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Overdrive Conversion


Mike Bird
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Hello

I am very new to all of this and have recently bought a 1969 Mk2 Vitesse. It doesn't have overdrive. It goes really well but as you would expect reaches around 4500 revs at 70mph not that I intend to scream around in it. I am planning a European trip next year and would value opinions on whether fitting a new gear box with overdrive would be worth it. I don't think it is a job I could do myself. How much should this sort of thing cost and what are the various options?

 

Thanks

Mike

 

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https://www.canleyclassics.com/technical-archive/different-differentials/

 

that chart gives the details about revs and speeds. So you are correct in that 70 is about 4.5K revs, which is a bit painful on long journeys.

 

A full recon OD conversion fitted is around £2K I think, and sounds a lot of cash.

 

Other alternatives would be to fit a 3.63 or even the GT6 non-od diff at 3.27, but that one takes away a bit of nippyness. A cheaper alternative, but ultimately not as good.

 

If you are prepared to risk a secondhand setup, they seem to fetch about £500 unless you get lucky, and are not overly difficult to fit. But you may then find the gearbox and/or diff need an overhaul, or the prop is vibrating etc. A real case or you pays your money and you take your choice! Or sometimes they do turn up from a reliable source with some sort of confirmation of how good they are (I have sold overdrive boxes over the years, but most people didn't believe I had driven the car they came from and could say they were definitely good). Beware some sellers, if they say untested it may mean literally that, they have no idea, but I get the feeling some they know are definitely knackered.

 

Sorry for the ramble.... (I do like overdrive!)

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Hello Mike.

 

Clive is spot-on and so are you in saying that O/D really is a necessary bit of kit; having driven numerous long journeys here and abroad it really does assist on all aspects - many of which you have already alluded to.

 

As an example here is a unit on Ebay that has been sitting around for some time (1970 !!) - the Q is, do you go for something like this quizzing the seller or spend way up the line for something tried and tested ?? Pay your money, take your chance. One thing I would say is that g/boxes and o/d units do not suffer the same type of problems as engines do by seizing up through lack of turning over - although you can expect a fair bit of dirt to have collected over the last forty years; if indeed the unit has been kept where it says it has been. It certainly does not look as though it has been stored outside exposed to the elements.   

 

http://www.ebay.co.uk/itm/Triumph-Vitesse-gearbox-and-overdrive-unit-/322535266924?fits=Car+Make%3ATriumph%7CModel%3AVitesse&hash=item4b18999a6c:g:WJoAAOSw8lpZLFX5

 

What I would say is you are in Beds and the unit is in Derbyshire so perhaps a drive to the Peak District this weekend to have a look at it may be prudent prior to speaking with the owner.

 

Some units do come up in The Courier and if you are a TSSC member you will be aware of that.

 

Personally, I would stick to the O/D option rather than going the change of diff route.

 

Good luck.

 

Richard.

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On the other hand.....

 

I've often toyed with the idea of fitting an overdrive but I would need, a different rear axel ratio, a shortened prop shaft, a different speedo and do I have the right gearbox? It's a lorra work just for motorway cruising and I have another car just for that. The armchair on wheels.

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Parts you need for the overdrive are as follows-

 

Gearbox with overdrive.

Overdrive mounting plate. Mounter on the chassis.

Overdrive rubber mount. Mounts the overdrive on the plate. 

Overdrive prop or shorten and balance yours from 47" to 43.5"

Overdrive wiring loom.

Overdrive relay.

Overdrive steering column switch.*

Overdrive steering column switch cow.* To hold the switch on the steering column.

Overdrive remote gear change assemble * and overdrive cut out switch.

Bracket to hold cut out switch.

Overdrive speedo cable.

Speedo angle drive.

 

* If you want to use the later GT6 type gearlever with overdrive switch you not require the normal column set up.

 

You will need to cut around 3.5" off the prop tunnel to gain access to the prop u/j as the overdrive gearbox is longer and prop is shorter. To cover up the u/j and prevent anything coming contact with the prop you will need to make up a removeable cover. This is standard on the factory fitted overdrive cars.

 

 

Think that's the lot.

Dave

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On the other hand.....

 

I've often toyed with the idea of fitting an overdrive but I would need, a different rear axel ratio, a shortened prop shaft, a different speedo and do I have the right gearbox? It's a lorra work just for motorway cruising and I have another car just for that. The armchair on wheels.

Doug, an overdrive setup as described (assuming you have the 3.27 diff, and would change to 3.89) would give you exactly the same cruising revs as you have at the moment, so from that point if view is not worth doing.But then again it would make your car feel livelier in the lower gears, and make the gears feel a bit closer together, but that isn't a huge issue with a GT6.

However, if you want it even more relaxed on the motorway, just fit an OD box and keep your diff.

