overdrive applications

I've noticed a few J-type overdrives up for grabs on ebay lately. Mostly from Volvo, but a few for Tr6. would either of these be useable on a GT6 MkIII, and how much modifying would have to take place to adapt them>

Many thanks,

Hym

Hi there. An overdrive unit will not bolt on to an existing non-OD transmission, you need to modify the existing gearbox with a different main-shaft. The OD units themselves also varied a bit is size and may not just drop right into place.

Didn't GT6+ come with D-type OD originally guys?

In addition to the right mainshaft (which has to match the transmission gear set as well as the OD model), you also need the right adapter housing.

But later Spitfires used a J-type, so you might be able to retrofit the combination into your GT6. I see John Esposito has the Spit 1500 gearbox listed as "Spitfire/GT6":
https://www.quantumechanics.com/categories.php?op=newindex&catid=5

Many of the J-types listed on eBay "for TR6" are actually Volvo units. The sellers have learned that they get more money when they advertise them for TR6, and generally don't bother to mention that they have the wrong (or no) driveshaft flange, speedometer drive, hydraulic pressure, etc. and of course lack the mainshaft & adapter to install them to a non-OD gearbox.

I installed a Volvo J-type in my MKII GT6, quite a fabrication project but it worked great. No GT6 was delivered with a J-type, only D-types (Andrew, that's right, isn't it?).

J-type gearbox is longer than the D-type so new gearbox mounts must be fabricated. Made mine out of angle iron, drilled new holes a few inches aft of the stock mounting holes. Wiring harness for the J-type is easily fabricated and you'll need a different angle drive for the speedo. Quantum has them, as do other vendors. Also, the propshaft is shorter, obviously, but the stock one can be cut to fit.

Nothing at all wrong the a D-type, they're easier to install, significantly less expensive, and correct for the car (GT6). Unless you drive the car very hard or thrash it on a regular basis, I'd go with the D-type for all the above reasons.