GT6 Overdrive diff.

[brian GT6.]

I have a GT6 overdrive diff from my 1971 mk3. I got a reconditioned one from Jigsaw a few years ago and never sent mine back so paid the premium. Before it came out it was working, a little noisy and a bit leaky. I didn't need to recon it but did a full mechanical recondition including engine.

 

 

If anyone wants it for 50 quid they can collect from shoeburyness in essex.

 

Many thanks Brian

[Wagger]

You need to post 'For sale' in the Courier really.

However, do you know the ratio? Rotate the prop flange 3.5 turns and check if the other two rotate almost once. That would be 3.63:1

If another half turn make them go just past one turn, then it is 3.89:1.

I am looking for a 3.27:1 which would be approx 3.25 turns of the prop for one turn of the other two.

The term 'Overdrive diff' usually refers to differentials that include a built in overdrive. Some Transits had these.

[Josef]

I’d understand this to mean the diff is that as fitted to an overdrive equipped GT6, which would make it the same as all Vitesse at 3.89.

[johny]

9 hours ago, Josef said:

I’d understand this to mean the diff is that as fitted to an overdrive equipped GT6, which would make it the same as all Vitesse at 3.89.

Yes but must be physically confirmed as suggested by Waggy...

[JohnD]

Wagger is, of course, correct, but it may be easer to do the check the other way around!

Mark input and an output flange, then turn both output flanges together and in the same direction.     Count the turns of the input flange.   

Of the possible gearings:

                     Just over 4 turns = 4.11:1

                      Just under 4 turns = 3.89:1

                      About 3 1/2 turns = 3.63:1

I believe that 4.55 (Courier van) and  3.27 (?) exist too.

John

[dougbgt6]

3.27 is late non OD GT6 mk3 like wot I got.

Doug

Edited November 13, 2023 by dougbgt6

[johny]

Think all GT6 mk1, 2 & 3 which didnt have OD should have got the 3.27 ratio diff. I suppose as the fully loaded weight of the Vitesse could be quite a bit more than that of the GT6 it was thought it would struggle with the alternative ratio...

[dougbgt6]

We've been down this rabbit hole before! There's a long thread on here about it, somewhere. I remember it mentions the GT6/Spitfire site 

https://triumphspitfire.com/

Here is a clip from it. Notice the 3.27:1 German GT6 with overdrive. Presumably sold with the Autobahn in mind. Some of these were allegedly sold in Switzerland which sounds odd except Switzerland is quite flat in the middle. And the list also notes 3.27:1 Swiss Spitfires. 3.27:1  with OD is also a popular mode in the USA for quite motorway cruising.

3.27:1 were stamped KC and 3.89:1 KD. I have come across references to early GT6 mk2s, non overdrive, being fitted with KDs as standard. 

 

Doug

Edited November 14, 2023 by dougbgt6

[johny]

Yes that list agrees with the WSM but I dare say you could request something different from the factory or if they had run out of one ratio theyd stick in another😁

[GrahamB]

 

26 minutes ago, johny said:

you could request something different from the factory

My father bought an Escort van.  As it was mainly for light loads, he requested a higher geared differential.  However, they did not change the speedometer drive  which lead to serious errors in the recorded speed and distance. So if you do change the differential, also consider the speedometer drive from the rear of the gearbox.

[Pete Lewis]

and good old triumph only had one speedo worm /pinion and changed the speedo TPM to suit the options 

if you have a  j type od then different pinions are available .

pete

 

[johny]

Ha yes not so difficult if its a GT6 as both speedo should be available but I believe the Vitesse speedo facia was different so there will be only the 3.89 one☹️

[dougbgt6]

I have to say I don't like the look of the 3.27:1 TR7 automatic.

db

[Peter Truman]

5 hours ago, johny said:

Ha yes not so difficult if its a GT6 as both speedo should be available but I believe the Vitesse speedo facia was different so there will be only the 3.89 one☹️

Whats the option of putting a Vitesse speedo face on the GT6 3.27 speedo innards

[johny]

Entirely possible but it means taking off the needle which I understand is a bit difficult....

[Colin Lindsay]

12 hours ago, johny said:

Entirely possible but it means taking off the needle which I understand is a bit difficult....

Taking it off is easy, putting it back is the difficult bit! 

I've used different speedo drives from overdrive-repairs before now, they supply a range to suit different applications.

https://www.overdrive-repairs.co.uk/product-category/j-type-spare-parts/

http://www.odspares.com

[johny]

I thought as you take it off the spring inside unwinds and youve got problems to get it back to the correct place/tension?

[Gully]

I fitted a 3.63 ratio diff to my GT6, which obviously means neither GT6 speedometer option is correct! I worked out the requisite TPM and asked Speedy Cables to recalibrate my speedo - they did so and it's spot on.

Gully 

[johny]

Quite a job as the speedo bit is ok to recalibrate but the mileometer/trip needs new gears of the right ratio🤔

[JohnD]

I've read that American gearboxes had different cable drive pinions for different diff ratios, same speedo for all.   But this was never done in the UK (?), so that a different speedo with different internal gearing was used.   But the OP  was about a Ford, so why didn't they do the first?

John

Edited November 20, 2023 by JohnD

[johny]

The thing is its not as straight forward as changing just the pinnion is it? Its internal drive gear on the mainshaft must also be different so if you buy a replacement gearbox youve got to make sure its suitable for your diff ratio.... 

[JohnD]

Johny did you read my previous?  AFAIK, all UK makers used the same pinion at the gearbox end of the speedo cable.   The different gearing for different differential ratios was inside the speedo, given by the tiny code numbers on the dial.

It was US manufacturers who used the same speedo for all, and changed the pinion at the gearbox.

John

Edited November 20, 2023 by JohnD

[johny]

Ah I did think you meant Triumphs US gearboxes but anyway Triumph did go the common speedos in the UK at the end (all 1000TPM) so I was just pointing out a possible disadvantage with that arrangement...

[Colin Lindsay]

1 hour ago, johny said:

Ah I did think you meant Triumphs US gearboxes but anyway Triumph did go the common speedos in the UK at the end (all 1000TPM) so I was just pointing out a possible disadvantage with that arrangement...

According to the overdrive website I linked, Triumph saloons (Heralds, Toledos, Dolomites?) had 17 tooth, USA Spitfire and UK Stag had 18, UK Spitfire and Dolomite 1850 had 19 and TR6 had 20, but no other clues as to when in production. Consequently as I used the US Spitfire MK3 J-type O/D box in the UK Herald, I swapped the drive for a 17 tooth version. I've absolutely no idea of how accurate it is.

[johny]

Can you confirm Colin that you had to change both the pinion an mainshaft gear as a pair?