Mk5 Overdrive Conversion

Hi everyone, there have been a few questions around the overdrive fitment I did to my Mk5.

My priority was to keep the car as standard as possible, and the only issue I was concerned about was cruising speed, and the limitations of the low-revving pushrod engine combined with 4 speed gearbox.

Fitting an overdrive means you can retain your original car gearbox, I was not prepared to start fitting a different gearbox from other cars. You need a donor moss box with overdrive. Our mk5 gearbox was in excellent condition, with no gear or synchro wear apparent. Therefore, we stripped the donor mk7M/mk8 gearbox, so we could use the longer main shaft. We stripped the mk5 gearbox, and re-assembled using the longer main shaft, and then the overdrive just bolts onto the rear.

I will try and take some pics this weekend, as ironically we have had some oil pressure issues with the overdrive, I have done 1,000 miles of running in, so am now doing the first major service. The oil pressure issue has been identified as the O ring seals we fitted to the accumulator piston - within 1,000 miles the new seals have shrunk! Cheap chinese rubbish from a reputable bearing supplier. We managed to locate some high temperature O rings, not easy as its a size that is not commonly utilised in any other applications.

Fantastic! Thank you so much for doing this. I’m really looking forward to seeing what you have done including the wiring as well as the switch on the dashboard.

Tim

Same here. I’ll be yanking the engine today to have the rod journals ground and clean everything. It would be the perfect time to upgrade the tranny.

Wayne

I borrowed a J type Laycock OD and was trying to find a place under north to remotely set it. I was told by a Laycock rebuilding shop that only the A will fit the MK V. Would have to swap main shafts in the tranny and all said would be about $2800 which is more then I want to spend to increase speed by 25-28% .
I was hoping to mount it after the center bearing but I don’t think there’s enough room for suspension jumps. Attaching it to the pumpkin has its own draw backs. Now I’m considering swapping the tranny for a Volvo with the OD already connected. I’d have to change the clutch plate or change the spline one this one. Plus adapt a bracket for the starter motor but it’s another possible route.
Has any one here successfully adapted a J type to a MK V?

Here is a picture of a box from an XK140 with a serial number beginning JLE.

Here is one which I think is from a Mark VII, can’t tell from this angle. If so, it would have a serial number beginning JLN.
OD trans Mk7 or 140

Either of these would fit your Mark V bell housing, no need to swap the mainshaft.
Whether or not they would fit in the chassis, I don’t know, but I have my doubts. Certainly you would have to engineer up some sort of a strong rear mount.
PICT0035

Certainly looks like the X frame would need massive modification: not a trivial task.

At this point, I’m just casting about to see what might work.
Today I removed the tranny, clutch and flywheel in order to get the crank out. Need to have the rod journals ground to fit new big end rod bearings and instal the pistons. I found that the spline in the clutch plate fits a Volvo tranny shaft and the complete tranny with J type OD is within 1/2” in length of the original tranny. If I can make a mounting plate to mate this to the bell housing, it might be a solution to the OD problem.
On another note, I’m finding that the engine has black paint over a greenish blue paint with no other color below that. I’m even finding it under the end plates and in places that haven’t been touched in a long time if at all before. Seems there wasn’t much care taken in prepping the blue paint before painting the black as the two colors have separated in a lot of places. It just makes me wonder how many times this engine has been opened.

I agree. You remember the discussion we had about the rear gearbox mount, the sandwich type vs the spindle type?

Perhaps it could be modified to suit?

Nevertheless, I’m really keen to see how the overdrive has been fitted successfully.

Tim

I would be interested to know if this would be an achievable modification for the Mk IV. Noting the need to devise a suitable rear mount, does the tunnel need modifying too? What would be nice would be to have the later '60s era all-synchro box with O/D.

Peter

Early E Types has the double spindle style of rear mount… worked fine.