Jaguar all-synchro 4-speed plus o/d torque capacity

I realise that the V12 E and XJS both had the Jag 4- speed gearbox (no o/d).

Is there any available data on the torque capacity of the jag gearbox?

How about with an overdrive?

Can’t answer with actual tech. data but l ran the 4 speed with OD in a series 1 SWB XJ12 for 20 odd years with absolutely no issues regarding strength. I had the original kick down micro switch connected to the OD, so it did get the occasional hard boot from the right foot, never missed a beat.

Thanks Baxtor - i like the idea of the kick down. very clever!

What about ratios? Did you use a series 1 box (2.93 first gear)- same as most of the E-types. I assume your diff was 3.31?

Yes my final drive was 3.31 and l am fairly certain it was stock series 1 box. It started life in my car with a makeshift clutch release set up until l swapped out all the internals into series 3 e type casing with the integral slave cylinder mount.


i’m looking at using the same box in a series 2 xj. It has the 3.07 diff- do you think the first would be too tall?

I think my series 1 was 3.31 but taking off was never a problem for the 12 anyway. Can’t recall ever stalling that motor with the original jag flywheel and once moving it just went. Long legs for cruising with that gearing.

I fitted a S1 MOD to my 3.8 ‘66 ‘S’ type that had a 3.07 diff, took a bit to get moving on a hill start but otherwise was OK.

Russell, have no idea of torque capacity, but the highest rated one would be the EJ box from the S1 4.2 E
GEARSET - ALL SYN.pdf (268.8 KB)

Thanks Norman. That PDF is very useful

I note that you give KB or KBN for the 3.238 first XJ6 XJS box

I thought it might be KP ir KPN


And IIRC ”KGN” on Series 2 XJ6.


Hi Russell, I do have a genuine XJS Trans in stock, definitely KB. I don’t believe the XJ would have the same designation as it was only listed for the 6 cyl engine.

Unfortunately the XJ and XJS workshop manual doesnt give the prefix (like the E-type manual does for example), so great to have a box to confirm. The XJS and later XJ6 do have the exact same ratios according to the books tho

I dont suppose you would have a reverse mainshaft gear and idler for that box in your spares? (i think the mainshaft reverse gear must be 36 teeth)


Russel, I bought out the spares from the Main Jaguar Dealer back in the late 90’S after British Leyland stabbed them in the back and opened up a lot of other Dealers in the Melbourne area. I do remember there was a XJS/XJ Cluster among the parts and ther may have been a Reverse Gear as well.
Unfortunately sold the Cluster ( real quick). I may still have the Reverse Gear.
Will check my Geabox parts. Let you know.

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happily i have a cluster and a first mainshaft gear, missing an idler and mainshaft reverse gear

To be honest Russell, I have so many trans in pieces that need to be sorted. Had a quick look for the mainshaft gear, definitely not, idler, I need to spend more time sorting. It may take some time, but will attempt to over the next few weeks.

many thanks

even knowing how many teeth they have would be good. I calculated 36 teeth on the mainshaft gear, and perhaps 19 or 20 on the idler