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J Type Overdrive 3 Rail Adaptor Plate


Nigel Clark
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As above, please has anyone got an adaptor plate to fit a J Type overdrive on a 3 rail gearbox they would be prepared to part with? I need one to complete a hybrid 1850 Dolomite 'box with J Type O/D for my 2.5 litre GT6. Most of the big parts suppliers list this item (part no. 313085) but all are currently out of stock.

Nigel

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I made a gt6 d type overdrive gearbox into a j type from a single rail jtype overdrive, the only bit i needed to do was turn down one of the shafts in the lathe. No other parts required except to get the correct length propshaft. Can't remember which shaft, will have to go through my photos.

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2 minutes ago, Mathew said:

I made a gt6 d type overdrive gearbox into a j type from a single rail jtype overdrive, the only bit i needed to do was turn down one of the shafts in the lathe. No other parts required except to get the correct length propshaft. Can't remember which shaft, will have to go through my photos.

That's interesting, though I thought the J Type O/D needs a different adaptor plate from the D Type because it's larger. There are different part numbers for these plates, 313085 for J Type and 305137 for the D Type, both to fit the three rail 'box. The single rail 'box uses yet another adaptor plate to fit the J Type, part no. RKC634 but as I understand, this won't fit the 3 rail gearbox case.

My gearbox will use 1850 Dolomite single rail internals, built into a 3 rail case to maintain the original gearchange, so to fit the J Type I need a shorter mainshaft (which I have) and adaptor plate 313085. The original GT6 O/D propshaft will then fit, as the overall length of the gearbox/adaptor/J Type O/D is the same as the original. If I've got that wrong, I would really like to be corrected, as I've got an RKC634 adaptor plate from a single rail 'box.

Nigel

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32 minutes ago, Nigel Clark said:

 gearchange, so to fit the J Type I need a shorter mainshaft (which I have) and adaptor plate 313085. 

Nigel

Thats what i did! Shortened the mainshaft! I remember how good it was , the quality of the metal as i machined it. Will dig up some pictures tonight and post them on.

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wow this a subject on which theres quite a few previous threads but heres my understanding:

the single rail design uses a larger mainshaft tip bearing to fit the input shaft and non standard short length mainshafts were made to accommodate this so hopefully you have one of these. If not a standard single rail mainshaft would mean the gearbox would need a standard single rail adaptor plate (has to be modified to fit a three rail case) which makes the complete unit 1" longer and a very tight fit.

So you do indeed need a J type adaptor as used in the last Spitfires (and Dolomite 1850?) before they came with the single rail box. 

However unfortunately I cant help you with sourcing one of these...

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Nigel, if you have fitted all single rail internals into a 3-rail case you use the single rail OD adaptor plate.

the original 3-rail adaptor is smaller so won't work with the longer large tip mainshaft. It is also f***ing expensive when you can find one. Seen them for £200 recently.

you do need to shave a tiny bit off top edge so the reverse light switch fits.

I had a thread on the subject recently, will try and find it.

I have 2 gearboxes built up in garage, one an original 3-rail 1850 small tip OD box and a single into 3-rail with large tip OD box.

Can get you photos 

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Nigel if I was going to do the job again I think I'd try the single rail adapter RKC634 as I think it would seal to the 3 rail case OK but just stick up above the top and the top hole in the adapter would be there but I don't think it would be open and thro more investigation reqd I think.

I have a Dolly 1850 single rail gearbox with J type here in the garage, but haven't stripped it down yet so I'm not too sure of what fits what, it's for a 3 rail case conversion to put in my Vitesse Mk2.

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I think Nigel has a large tip mainshaft but which is the length of a normal 3 rail item. Rimmers for one listed these as HD shafts but not sure if they were made from scratch or modified 3 or single rail shafts but mean a shorter adaptor is needed and the end result will be the same length as the standard box...

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Looks good and the only thing Id want to check is if the hole that runs through the top of the adaptor casting is actually open to the OD unit? Other than that the only problem might be the extra inch length which I believe makes it a very, very tight fit... 

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Thank you for all the replies!

I have a Dolomite 1850 single rail J Type O/D 'box with the larger mainshaft tip. My aim is to have this built into the 3 rail casing with the shorter overdrive adaptor plate used on late Spit Mk4s with J Type, creating a unit that's the same overall length as the original GT6 box plus D Type O/D. This way, I won't need to shorten the propshaft or cut away the transmission tunnel, and with a suitable rubber mounting and bracket, the J Type O/D will bolt to the existing chassis crossmember in much the same way as the D Type. Simples!

In order to achieve this, the complicated bits are:

- Shortened mainshaft with the large diameter tip of the 1850 single rail 'box. I've got this sorted.

- 313085 adaptor plate, which is shorter than the RKC634. I haven't yet managed to find one of these.

So unless anyone knows different, I've got a workable plan to upgrade the gearbox and overdrive on my GT6, but need to find the 313085 adaptor which is the missing piece of the jigsaw puzzle.

Nigel

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Sounds good but that adaptor is pretty rare as it was used in so few cars. I wonder if when none are available it might be possible to take an inch out of the middle of the standard single rail adaptor - not easy but it might be the only alternative to machining one from scratch...

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3 minutes ago, johny said:

Sounds good but that adaptor is pretty rare as it was used in so few cars. I wonder if when none are available it might be possible to take an inch out of the middle of the standard single rail adaptor - not easy but it might be the only alternative to machining one from scratch...

Apparently one of the major Triumph parts suppliers is currently having a batch of adaptors made, though timescale is unpredictable. I was hoping it might be possible to save time by finding one through the forum, though I realise it's a long shot. I will probably just have to be patient and wait for one from the new batch. Hope my D Type O/D will last long enough!

Nigel

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41 minutes ago, johny said:

Im surprised as the demand must be pretty low cos finding the J type OD themselves isnt easy now. Brace yourself for the price if some adaptors are made😄

£112 on Canley Classics website - ouch!

But I've already got a freshly rebuilt J Type on the bench waiting to be fitted.

Nigel

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Just to confirm - are they unique to Triumph overdrives (as in: is the Triumph version made specially for Triumphs only?) or do they differ between marques?

I've found various other brands online, second-hand -Jaguar for one - so am wondering if one will fit any J-type or does the type of car matter?

 

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Canley's is where I got mine from, the price is okay for a niche low volume part. As far as I know, the rationale for remaking them is that the supply of original adaptors dried up sometime ago. But maybe some specialists have some stashed away.
 

On 30/03/2021 at 14:19, Nigel Clark said:

That's interesting, though I thought the J Type O/D needs a different adaptor plate from the D Type because it's larger. There are different part numbers for these plates, 313085 for J Type and 305137 for the D Type, both to fit the three rail 'box. 

This is correct you can use 313085 for both J-Type and D-type overdrives, but 305137 will only fit a D-type overdrive. Nick Jones confirmed this for me a while ago. The J-type adaptor has a bigger void on the OD side to clear the pump cam IIRC.

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