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MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - A-Series Overdrive

I was looking through the archives and there is a lot of information about fitting an overdrive gearbox to a 1500 but I was wandering if anyone has any information on fitting an overdrive gearbox to a 1275?
I know in theory its possible!!

The reason I ask is the 5 Speed conversions seem very expensive and I have a GT6 overdrive gearbox lying in my garage and I know of a dolomite sprint overdrive gearbox too.

I know that the ital/ marina used a triumph baised gearbox for the 1.3 all be it with a longer input shaft (I think it was the input shaft from a 1500/1850 dolomite?)

So all I might need is a ital/marina bellhousing to go from A-Series to triumph gearbox, clutch, propshaft and some fabrication work?

Or am I being too optomistic?
M Slater

I think you will find that the bellhousing used for the Ital was a large cast iron lump, and the used a larger engine backplate and clutch to match. I think that apart from the weight, there is a problem with clearance of the bellhousing with the chassis rails. But it can be done - possibly using a different bellhousing setup.

The overdrive box is nice to use, older and more noisy in design than a type 9, but driving with it is a different and more interesting experience.

Guy
Guy Weller

Which GT6 OD is it?
Brad (Sprite IV 1380)

Thanks Guy,

I was aware about the weight issue of the bellhousing but when you consider the 5 speed route it's no feather weight either. I'm not actually sure how heavy the bellhousing or overdrive gearbox are but i'm sure it can't be that bad.

Brad

I'm not sure which type it is i'm trying to find out. I think i have narrowed it down to either a "j" or "d" type.
M Slater

Easy way is to look at the solenoid, they are different. Post a picture of it & i'll check against my GT6 MK3.
Brad (Sprite IV 1380)

it does need some bashing of the tunel if i am correct (friend fitted an od to a 1500 midget)

But now my mind has gone a bit further
can there be an od fitted to the ribedcase gearbox??
is there and od unit that we can bolt (with some work) to the back and fit with an short propshaft?
Onno Könemann

All the information you need including weights are in this book.

What you had in mind is going to be the heaviest and most awkward conversion to make so probably the least desirable but probably the cheapest.

You would need to swap the input shaft for the longer one from the Marina/Ital box.

I suppose you already know the GT6 box has close ratios compared to the standard box.

However, you might be better off selling the box and going down the Ford based 5-speed route. A little more money for a lot better end result.


Daniel Thirteen-Twelve

Like Daniel says the T9 works well in the Sprite, i wouldn't change it for the OD box.
If you have the GT6 OD going spare i would like first refusal on it, if its the right type for me.
Brad (Sprite IV 1380)

I have an Ital bell housing and box. Sat unused since removed from the car (ital) in the early 90's. The Flywheel is *huge* aswell (also available)

I havent got apic of the flywheel to hand...but this has the clutch cover which dwarfs the midget one.

Free to anyone who can collect it.

Might be a long trek from Grampian though :-(
Dean Smith ('73 RWA)

That's what I was meaning - it isn't just a weight issue. The backplate, clutch and bellhousing are much larger than a normal A series an I think there would be problems fitting it within the Spridget chassis rails. I think it can be done, but with a different bellhousing arrangement, probably using the normal Triumph one with an adaptor back plate.
Guy Weller

If it is a GT6 overdrive then it is a D Type - the J type was never fitted to the GT6/Vitesse but to the later Spitfires and the TR6.

The GT6 gearbox already has close ratios (when compared with a Spitfire) but to fit it to a shorter bell housing would require a Dolomite 1850 input shaft.

WRT to bellhousings you can not use a Triumph one (without a whole raft of butchery) since the starter motor on an A Series is on the other side of the engine when compared to a Triumph.

I know Chris Groves does a Triumph 'single rail' gearbox bellhousing (as fitted to the 1500 Midget) to fit the A Series since a number of racers use this. This might be convertible to take the 3-rail Triumph gearbox.
Deborah Evans

Deborah,

Do you have any contact details for the Chris Groves?

This sounds interesting about the bellhousing to fit a triumph gearbox to an A-Series.

I knew this was never going to be a bolt in job or propably the most desirable conversion but like i said I have a few bits already lying about that may be of some use and will help to keep the cost down.

Or I could just not be soooo greedy and just buy a 5 speed kit, but where's the fun in that :-)
M Slater

Dean,

Can I get your email address please?
M Slater

A 3 rail Triumph gearbox won't fit very easily into the Spridget as the gearlever section makes it very tall.

