Qu1ckn1ck Posted March 26, 2019 Report Share Posted March 26, 2019 The differential in my GT6 is apparently fitted with the small dia quarter shafts and I am looking to replace it with a diff with the stronger larger dia quarter shafts. According to the excellent information provided by Canleys my existing diff probably has 5/16 bolt holes on input and output flanges whereas the replacement diff has 3/8 bolt holes. Does this mean I would have to change or modify the prop-shaft flange and the output shaft flanges to suit the slightly increased bolt sizes ? https://www.canleyclassics.com/technical-archive/different-differentials/ Link to comment Share on other sites More sharing options...
68vitesse Posted March 26, 2019 Report Share Posted March 26, 2019 Short answer is yes but check as when the diff was changed did they change the flanges on the drive shafts and prop, thought all GT6 had 3/8 as standard. Depends what previous owners did and easy enough to check. Regards Paul 1 Link to comment Share on other sites More sharing options...
Pete Lewis Posted March 26, 2019 Report Share Posted March 26, 2019 Dont think The bolt holes decide the diameter of the short shafts The shaft diameter was increased in early days , i sheared one on a 59 948 taking off in a hurry ,,¡¡¡¡ If the flange register is the same on both then drill the bolt holes to 3/8 and buy some beers with the saving If the small small dia shafts used a smaller spline it cant be swapped as the broached spline in the differential gear will be too small I dont remember the detail.. Pete 1 Link to comment Share on other sites More sharing options...
Clive Posted March 26, 2019 Report Share Posted March 26, 2019 Interesting, as the smaller bolts were only used on the 4.11 (plus the 4.55 and other assorted early herald) diffs. And had square input flange. The bolt size is NOT an issue. The Subaru diff I use on my spit with "plenty" of power used 8mm bolts (equivalent of 5/16) though I have used 12.9 grade which is a higher grade than the usual 8.8. Plus had to buy bolts that were too long and cut them down so the unthreaded shank went almost entirely through both flanges, you really do not want the joint betwreen the flanges to be on the threaded section. Small but important detail.... The small quarter shafts were used on early cars, with the mk3 spit and at the same time heralds getting the larger size which carried over to the later diffs, GT6, vitessse 2 litre etc. If you want to avoid changing flanges it is possible to drill the diff to accept the smaller flange bolts. Best done between the existing flange holes. I have had at least 3 diffs "through my hands" that have had this treatment, so must have been common "back in the day". But as your GT6 should have the larger flange both ends of the prop, it may be best to swap the flange. The car should be a bit more relaxed too, how much depends on what ratio you are fitting (unless you have had a 4.11 built into the later diff case) 1 Link to comment Share on other sites More sharing options...
Qu1ckn1ck Posted March 26, 2019 Author Report Share Posted March 26, 2019 Thanks for the suggestions. Both the engine and overdrive gearbox are certainly later replacements so it is likely that the diff, propshaft and drive shafts are also not the factory fitted originals in my GT6. Pete warned me some time ago when I was replacing diff seals and queried circlip sizes that I probably had the small dia shafts. Guess I will have to ignore the arthritis and climb underneath to check what I have actually got fitted. Link to comment Share on other sites More sharing options...
dougbgt6 Posted March 26, 2019 Report Share Posted March 26, 2019 8 minutes ago, Qu1ckn1ck said: arthritis Commiserations. I have regular arthritis plus gout, a sub species of arthritis, had a flair up last week. The weather's changed and I can't get out to do stuff on the car. So frustrating! Doug 2 Link to comment Share on other sites More sharing options...
poppyman Posted March 26, 2019 Report Share Posted March 26, 2019 Gout is about the only thing i dont have...... But you have to keep smiling and get on with the sh*t life throws at you At least we have our cars to moan at....... And as Doug says the weather has changed Tony. Link to comment Share on other sites More sharing options...
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