www.jec.org.uk

Good morning,

Im looking in to the prospect of fitting an overdrive unit to my V12 coupe, i was wondering if anyone knows the number of splines there are on the output shaft?

Many thanks for your help

Jay

that would be 32



BB

Many thanks BB,

Have a great weekend

All the best

J

I tried looking into this option some years ago. The idea being that an overdrive box could be fitted behind the GM400 and controlled via the cabin. This set-up could potentially provide up to 6 ratios.

Unfortunately, the limiting factor is the available space in the transmission tunnel!

The Gear Vendors kit https://www.gearvendors.com/ for example adds 13 inches behind the GM400 box and so the drive shaft would need to be shortened, assuming that there is room in the tunnel to take the width of the overdrive unit. I would be cautious about buying such a kit like this unless the vendor can prove he has supplied for several XJSs satisfactorily before.

I have the unit templates somewhere if you would like to check the tunnel dimensions.

Cheers, Rich

PS - I went for a getrag 265 manual conversion in the end

Hi Rich,

thanks for the information. yes it is the Gearvendors overdrive unit that im looking to install. my mechanic had the transmission tunnel covers off and measured the dimentions, there might be some modifiation to allow for the fit but he ensures me that it will be ok. The prop shaft will need cutting , balencing etc.

A couple of friends have the overdrive unit in thier Jensen interceptors and it does make a significant difference to crusing speeds.

My rear diff and GM400 need rebuilding as they are starting to moan a bit. so was going to have the car in the garage this spring ready for summer (as the theory goes)

will keep you posted.

Many thanks

Jay

Thanks for the update Jay.

I would be very interested in how this progresses as I'm not aware of it being done before to an XJS. If you do go ahead, please ensure that you / your mechanic take plenty of pictures as this would make a great article for the club magazine.

Cheers, Rich

Good Idea, thanks Rich. Will keep a form of a log.

Cheers

J