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Trying to identify a J Type Overdrive


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Hi All,

I have the opportunity of getting a J type overdrive unit with the Serial Number of 25 115869 010289. I have managed to identify that it has a Triumph Spitfire rear housing, as apparently they are unique in having a mounting point for the gearstick housing. But the number doesn't seem right for a Spitfire. I have found only one reference to the number on line, and that was someone guessing it was for a GT6, which makes sense. Is anyone out there in the Sports six community in the know? 

I was maybe hoping to use it on my Vitesse 2.0Ltr Mk2. So a Spitfire one might not handle the torque of a six. This is probably more out of curiosity, as I haven't got the adapter housing, main shaft and oil pump cam. And they look an exorbitant cost. And I'll have to strip it down and see if it's worn out before spending.. 

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Any J  type OD will be stronger than the D type. 

The J type for a GT6 is a bit mythical. But the vast majority of J type boxes will be the ones built up by Kippings and canleys. And a fair number from early Dolly 1850s, though these are still weak the same as Gt6 and Vitesse boxes.

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Jtype as OE was only used on  late mkIV and 1500 spitfires 

yes its used on other models not in the small chassis model range  like 2000 saloons and Dollys 

it has a tail casing to support the single rail gear remote  used on later spitty's   certainly never OE fitment on any  GT6 or Vittesse 

a good few have modified and fitted a J type to replace a D type or to  just make the conversion by adding am OD

you do need a dedicated adaptor plate to fit a J to a 3 rail gearbox  they are available 

so fitting a D type is very much less hassle  and there are many GT6 running very well on Dtypes 

if you up the engine to a 2.5 then a J is the stronger of the two 

both types were commonly used on far more powerful cars made in the hey days of british makers 

J Type Overdrive Assembly : Canley Classics

J Type Overdrive Unit - Tower, End Cover : Canley Classics

Pete

 

 

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1 hour ago, Gary Sweetland said:

I have managed to identify that it has a Triumph Spitfire rear housing, as apparently they are unique in having a mounting point for the gearstick housing.

...

I have found only one reference to the number on line, and that was someone guessing it was for a GT6,

It's definitely not GT6. The gearstick mounting is for a single-rail gearbox, which was never fitted to GT6. It could be for a later Dolomite or a 1500 Spitfire.

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If J-type it definitely won't be GT6 however if single-rail it could be Spitfire, but possibly Dolomite 1850.

The serial number means that 25 is the ratio by which it reduces (25%) and the centre number is the actual model - 115869. The last number is your own particular unit's serial number. I can't identify the actual model but a call to Overdrive Spares may help:

http://www.odspares.com

 

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Gary,

Fitting a J-type overdrive to replace a D-type is no small undertaking.

I did it on my GT6 but only because I'd fitted a 2.5 litre engine and believed the D-type would never survive the extra torque. To do this conversion while maintaining the original length of the overdrive and gearbox assembly (to avoid cutting the transmission tunnel, extending the tunnel cover and shortening the propshaft), you will need:

- Custom shortened mainshaft. Mine was made for 2Spec Transmissions in Kenilworth. Paul Hughes may still have a few in stock.

- Short overdrive adaptor plate part no. 313085 from a late Spit Mk4. These are rarely available secondhand, no new old stock, but Canley Classics recently had a new batch made and may have stock.

- Custom crossmember for overdrive mount to chassis. I made one from 3mm steel plate and redrilled the chassis to suit.

Having done this, my stronger overdrive and gearbox assembly fits perfectly without cutting any bodywork, tunnel cover or chassis, so it's possible to go back to original should I ever want to. There's enough work and cost involved in this conversion that I really wouldn't bother for a 2-litre Vitesse or GT6, but that's just my view. It's worth adding, the J-type in much has a much fiercer engage/disengage than the old D-type, making it essential to declutch when operating the overdrive.

Nigel

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6 hours ago, Pete Lewis said:

if you ever want to fit an overdrive you have to replace the gearbox mainshaft as they are very different 

you dont just bolt it  on the back of the existing  the box has to be stripped to replace the mainshaft 

Pete

Thanks Pete. I did know the shafts were different. And the J type is metric as opposed to the imperial D type.  I realise it’s major surgery to even adapt it to the more conventional D type.

I’m a retired engineer having worked with repairing and making car parts for most of my career. And have a workshop with a lathe and a mill. So for a laugh, I was considering making the parts. The housing would be easy from solid Aluminium. But hardening the shaft would be expensive and I’ve no idea how much to harden it. 
My other idea was to try and adapt a BMW N47 6 speed box. They’re only a couple of inches longer than the Vitesse box. But I think the issue might be they’re a bit fatter. And it’s tight on the chassis with a Vitesse. 🤔

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6 hours ago, Nigel Clark said:

worth adding, the J-type in much has a much fiercer engage/disengage than the old D-type, making it essential to declutch when operating the overdrive.

That's what I do on Vit with J Type. As I'm a bit of " wrap these old timers in cotton wool ", sort of person (can't afford extra expense/hassle).

It does take the romance out of those, "foot to the floor changes", though!.

Probably being over cautious, with any small amount of wear, that my low annual mileage incurs anyway?.

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14 minutes ago, Gary Sweetland said:

I’m a retired engineer having worked with repairing and making car parts for most of my career. And have a workshop with a lathe and a mill. So for a laugh, I was considering making the parts. The housing would be easy from solid Aluminium. But hardening the shaft would be expensive and I’ve no idea how much to harden it. 
My other idea was to try and adapt a BMW N47 6 speed box. They’re only a couple of inches longer than the Vitesse box. But I think the issue might be they’re a bit fatter. And it’s tight on the chassis with a Vitesse. 🤔

 yes the hardening is a mystery to many after market manufacturers of components today as well, in fact some dont even seem to have heard of it😁

And if you can come up with a good, straight forward and readily available gearbox swop for the small chassis Triumphs you`ll have a busy retirement👍

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