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GT6 Overdrive diff.


brian GT6.

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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

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  • 2 weeks later...

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.

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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

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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. 

 

image.png.ef4e09914a3a38d64050851927511951.png

Doug

Edited by dougbgt6
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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.

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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

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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

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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 by JohnD
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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 by JohnD
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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.

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