Differential query

[dxke38]

Hi guys, just wondered if anyone could tell me when the diff that required brackets for the rear calipers was fitted, was it just on 4.2 cars or did some 3.8 cars have them. Many thanks
Derek

[Allrand]

My Jan 64 3.8 doesn't have separate brackets, but differs from earlier cars in that the bolts go in fom the diff side outwards through ears in the diff casting into the calipers, they don't attach from the outside through the holes in the disc.

[dxke38]

Thank you Randall, thats interesting. My car is from March 64 and does have the brackets, I wondered if it was the original diff as it is a 3.54 ratio which was unusual for a UK car.
Derek

[mgcjag]

Hi Derek......look at the rear of the diff...bottom right corner below the cover they are typically stamped with last 2 digits of the year......eg 69...1969.....Steve

[dxke38]

Hi Steve, I've got the diff out and on the bench so I've just scraped the paint off to check for a number. All it has in that position is H1 stamped upside down, any thoughts?
Derek

[mgcjag]

Hi Derek...photo of diff number....maybee it wasnt on earlier cars this is from a 69 car....i also have the same from a 68 and a 70 car....Steve

[dxke38]

Thank you Steve, nothing like that on this diff, I'll probably have to do more research. I've been running the car with a spare 3.07 diff thats much earlier without brackets but would like to go back to the original spec for the car. Time to get a Heritage Cert perhaps.
Derek

[44DHR]

Hi Derek,
Just been checking through my records and looking at some diffs that I have here. This my understanding of the situation.
It’s not so much about the differential, (Diff) that you have, but more what brake calipers and brake discs were originally fitted.
There were three main types of calipers over the years and this required the use of the Adapter Plate C22859 to be fitted between around 1965 to 1970 to cover the change over period for the calipers before the Diff design caught up with the changes to the brakes being used.
The first calipers were Dunlop with twin bolt on cylinders and these mounted directly to the diff using the thin brake discs, with no Adapter Plate between the caliper and the Diff.
There was then the change to a later type of Dunlop caliper - still with the twin bolt on calipers but a different design of how this was mounted - and the introduction of a Girling caliper which had integral cylinders and this was the period around 1965 to 1970 which required the Adapter Plate. From 1970 onwards only the Girling caliper with integral cylinders were used and then at that point the Diff output shaft housings were modified to permit the calipers to bolt directly into them without the need for an Adapter Plate.
Hope this helps,
Regards,
Dave

[dxke38]

Thank you Dave, my original diff is 3.54 ratio, has Dunlop brake calipers and has the adapter brackets. My query is, is this correct for a March 64 3.8 car. The spare 3.07 diff currently on the car is earlier and has no adapters. I have recently found another query in that the lower rear wishbones fitted have the thinner inner trunnion arms, so even more dubious. Any info gratefully received.
Derek

[44DHR]

Hi Derek,
It’s a bit hard to diagnose an issue remotely and I’m assuming that your car is a UK supplied car, but your spare diff with the 3.07 ratio, without adapters would be more likely correct for a 3.8 litre car. The 3.07 ratio was finally adopted for UK cars in October 1962, but just to confuse us, an optional ratio was offered of 3.54 for the 3.8 litre cars. However, your 1964 car, albeit March, is in the year the 4.2 litre car was introduced in October 1964 so - as we know with Jaguar - there could well be some from of transition with various parts being used with regard to the brake calipers.
To explain the transition to the use of the Adapter Plate C22859 (Green in the photo below) and the two sorts of Dunlop calipers which used the twin bolt on cylinders, have a look at the photo below which confirms the original 3.8 litre calipers bolted directly into the diff. You can see the difference in the two caliper designs and how the later design of calipers bolted into the Adapter Plates - which themselves were bolted to the diff. However these later design of calipers were used on the “late” 3.8 litre cars like yours as well as the 4.2 litre cars. I hope this helps to show this.



The thinner inner trunnions arms is more of a mystery and would also point to a 3.8 litre much earlier than your 1964 car. With acknowledgment to the excellent Dr. Haddock book, the thinner inner trunnions arms also have a different brace plate as the mounting bolts are differently spaced to the later brace plate. Dr Haddock said this change to these mountings at the inner fulcrum shafts was about April 1963 - so your car “should” have the thicker inner trunnions mountings with the much smaller central hole around the Diff sump plug.



The thinner inner trunnion brackets have wider mountings fixings to match the different early brace plate. This photo of these thinner brackets comes from a previous post on here “Rear inner fulcrum shaft mounts”, where at post #17 I posted this photo of said brackets with the extended lug :-



In conclusion, your original diff could be correct if it was a “transition” car being one of the last 3.8 litre cars to use the “new” Dunlop brakes and having an optional 3.54 diff. Your spare diff looks to be the standard 3.07 ratio, with the early Dunlop calipers, but if it has the larger hole in the brace plate signifying the thinner trunnions brackets, this would point to a 3.8 litre car before April 1963.
Regards,
Dave

[dxke38]

Thank you very much Dave, you've given me all the info I needed. I'm now reassured that my car has'nt had any alteration re diff in the past and is indeed a transition car. I wonder why the original owner opted for the 3.54 though. Thanks again.
Derek