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Differential flange bolt sizes


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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 ?


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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..




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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)


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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.

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8 minutes ago, Qu1ckn1ck said:


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! :angry:



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