it. It had a huge and very heavy bellhousing which I also had to swap, so it may have been from a Dolly. The real bonus was when I got there I found it had been fully reconditioned as well, which made my 40 a double bargain!
Guy W

Just as well Guy. That's a long drive in one night. ;)
Lawrence Slater

If it was huge and had a weird looking lug on the top of it, that was from a Marina Guy.

The 1300 and 1800 used the same box with very heavy cast iron bellhousings and long input shafts.
SR Smith 1

About 450 l think, though probably add another 50 miles driving around the rabbit warren of roads of M K !! Still well worth it. I loved that gearbox conversion. Much better suited to the car than a T9 as the select few of others who have done them will agree.
Guy W

Agree with you Guy, having driven both, the overdrive really suits the car and a relatively easy conversion.
SR Smith 1

Re. difference between 1500 Spitfire and 1500 Midget gearboxes.

My understanding is that they share the same external housing, but the gear ratios between the Spitfire and Midget are subtly different, making the internal gears of each type incompatible in the 'mix and match repair' sense.
Andy Hock

Further to my preceding post, I think the following gear ratios are correct (but I will double check later).

Top (Midget 1.0) (Spit 1.0)
3rd (Midget 1.43) (Spit 1.39)
2nd (Midget 2.11) (Spit 2.16)
1st (Midget 3.41) (Spit 3.5)
Rvse (Midget 3.75) (Spit ?)

Many years ago, I was warned not to mix gear drives from the 1500 Midget and 1500 Spitfire gearboxes. I was told that 'cross bred' gears will work for a short while, but will definitely result in a seized/damaged gearbox.
Andy Hock

That is a valid point Andy of course.

Not a good idea mixing the actual gears themselves.

I fitted the spit gearbox and overdrive complete all those years ago.

Depending on how the supplier reacts to the e-mail detailing all this will dictate the best course of action'

It is of course possible to fit the 3rd motion shaft to the a Midget casing and use the rear casing and overdrive to adapt a Midget unit to overdrive changing the clutch centre plate for the MG one.
SR Smith 1

I mentioned that with mine I swapped the input shaft - complete with the gear on the end - because the O/D box I had came with a longer input shaft. So I was introducing a different input gear (from a Midget) into the new box. It appeared the same - same number of gear teeth but I did wonder for a while if it would wear faster if the were not correctly matched. Ideally one shouldn't mix components in this way. But I did many tens of thousands of miles without any problems and understand the O/D box is still performing well 12 years and at least 3 owners on since I sold it.

Internal gears are different hough, so presumably so is the layshaft gear itself. Steve, I wonder if you have the correct layshaft gear?

I also have some vague recollection that there are two alternative needle roller kits for the layshaft. Different numbers and diameters of the needles. Getting the wrong one supplied with your reconditioning kit could have resulted in a poor alignment of layshaft and gears.
Guy W

This thread was discussed between 01/11/2015 and 03/11/2015

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