Fitting a J type overdrive to an SP 250

[d.wade.j]

Has anyone fitted a J type overdrive to the SP250? If so could you explain what is required?

[JohnM]

Yes, many of us have, and it's quite straightforward.
If you wait, Wilf Stephens (Dragon on here) will reply (he's been at the AGM at Gaydon today) and he will offer you the complete write-up on how to do it.
Cheers,

[Dragon]

Hi David, and welcome to the Forum.
Like the rest of us, I am still learning and don't yet know if I can safely attach a document to an e-mail sent via the new forum, so could you please e-mail me so I get your e-mail address?
Then I will send you the illustrated write-up on the J-type overdrive conversion, and the list of other articles currently available.

[Jez]

Hi David,

I'm in the process of doing exactly that. Had the original gearbox refurbed and the overdrive added by Overdrive Repair Services just outside Sheffield. Got it refitted into the car this weekend and now need to work through putting everything back.

Getting the gearbox out and the new one in is definitely a two person job and needs a fair bit of jiggling and messing out but it does give in eventually! Access is from the top so remove seats and fibreglass tunnel for access. Remove clutch slave and bracket, speedo drive, and prop shaft. The starter also needs unbolting but can be left resting inside the car.

When removing or reinstalling, note that the gearbox itself is narrower than it is deep so we found it easier to move it on its side, turning it for final fitment. The guys at Overdrive Repair Services provided the new gearbox mount and a suitable bracket which matched exactly with existing chassis holes.

You will also need to reshape the transmission tunnel a little to allow for the extra width of the overdrive. It's only a few mm, so I think cutting out the relevant section and then reattaching it as a raised cover over the hole would do it. There's plenty of advice and experience on the forum to help.

Happy to help in any way I can.

Cheers - Jez

[Vortex O'Plinth]

A point to note re installation is that with the 'J' type overdrive fitted, the original gearbox to chassis mounts are usually replaced by a Triumph Spitfire item. To preserve the original driveline geometry, the base of this mount should fit flush or actually a few mm below the top of the chassis. The original gearbox mount bracket can be replaced with a flat plate, but this needs to have a 'joggle' to lower the new overdrive mount to the correct level.


Before


After


Triumph Spitfire overdrive mount


New mounting plate


Unit installed

After fitting a 'J' type overdrive some owners have also fitted a right angle drive to the speedo cable take-off. Part of the reason for this can be that unless the original driveline has been maintained, the take-off is positioned too high and the cable fouls the body. I had no problem fitting the cable although the original will now be too short; replacing it with the cable for a LH drive SP solves this problem.

If the overdrive unit is supplied by Overdrive Repair Services for the SP, they will have selected an appropriate drive pinion for the speedo take off and your speedo will not need recalibration. If it comes from another source you may need to check this.

[d.wade.j]

Thank you to all who have replied, I now have Wilf's explanation of fitting the overdrive and am dismantling my gearbox in preparation to send the mainshaft for modification. I should not have a problem in refitting as the body is currently off and I will fit refurbished engine and gearbox before refitting this.