J Type Overdrive fitting

[JSabah]

Some time ago I purchased a J Type overdrive but it has just been sitting waiting. I also have had an output shaft made. Today I started to remove my gearbox to send to the rebuilder along with the OD, shaft, etc. but before I go any further with the removal, I want to make sure I’m on the right track and have some questions for those who are more experienced than I.

What is “A” in the picture below?

It doesn’t appear that the existing rear mount will work with the OD unit. Is there another support/mounts that I should be getting? Does the metal cross brace need to be relocated? (See my red pen and “C” in the pictures)

Will I need a longer speedo cable (see ”B” )? Is so where are they available or from what other model?

Looking at the bottom of the OD unit, I don’t even see where the mounts would be located...

Finally, I’ve attached a picture of the OD label/plate for information

[classiclife]

Hello,

Item A is likely to be the reverse light switch - you can check that by connecting a multimeter continuity probe on the terminals and selecting reverse and then disengage.

You will need a longer cable, OD units by their very nature increase the transmission length and shorten the prop shaft length; the latter to bear in mind as well. I suggest you use the original cable as a guide and then add the additional length you require. Once you have that total figure, you can contact any speedo cable company to get a stock item or have one made. Ensure you allow for gentle curvature on the cable when collating your new cable length.

J Type OD's usually have a bespoke fitting plate and mounting.

The other factor is that you will need to recalibrate your speedo to match the new transmission ratio.

Could not see the OD info plate photo.

Regards.

Richard.

[Vortex O'Plinth]

Probably the easiest way to replace the gearbox/overdrive mounts is to use a Triumph overdrive mount. This needs to sit on a plate straddling the chassis cruciform. This plate should be joggled to lower the gearbox/overdrive and maintain the original drive line. If you don't do this, the back of the engine may foul the body and the speedo drive cable will foul the floor.

This shot shows the mount and plate from underneath the car....

...and this rather less informative one the installation from above....

You can obtain the joggled plate from Robert Grinter and the speedo drive cable from a LHD SP250 (obtainable from Barry Thorne) is an ideal replacement for the original.

The need to recalibrate the speedo will depend on the provenance of the overdrive. I got mine from Overdrive Repair Services and the drive pinon was perfect for the original speedo calibration.

[Ian Slade]

The micro switch may not be pertinent for fitting to the SP, some cars had the switch for engaging the overdrive in 3rd and 4th gear only, I doubt the selector rods on the SP would extend to that part of the box to opperate the switch. The speedo drive is on the O/P shaft and if the gearing is the same as SP the speedo will read correctly.

[Vortex O'Plinth]

I've just realised Josh that your car is probably LHD, so my advice re the speedo drive cable is inapplicable. Richards advice is more relevant.

[Ian Hastings]

I don't think that the A switch you have highlighted will be relevant to the SP. The switch to engage the OD on the SP box will have to be fitted to the g/box top cover to engage with the selector rods. You can also fit one to operate the OD in 2nd gear but that is gilding the lily a bit.
The rear OD mounts are for Spitfires and available on Ebay or other Triumph sources. You will also need to make up a lowered plate to attach the mount to the chassis.
You will probably get away with your speedo cable but you will need to fit a right angle drive to the speedo fitting on the g/box in order to clear the floor and chassis as Vortex says. Right angle drives and speedo cables can be had from BT.
You should not have to recalibrate the speedo.
When assembling the OD onto the g/box it helps to drill a small hole through the cast alloy webs to allow oil levels to equalize between the g/box and OD compartments. Failing that the oil level must be kept about 1/2" above normal level to allow oil to flow over the web so the OD pump is not starved of oil.
The original propshaft is perfectly fine for fitting with a J-Type OD, the A-Type OD required the shaft to be shortened.
You will have to have the rear drive flange on the OD machined to accommodate the locating lip on the propshaft flange. (It may be on the OD flange - I can't remember.)
After all that you will need to reshape the fibreglass gearbox tunnel to create space for the extra width of the OD at the rear. This is covered well in other posts on this forum.
There are comprehensive conversion details in one of the "technical articles" by Wilf Stevens on this web site. If you cannot source that I can send you a copy.

[Paulkennedy]

Ian, I have A type overdrive on my SP, provided by OS, Sheffield and the prop did not need to be shortened.
Paul

[JSabah]

Thank you all for the prompt and complete responses. My apologies for my brain fart but now I have reviewed Wilfs articles and (re)opened my box from Overdrive Repair Service and was pleased to find that I had, in addition to having the output shaft made, purchased the adapter, gaskets, and pump driver eccentric cam, its drive key and circlip. I also have a Lucas switch, so it appears that the only parts that I’m missing should be (1) the right angle speedo adapter and (2) the rear rubber mounting - I’ll have to make the bracket/support as per Wilfs drawing.... and possibly some O rings.

So I’ll proceed with the gearbox removal

[Ian Hastings]

Paul
That's good to know, I was advised by OS not to go for an A Type for that very reason plus the tunnel needed much more modification.
Ian

[Jez]

Take a careful look on whether you need the 90 degree speedo drive - I had purchased one but found it didn't fit, and I've seen a number of reports of reliability issues. On my car (rhd) there is sufficient space for the lhd speedo cable to (just) exit under the chassis member without any excessive curvature.

If fitted, the switch on top of a standard gearbox should be for reversing lights. On an overdrive gearbox an addeitional switch limits the overdrive to being activated in 3rd/4th only. The circuit is essential ignition feed to dashboard switch to gearbox switch to solenoid, returning via the engine earth. Use some spiral wrap or similar to protect wires under the car. The current draw of the J type doesn't really mandate use of a relay, although you could fit one to reduce current through the dashboard switch.

Cheers - Jez