If you want the best of everything, fit an OD with a 3.63!! Never easy is it? And don't get me started on 5 speed conversions.....(too much faff I reckon, butmore efficient than OD)

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I kept the original non-O/D diff on my GT6 Mk1 and found it very relaxed, however I tried a 3:63 diff and didn't like it at all. I went back to the original once it was rebuilt. My gearbox was second-hand from the classifieds, cost me £180, and has lasted about thirteen years so far, so don't be put off buying a second-hand box.

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Only thing I would add is use a J type overdrive, if you have a choice, not the D type which was a factory fitting. Early 1850 Dolomites used the Vitesse/GT6 gearbox with the J type overdrive just a different bell housing.

 

Run a J type in my MK1 2L Vitesse with the 3.89 diff 70mph is just under 3500rpm.

 

Regards

Paul

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Quite right - I forgot to say mine was a Dolomite 18/50 box with J-type. All it needs is the Dolomite clutch plate rather than the GT6 version.

The 3 rail dolomite box is more or less identical to the GT6/vitesse. Only difference is it uses the J type (and therefor a different mainshaft internally) but it uses the same clutch as the Gt6 /Vitesse

 

The later single rail dolomite box is a different beast, and indeed uses the "fine spline" clutch plate. It is also a little longer overall, so needs a special propshaft.

 

Colin, your prices were accurate for 13 years ago. Todays prices have gone up considerably. I sold a 3 rail dolomite box last year at Stoneleigh for £200, hadit for ages and decided I was never going to need it. Only later realised I had substantially under valued it. A trader snapped it up PDQ, suspect it was resold later in the day, as often happens. Ah, if only I had kept all those parts I sold, or even threw away.......

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Do not know who built it but the unit I use is 3 rail box, reverse up by first, with J type with a fine input shaft, I also have a 3 rail with J type which has the coarse input shaft. This J type suffers from slip on moving off so needs an overhaul which I is on a list which never seems to get any smaller.

 

Regards

 

Paul

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Same engine clutch works fine with the same gearbox and different overdrive which is now in the car so slip is caused by the overdrive. I checked the oil level before the change and used the same solenoid so assumed it was the overdrive clutch. Overdrive currently sitting in a bucket in the spare bedroom do not know if I will take it apart for a look or just get it refurbished.

Last one I had overhauled I was asked which car it was going in so the oil pressure could be set, rather than the usual suspects one size fits all, it is the one in use very smooth on engaging.

As I understand it the higher the engine power the higher the oil pressure needed.

 

Regards

 

Paul

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Yes the pump pressure valve is adjustable to apply more or less pressure to the cone linings, but the hydraulic pressure only engages.  there are also differing pump capacities available 

 

the Off pressure is controlled by springs , again these have a variety of specifications 

there are two  cone  lining that have to 'grip'   for the  on and the off. ( or in or out if you prefer)

 

more powerful  need more , smaller engine need 'less'  A types D types and J types were used extensively across a whole  motor industry.  In Commer we fitted them to our PB vans  so not just cars 

 

Pete

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Do not know who built it but the unit I use is 3 rail box, reverse up by first, with J type with a fine input shaft, I also have a 3 rail with J type which has the coarse input shaft. This J type suffers from slip on moving off so needs an overhaul which I is on a list which never seems to get any smaller.

 

Regards

 

Paul

A hybrid box, the single rail internals into a 3 rail case. I know a chap who has built a few. Uses the larger mainshaft bearing or something so stronger. But as you observe, uses the fine input shaft.

I had clutch slip on a  j type, butback then secondhand units were £25....Bit more now!

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Having driven the vitesse with the 4 speed box a bit more, it seems that the gear ratios allow me to use 4th gear at quite low speeds and I rarely need 2nd. 3rd gear if fine for most corners. I am now wondering if a change to the differential might be a simple option so that 4th gear only comes into play at slightly higher speeds which would lower the revs at the top end. I realise that there would still only be 4 gears but it would make for better cruising and probably a simpler job. Has anyone done this?

Mike

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Many up the ratio, its a compromise and can be a cheaper simpler option

 

try a 3.63 :1 its a reasonably easy change , dont go too high a 3.27 doesnt like too much torque

and can make hill starts when fully laden a long clutch slip start , eg in saloon 4 up and boot full .

 

due to poor triumph gearbox ratio's 3rd OD and 4th are identical so all you gain in a split shift in 3rd is the speed of change under power, when overtaking etc. Can give you a buzz.

 

Pete

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  • 3 weeks later...

Hi Mike,

I have a GT6 MK2 with 3.27 diff.  Just before Christmas I paid Mike Papworth to install one of his 25% J type overdrive kits.  Very happy with the outcome. Now 2000 revs at 50 mph. Did two European tours this year and glad I had the OD fitted.

Even the 6 hour trip to the Peak Valley Run this year was so more relaxing with the overdrive.  Especially pulling the trailer full of camping gear at 50 mph.

Regards,

Nigel

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