Is there a posi unit from a v12 xjs that would work in a x305 rear diff? I imagine if any it would be the gkn 1987+ not the Dana or old Salisbury unit. 305 diff is the same as any x3xx differential.

 

It's not a drop in. The entire core units will fit, but there is a Jurid coupling to the driveshaft, not a flange on an X305, so that pin would need to be ground off the pinion gear and a flange substituted.

 

Terrific. Although I am unclear, we are talking about the 87+ gkn units right? And what do you mean by entire core units? The whole differential? Probably not what you mean, but what do you mean specifically by that? Thanks a ton!

 

Yes, basically everything inside the differential case - pinion gear, ring gear, side gears, carrier and limited slip components can be moved into the older case as one lump, but the internals are different enough you can't mix and match old and new gears and carriers for example. The internals need to be all old XJS or all new X300.

The ratio will be different though, an older (pre 1993) XJS will have a 2.88 axle, the later X305 is a 3.58. The late XJS 6.0 V12 with 4 speed transmissions were a 3.54 ratio, with a 3 speed transmission was 2.88.

 

Makes sense. With inboard to outboard brakes, will output shafts be different? And I imagine you are saying I can't put a 88 gkn posi unit into my 96 x305 ring gear? I would probably never do a 2.88.

 

I believe the output shafts are interchangable. They are not the same, the later XJ40/X300 have a different bearing arrangement, but I believe they can be interchanged. Not 100% sure on that.

No, you can't put just the 88 posi into a 96.

 

Okay so I know what I need to do now, either swap 6 liter xjs guts into my diff, or bolt in a posi xj12 xj81 diff. Thank you. Correct me if I'm wrong.

 

The XJ81 is the same outer case as the X305. It's a different suspension from the XJS, so the diffs don't interchange between that and an XJS

As far as I know, all the XJS were posi from the factory, so you should already have it.

 

Don't be mad I came here to ask a question related the xjs although I have a x305. Thanks again, now I need to find parts, having challenges with that already.

 

Ah, I see. Some of the XJ40/X300 did come with limited slip. It was optional in some markets for the 6 cylinder cars, so you may be able to just swap the whole diff assembly. Ratios will vary by market too, for example early USA market XJ40's were 2.88, Canada and nearly the rest of the world was 3.54, soon changing to 3.58. Look at junkyards for the 87 and on 6 cylinder sedans and you might find what you are looking for.

The 6 cylinder XJR (1995-97) were all limited slip, but with a 3.27 ratio. Other optional ratios for various markets were 3.77 and 4.09.

The V8 cars (1998 and on) use a different diff, so those won't be of any use to you.

 

Found a xjs diff, asking $1250. But I'll look for the xj6 stuff too.

 

PROBLEM SOLVED. Coventry west will take my open diff, and send me a rebuilt posi unit for $895 plus shipping which is $100. Bam!

 

Coventry West is good, they are a quality rebuilder.