changing 72 gt6 to overdrive

[endurodavek]

im looking to change my 72 gt6 to an overdrive,  I bought a rebuilt gt6 trans with a d type. not sure what rear end it has, does it need a specific one or can I just swop out the trans? and does any body know what length the driveshaft should be?

[thebrookster]

Whilst I struggle to recall GT6 details (not yet having had the chance to own one of these) the only thing you will really need for the swap is a shorter propshaft, which should not be too difficult to pick up.

The diff is likely to be a higher ratio than would normally be used, according to James' site mintylamb your GT6 would likely have a 3.27 diff, whilst the OD car would have had a 3.89 diff. This will mean less acceleration, however your fuel consumption will be pretty good, certainly for long distance motorway driving!! This is of course assuming someone has not previously changed anything of course..............

For Propshaft someone like Canley Classics will have these in stock I would suspect?

Hope this helps,

Cheers,

Phil

[Richard B]

or a local propshaft specialist can shorten/make one for you.

A 2litre with a 3.27 & overdirve will be long legged. Yours may already have a different diff (see how it goes).

You might want to consider changing to a 3.63 (Spitfire 1500 diff).

3.27 with overdrive is popular with 2.5 engines in GT6's as they do not rev as high.

[endurodavek]

thanks I like the 2.5 idea, is there a code number  on the rear that would tell me what is ? as it is on the freeway I'm turning about 3800 rpms just to keep out of the way in the slow lane

[Richard B]

James C's website

Minty lamb is you friend, punch in the numbers and see what comes out. GT6 O/D is 3.89 in UK, GT6 Non-O/D is 3.27 in UK. Overseas models varied.

http://www.mintylamb.co.uk/gearspeed/

[thescrapman]

endurodavek wrote:

thanks I like the 2.5 idea, is there a code number  on the rear that would tell me what is ? as it is on the freeway I'm turning about 3800 rpms just to keep out of the way in the slow lane

That sounds more like a 3.89.

3800k on a 3.89 is just under 70.

Might be a 4.11 from a Mk3 spitfire or (US) Mk4.

3800k on a 4.11 is 62mph.

[endurodavek]

thanks, I forgot to say I am in the US hope that helps.

[endurodavek]

I think 3800 is about 62, the tack kind of bounces a little 70 is 4k,  O and one thing my speedo reads  about 10 mph  faster than im going and after 50 the difference gets bigger what does this say

[junkuser]

"I think 3800 is about 62, the tack kind of bounces a little 70 is 4k,  O and one thing my speedo reads  about 10 mph  faster than im going and after 50 the difference gets bigger what does this say"

Sounds like the hair spring is staked in incorrect position.

[rotoflex]

You don't have to swap out the diff when you do the trans & propshaft.
You may wish to save that shopping trip & wrestling match for later.
Don't forget you will also need OD wiring haness, relay, extra inhibitor switch on trans, & OD activation switch.

[BrendanD]

rotoflex wrote:

You don't have to swap out the diff when you do the trans & propshaft.
You may wish to save that shopping trip & wrestling match for later.
Don't forget you will also need OD wiring haness, relay, extra inhibitor switch on trans, & OD activation switch.

and a right angle drive for the speedo cable

[Tim Bancroft]

If a J type overdrive, the wiring is very simple and you will not need a relay.

Have a good look at the state of the thin wires that will pass up through the gearlever, replace if any sign of damage. It is possible to buy new gearlever top switches as they were fitted to many BL cars that had overdrive. I'd be tempted to fit the type of switch fitted to earlier models.

[endurodavek]

thanks every body,  I bought  a complete kit suppose to have come with every thing needed but the prop shaft. I got it about 3 years ago as my trans was still good just wanted the rpm lower in top gear. now 2nd gear grinds when  going in and won't go into 4 so its time thanks again dave.

[endurodavek]

junkuser wrote:

"I think 3800 is about 62, the tack kind of bounces a little 70 is 4k,  O and one thing my speedo reads  about 10 mph  faster than im going and after 50 the difference gets bigger what does this say"

Sounds like the hair spring is staked in incorrect position.

can you explain how to adjust the spring stake? thanks

[junkuser]

Not what I regard as a home workshop job, but then again I don't wish to underestimate anyone's skills.

My suggestion assumed that the speedo was the correct one for the drive gear ratio, diff ratio and tyre size fitted to your car.
Are your odometer readings coinciding with measured distances? (Some of our expressways have accurate distance markers which are handy for this)

If so, the fact that the speed reading seems to be changing as a percentage higher than true points to the hair spring being set with too little tension.  Re-setting this is really a specialist's job.

[cook1e]

This page from Canley Classics website gives the lengths of the various props fitted to small chassis Triumphs. GT6 with overdrive is the shortest of the lot at 36 inches:-   http://www.canleyclassics.com/?xhtml=xhtml/infodatabase/propshafts.html&xsl=infodatabase.xsl

[endurodavek]

ok thanks it doesn't bother me much so ill leave it alone. I haven't checked it yet but the miles do add up quick , I think some things have bin changed in the drivetrain, from what I can tell I do have a 3.89 diff. so when I change the trans out for a rebuilt d type will see what happens

I want to thank every one for their help . Dave

[Docman]

Call John at QuantumMechanics and he'll set you up with everything you need except finding a shop to shorten your driveshaft.
http://www.quantumechanics.com/qm-htm/spitfire1.htm

That's the best upgrade I've made to my car.
I have the 3.27 original differential and the car still has plenty of torque to accelerate in O/D from 40 mph.

[endurodavek]

thanks, I will look them up. it will be a few months before the swap, I had a line on a 2.5 a year ago, I've got to look him up again. and find out what upgrades to do.