Gt6 Overdrive conversion

[Alex Lowe]

Hi All

 

I'm thinking about trying to convert my current non OD gearbox to an OD unit and was wondering all the components I would need to do this whether it would be an original D type unit or a better J type (which if my understanding is correct are stronger). On the face of it doesn't seem like a massively complicated job just not sure on what exact parts would be required from which triumph models.

 

 

[Clive]

You could strip your box, fit a new overdrive mainshaft (needs a complete stripdown). The the adapter plate and overdrive. Don't forget the pump cam. 

Or buy a complete overdrive gearbox.

Then:

Overdrive propshaft

Overdrive speedo cable and angle drive

Rubber mount and matching mounting plate

Overdrive switch (mk1+2 is a column switch, mk3 gearstick/switch)

wiring loom and if d type, a relay. OD solenoid if missing off the overdrive

I may have forgotten something.

D vJ type. Yes, J type is stronger, but a GT6 is unlikely to trouble a d type, and the J type boxes are rather harder to find. But if converting your box, that wouldn't such an issue (I think!)

 

[Alex Lowe]

Thanks for your reply Clive I thought it was something similar to that. I think finding a main shaft would be the biggest problem. Are the main shafts from the dolomite 1850 boxes compatible I feel like I read that somewhere but could be incorrect. 

[Clive]

I believe so. However, I know from 2nd hand experience there can be subtle, but important differences. Even better would be to get a mainshaft with the larger tip and machine? the input shaft.

But check with suppliers. Canley Classics or Mike Papworth would be my first choices to call.

[thescrapman]

You can fit a complete single rail 1850 box into a 3-rail case. You need an inch off your propshaft.

If you can find a complete 3-rail 1850 box it just fits straight in. Very rare box.

Mainshafts are currently very expensive. Short adaptor plates for J-type are extortionate.

[dougbgt6]

Alex,

Differential?

The standard diffs are a different ratio from the OD diffs. There's a lot of debate on here as to acceptability of using standard diffs with OD. Worth having a search. 

 If I recall late MK3 diffs would definitely have to be changed.

Doug

 

[Alex Lowe]

Hmmmm I would rather keep my original box if at all possible so by the sounds of it the main parts I would need to acquire would be a D type overdrive and main shaft? 

 

And my car is a 74 reg so defiantly a late model I just did my seals on the diff and although i didn't check the code i noted a blob of red paint on the casing 

[Clive]

8 hours ago, Alex Lowe said:

Hmmmm I would rather keep my original box if at all possible so by the sounds of it the main parts I would need to acquire would be a D type overdrive and main shaft? 

 

And my car is a 74 reg so defiantly a late model I just did my seals on the diff and although i didn't check the code i noted a blob of red paint on the casing 

Diffs are always a debate.

The non OD cars came with a 3.27 diff, the OD with 3.89.

That means that an overdrive car and non od both do the same speed/1000rpm in their highest gear. 

 

https://www.canleyclassics.com/?archive=different-differentials  has all the data

The 3.89 diff means the OD cars are sportier to drive, brisker off the line etc. But some like a 3.27 with overdrive (it will do 75mph at 3k) as it feels more relaxed on the motorway. The inbetween spitfire 1500 diff (3.63) is a common compromise. 

So no need to change the diff. It just means you will have an extra gear after 4th.

Assuming it still has the original diff.... the ratio can be determined manually, see the above Canley page, or by checking the prefix stamped on teh underside if the diff

http://www.wolfitt.com/numbers_and_ratios.htm