Someone has fitted the enormous Marina/Ital bellhousing into a Spridget but I don't have any photos of it.

I forgot about the Chris Groves bellhousing for fitting a Triumph box to an A-series only having seen and heard of one a few weeks back. Deb can you send me contact details for him? However, even with a Chris Groves bellhousing you still need the single rail o/d box (I have one which didn't sell on e-bay for £100 and needs a rebuild) or will need to fabricate a new tunnel.

Daniel Thirteen-Twelve

I had a chat with some people today regarding Triumph 'boxes and A Series powerplants.

I was correct in my assertion that you can bolt a 3-rail box (alà Spitfire or GT6) to the bell housing for a single-rail box (alà Spitfire 1500/Midget 1500/Dolomite/Marina). You can not connect a single rail 'box to a 3-rail bellhousing because the latter doesn't have the drilling for the rail.

This being the case, then the use of the Chris Groves bellhousing becomes an option for use with a 3 rail box.



"A 3 rail Triumph gearbox won't fit very easily into the Spridget as the gearlever section makes it very tall."

Having had a closer look at both the single and 3 rail boxes today (I work part time for a Triumph 'racestoration' company setting up race cars), I can tell you that there is little difference in height between them. The single rail is actually the taller 'box but the 3-rail has a selector housing on top so transmission tunnel butchery wouldn't be a MAJOR worry (certainly no worse than fitting a Ford Type 9).

The single rail 'box with O/D would be easier to fit since you'd only have to fab up the O/D mounts and shorten the prop.

HOWEVER the 3-rail 'box is FAR lighter and has a FAR sweeter and more positive shift. Furthermore you can't really effectively close-ratio a single rail box (unless you use Chris Groves ratios which are around £1800 a set!) and first gear will always remain relatively 'tall'.

The major advantage of the Triumph 'box over the Spridget 'box is strength. We've already blown up 2 'boxes in the race car in 2 years, yet in 6 years of racing Spitfires (with GT6 ratios) I never damaged a 'box.



WRT to fitting a GT6 'box then, if you were fitting it to a Triumph 4 pot with a 3-rail bellhousing you'd need to modify the input shaft to that of a Dolomite 1850 with a 10 spline clutch plate (6.5" as standard or 7.5" AP racing clutch).

Using a single rail bellhousing (which is deeper) you have to use a 'mimic' of a single rail input shaft and the 7.25" 20 spline clutch plate (although I do know of a racing Spitfire where a 20 spline shaft has been modded to 10 spline to take the AP Formula 3 'Lug Drive' 7.5" clutch). Jigsaw Racing (http://www.jigsawracingservices.co.uk/contactus.htm) sell such a shaft specially made to order although it does require a 'box rebuild because the input shaft bearing needs to be modified.



Unfortunately I don't have Chris Groves' contact details to hand at present - I suggest asking on the Midget Challenge forum: http://www.mgmidgetchallenge.co.uk/discussion/

Deborah Evans

Deb,

How does the 3 rail box compare to the single rail box when you measure from the centre of the input shaft to the top of the box?
Daniel Thirteen-Twelve

Of I get chance (probably over lunch) I'll measure them up tomorrow Daniel.
Deborah Evans

M.....dean at eatworms.org.uk

Dean Smith ('73 RWA)

Daniel,

Did you speak to Chris about the bellhousing going from A-series to Triumph gearbox?
M Slater

I need to ring him
Daniel Thirteen-Twelve

Sorry for the late reply Daniel, things have been a wee bit hectic this last week.

The Triumph single-rail 'box is 95mm from the centre of the input shaft to the tip of the 'box. However there is a bolt head on the top of the box that stand proud by 10mm.


The 3-rail 'box is 95mm from the centre of the input shaft to the top of the 'box. The selector housing sits on top of the 'box and increases the overall height by another 35mm.

The major difference in 'height' between the two is that the former protrudes further below the input shaft - ie it is deeper overall.

Having looked at a Sprite in the workshop there might be some cutting to the roof of the tunnel but I estimate not significantly more than for the Ford Type 9 conversion. SIGNIFICANTLY the gearbox tunnel cross member will NOT need to be cut and plated and there is no requirement for an additional lower stiffening plate (unlike the Ford conversion).

Additionally if gear lever position became an issue then it is relatively easy to shorten or extend the selector housing on the 3-rail 'box - I know several race cars where this was done.
Deborah Evans

This thread was discussed between 05/08/2009 and 14/08/2009

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.