Me and a couple friends are buying a 1979 XJS. The seller says the only issue is the differential, so we were wondering if the differential from a similar model would be compatible.
For example, would a differential from a 1982 XJS or a 1995 XJS be compatible. Or would there be any issues with bringing them over.

 

All XJS diffs are compatible from a fitting point of view, the only thing is that the actual ratios in various models changed. The 1979 car is a pre HE, and had a lower diff ratio that the HE models that followed. 6 cylinder cars again had different ratios, as did the later V12 facelift cars with the 4 speed autobox.

 

AGREE

That 1979 would be 3.07:1 as best I remember.

The HE that Greg mentioned are 2.88:1. They will fit, but the speedo etc will be out a tad.

Be careful when hunting a replacement. Yours is what is know as a GKN/Salisbury diff, and has an oil drain plug. There were DANA units (1984 -1986) used, and they have NO drain plug, and parts are not easy for them.

If you get onto the late 1980's, Jag used the diff cover for the speedo sender unit, not a real issue as yours will be cable speedo, just saying.

 

Almost. The input Flange on the face lift diffs is different. (3.54 from 1993+) The diff input flange on has a lip that the drive shaft fits into. That lip is different on the face-lift diffs with the 3.54 so it wont bolt up. The u-joint is the same so if you get a later diff with a piece of the drive shaft you can swap the flange on the driveshaft side to match.

If you have a 3.07, fix it. It's a good ratio for US roads. The later 2.88 will make the car feel lazy and the 3.54 will be a bit to high for highway cruising with the OE 3 speed transmission.


BTW I'll be in CT this weekend, on the track with 2 Jag's at Limerock.