brian GT6. Posted November 1, 2023 Report Share Posted November 1, 2023 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 Link to comment Share on other sites More sharing options...
Wagger Posted November 12, 2023 Report Share Posted November 12, 2023 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. Link to comment Share on other sites More sharing options...
Josef Posted November 12, 2023 Report Share Posted November 12, 2023 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. Link to comment Share on other sites More sharing options...
johny Posted November 13, 2023 Report Share Posted November 13, 2023 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... Link to comment Share on other sites More sharing options...
JohnD Posted November 13, 2023 Report Share Posted November 13, 2023 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 1 Link to comment Share on other sites More sharing options...
dougbgt6 Posted November 13, 2023 Report Share Posted November 13, 2023 (edited) 3.27 is late non OD GT6 mk3 like wot I got. Doug Edited November 13, 2023 by dougbgt6 Link to comment Share on other sites More sharing options...
johny Posted November 13, 2023 Report Share Posted November 13, 2023 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... Link to comment Share on other sites More sharing options...
dougbgt6 Posted November 14, 2023 Report Share Posted November 14, 2023 (edited) 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 Link to comment Share on other sites More sharing options...
johny Posted November 14, 2023 Report Share Posted November 14, 2023 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😁 Link to comment Share on other sites More sharing options...
GrahamB Posted November 14, 2023 Report Share Posted November 14, 2023 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. 1 Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 14, 2023 Report Share Posted November 14, 2023 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 Link to comment Share on other sites More sharing options...
johny Posted November 14, 2023 Report Share Posted November 14, 2023 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☹️ Link to comment Share on other sites More sharing options...
dougbgt6 Posted November 14, 2023 Report Share Posted November 14, 2023 I have to say I don't like the look of the 3.27:1 TR7 automatic. db Link to comment Share on other sites More sharing options...
Peter Truman Posted November 14, 2023 Report Share Posted November 14, 2023 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 Link to comment Share on other sites More sharing options...
johny Posted November 14, 2023 Report Share Posted November 14, 2023 Entirely possible but it means taking off the needle which I understand is a bit difficult.... Link to comment Share on other sites More sharing options...
Colin Lindsay Posted November 15, 2023 Report Share Posted November 15, 2023 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 Link to comment Share on other sites More sharing options...
johny Posted November 15, 2023 Report Share Posted November 15, 2023 I thought as you take it off the spring inside unwinds and youve got problems to get it back to the correct place/tension? Link to comment Share on other sites More sharing options...
Gully Posted November 15, 2023 Report Share Posted November 15, 2023 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 Link to comment Share on other sites More sharing options...
johny Posted November 15, 2023 Report Share Posted November 15, 2023 Quite a job as the speedo bit is ok to recalibrate but the mileometer/trip needs new gears of the right ratio🤔 Link to comment Share on other sites More sharing options...
JohnD Posted November 20, 2023 Report Share Posted November 20, 2023 (edited) 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 Link to comment Share on other sites More sharing options...
johny Posted November 20, 2023 Report Share Posted November 20, 2023 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.... Link to comment Share on other sites More sharing options...
JohnD Posted November 20, 2023 Report Share Posted November 20, 2023 (edited) 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 Link to comment Share on other sites More sharing options...
johny Posted November 20, 2023 Report Share Posted November 20, 2023 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... Link to comment Share on other sites More sharing options...
Colin Lindsay Posted November 20, 2023 Report Share Posted November 20, 2023 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. Link to comment Share on other sites More sharing options...
johny Posted November 20, 2023 Report Share Posted November 20, 2023 Can you confirm Colin that you had to change both the pinion an mainshaft gear as a pair? Link to comment Share on other sites More sharing options...
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