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Can a Single Rail J Type Adapter be used on a 3 Rail Box

Peter Truman

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I have previously successfully converted a Spit 1500 single rail internals into a 3 rail GT6 box case and attached a J Type overdrive for my daughters Spit Mk2. I used a Spit MkIV adapter plate 313085 and an additional 1inch adapter to accommodate the longer mainshaft RKC767, with the bigger input spigot.

I have a Dolly 1850 single rail gearbox with J Type overdrive which I want to similarly covert to a 3 Rail box/case for my Mk2 Vitesse. I don't want to go down the route of using the SpitIV adapter plate 313085 and the more I look at the photos of 313085 and the Single Rail adapter RKC634 the gearbox case mating flange is similar except for its upper extension for the single rail selector rod.

The circular overdrive mounting flange looks similar except for this selector rod extension,  also could this upper hole for the selector rod be plugged and the later single rail adapter RKC634 be used on the 3 rail box conversion in place of the Spit IV adapter 313085 plus 1 inch annulus spacer. The extra width of RKC634 would also negate the need for the 1in spacer annulus.

if the 3 rail gear selector arm and rod foul the top of RKC634 there is the possibility of plugging the top hole at the overdrive circular flange end and trimming back the unwanted top of RKC634 at the overdrive end. The gearbox mounting bolts of RKC634 and 313085 appear to be the same re number and location/alignment.

I haven't seen any Spit MkIV adapters 313085 on ebay for a long time & I know Canley sell a J Type conversion of 313085 but it is expensive ( 98GBP plus overseas shipping), and I'd still need a 1inch adapter plate. I do have the J Type conversion new mainshaft TKC899 small tip & D Type overdrive adapter 305137 spare that I could sell to defray Canleys cost!

I need to get both the RK634 & 313085 gaskets and check the various hole alignments and mounting bolt positions!

Anyone have experience of such a modification for adapter plate RKC634.


Peter T

J Type Single Rail Adapter, Gearbox End.jpg

J Type Adapter RKC634 internal view.jpg

J Type Adapter RKC634 Top View with single rai selector tube.jpg

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I'm pretty sure this is possible, I've seen photos of such a thing on sideways...


Not the route I've gone down for my Dolomite 1850 gearbox in a 3-rail case, I bought 313085 from Canley's and a hybrid mainshaft from Mike Papworth. That takes care of the 1" difference but is definitely not cheap. Still not actually built it yet either...

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thanks for that answered all the questions re adapter plate. I brought on special a modified J type conversion prop shaft for the Vitesse from Rimmers eons ago if I go down the Single rail  Dolly 1850 gearbox with J OD, into 3 rail box, I'll have to get that shortened by 1inch, but its a plan!

Sidways Technologies interesting, a lot of knowledge & experience but also appears to be some deep baggage!

You learn something everyday like Spirlock Snap Rings, in place of circlips

Thanks again

Peter T

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