Jump to content

Fitting Overdrive


Recommended Posts

I’ve just bought a 3 Rail Gearbox with D-Type overdrive to fit into my 13/60 Herald, I just have a few questions that I’m sure someone can help with.

•     How can I identify if it is a 3 or 4 Sync Box?
•     If it’s a 3 Sync box is it simple to convert to 4 Sync?
•     Presumably you have to modify the propshaft tunnel to allow the extra length of the gearbox?
•     Can the existing Gearbox tunnel be used?
•     Does anyone know what length the shorter prop has to be from flange to flange?

Many thanks

Link to comment
Share on other sites

Hi I can answer some of your questions but I expect you will get some better advice later. First is the input shaft the same length? This will define if it is suitable without modification as it could have been from many different cars.. Spitfire hopefully if you have a herald.  Three rail boxes were fitted to herald vitesse and dolomite plusnothers probably.
Simple check to see if it is a 3 or 4 synchro is to see if it has a reverse light switch on the top of the selector cover.  I think the 3 synch ones never had them.
You should not need to modify the tunnel but may need to make a cut to the floor section behind the tunnel to be able to get to the prop bolts as these will be further back.  You do on a vitesse but I am not sure about Herald.
I think Canleys will sell the correct prop for this conversion or else you could get yours shortened. You also need to consider the speedo cable and wiring..  Hopefully your box will have the angle drive on the speedo connection as the after market ones are less than perfect.  Lastly you cannot convert 3 yo 4 synchro the case is even different.


Link to comment
Share on other sites

MKlll Spit is 3 synchro with reverse light switch.  The way of telling them apart is the 3 synchro boxes have a square rear flange and the 4 synchro has a round one.  Not easy to convert a 3 synchro to a 4 synchro.  Incidentally if it is a 3 synchro box, I'd be interested in buying it off you.  Check the input shaft is the same as this'll tell you whether it is a Vitesse 'box or a Spitfire 'box.  If larger input shaft then it is from a Vitesse and won't fit without other mods such as modifying the flywheel and clutch and the ratios will be unsuitable anyway...


Link to comment
Share on other sites

Thanks for the replies.

The flange is round, so looks as if it's a 4 sync box, that's good news.

The gearbox is out of a spitfire so splines and input shaft length should be the same as the Herald. It came with a new solenoid, original angle drive, longer speedo cable and switch, so I only need mounting and mounting plate and sort wiring.

I did buy a sliding prop a while back to replace my solid one, only when I tried to fit it to my non OD box did I realise it was shorter so I'm hoping it's the right one for an OD. If anyone does know what the length should be it'll save me having to wait until I fit it to find out.


Link to comment
Share on other sites

1218 wrote:
The way of telling them apart is the 3 synchro boxes have a square rear flange and the 4 synchro has a round one.

Hi Mark,

The gearbox has a round flange, but after popping the top off I can only see 3 Synchro Rings (image below), am I missing something here or do I just happen to have a 3 Synchro box with a round flange?

9110 wrote:
If you go to the Canley Classics website, technical info section, it lists the triumph models and required propshaft lengths. Cheers, Dave.

Great Info, thanks. Presumably the measurements quoted are from/to outer edge of the flanges?

The sliding prop I have measures 43 1/2" (retracted) to 45 1/4" (extended), so the only car this could possibly fit would be a Vitesse with OD I guess? Or could just not for a Triumph?

With the bundle of bits I got with the gearbox was a clutch driven plate, it has the correct spline for the gearbox and exactly the same as my non-OD box, but it has an OD of around 7 1/8" as opposed to the 6 1/2" for a standard Herald.

Any ideas what the large clutch plate is from? Should I be using this or will a standard Herald clutch be fine?


Link to comment
Share on other sites

That's a 3 synchro box, as in Heralds and early Spits. The number, FC, indicates that it starting life in a mk1 or 2 Spit ( or at least the casing did!)
Standard Herald clutch, you'll need a Vitesse O/D speedo cable and angle drive as well as the mounting plate to bolt the overdrive to.


Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...