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Single rail to 3 rail


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Hi guys/girls, about to do an engine rebuild on late vitesse 1600, as it’s all coming out thought I might look at gearbox too.  It currently has a single rail gearbox with d type overdrive, makes lots of whirring/whining in 1,2&3 gears, but works fine. Now I have a pretty good 3 rail gearbox I took out of a late vitesse 2.0. The 3 rail in my Gt6 seems much more robust than the single rail. 
so question is, how easy is it to swap between the two? I would hope to put the overdrive on the 3 rail. Assuming shaft splines and bell housing are the all the same? 
Any advice greatly appreciated. 



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the base gears etc is pretty much the same as far as is one more robust than the other 

single rail has the chance of a 18mm mainshaft spigot rather than the normal 1/2" of the three rails ( unless they are converted ) 

the single rail was a cheap replacement for use on later cars  the selection metods are pretty awfull

many parts look the same but have small vaiations and you wont find much that is swapable  unless you want some rattling good fits 

in hubs /sleeves  etc 

you wont fit 3 rail hubs to a 1 rail mainshaft the  diameters are a little different , likewise a 1 rail sync hub will be rattling fit on a 3 rail mainshaft 

and so on   tooth count will vary esp on reverse 


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having just done a refurb of PaulH's   3 rail it has a rattle fit 1/2nd hub so its my view this had a single rail hub fitted to a 3 rail mainshaft 

it may well cause problems  Mick dolphin has Hubs but  a slave box will be the simple cost effective solution so depending on just how well or not this works 

Im awaiting a revist sometime soon 

tooth profiles did change and you cant mix and match unless you wanta real good whiner



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Thanks guys, I hadn’t planned to change internals as the 3 rail is pretty good, just wanted/hoped to be able to bolt the OD on the 3 rail and hopefully the main shaft splines might be the same. But it sounds like there may be some tricky bits to deal with. I’ll have a look in a few weeks when I take it out.

no doubt further questions to come. 


Thanks hag 

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Are you sure you have a single-rail with a D-type? Such a combination shouldn't exist from the factory, even with aftermarket parks I've never heard of a single-rail mainshaft for a D-type overdrive.

I think actually you mean 3-synchro and 4-synchro gearboxes? Both should be 3-rail (outward appearance is the same)?

The 4-synchro is certainly no weaker than the 3-synchro, the straight cut first gear on the 3-synchro doesn't last indefinitely and the ones I've dismantled have been pretty badly worn.

Unfortunately you can't just bolt an OD to a non-OD gearbox, you will also need to change the main shaft, 3-synchro and 4-synchro mainshafts are different.

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I've put the single rail internals into a 3 rail 4 syncro case with a J type overdrive, there are some tings to watch out for;

  • the gearbox to overdrive adapter for the 3 rail MkIV J type box is too short & needs a 1 in annulus as well to mate the two as the later single rail gearbox is 1in longer, OR you can use the later single rail adapter it just overhangs above the gearbox at the top, I believe it does seal.
  • The 3 rail reverse lever (106254) doesn't mesh with the gear selectors and needs very serious adjustment! ie bending forwards OR possibly replace the longer single rail reverse spacer (22G1435) with the shorter 3 rail unit (Spit MkIV 3 rail 137687). I didn't do the replacement and the meshing of the layshaft to mainshaft gears would need to be checked? maybe that's what the MkIV 1st/2nd selector rod spacer below is about.
  • In the 3 rail gear selector shafts two additional spacer washers need to be installed one in the reverse rod (spacer washer 158585) and another in the 1st/2nd selector rod (spacer sleeve UKC0706).
  • you have to change the bellhousing scroll seal cover for the later mechanical rubber lip seal type cover (122566), easy changeover.
  • obviously the mainshafts have to be changed to accommodate the J type overdrive. 
  • Oh don't forget the 3 small pretension springs (137532) not fitted to earlier 3 rail gearbox's that preload the layshaft front thrust washer on the 4 syncro box's.
  • Then there's the clutch driven plate splines to check/changeover.,

The Dolly 1850 earlier 3 rail J type O/D unit is a lot easier conversion, but they are hard to get esp if your in Aus!!! In my single rail conversion I had to source parts from all over US, Canada, UK, NZ, and here in Aus. when I started thinking about it around 2005/8 John Kipping then resident in NZ did give me some pointers to watch out for!

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