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Dolly 1850 single rail gearbox in to mk3 GT6


Graham C
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Hello

I have obtained a 1850 Dolly Single rail gearbox with a j type OD to replace a 3 rail non OD gearbox. Besides prop shaft, rear support plate, OD support, 90 elbow driver, diff and needing a Dolly friction clutch (fine splinters type) have I missed anything. I am aware the GT6 bell housing needs altering to fit the selector shaft extending passed the gearbox, so have I missed anything like the main shaft spigot being a different size.

With the clutch planning on use the dolly friction plate with the GT6 clutch drive plate and clutch bearing, no sure if the dolly 1850 clutch parts will fit the GT6?

Will I also have problems with the Speedo miss reading as well?

Graham

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Not sure why you think you need a different diff?

You will need a propshaft and it is a special as the single rail OD box is 1" longer than the 3 rail OD box.

As you say, Dolly bell housing is not a direct fit on the single rail box as it lacks the housing area and bores for the selector shafts.  I don't remember the details (it was 25 plus years ago) but after looking at the problem I did not attempt to modify the bell housing but rebuild the singe rails internals into a three rail casing - which is straightforward.

Fine spline friction plate should work fine with GT6 cover. Ideally you would match make but sure it really matters. Use the GT6 release bearing and carrier (bearing may be same part)

The extra gearbox length makes things very tight at the back end.  I modified the non-OD mount bracket with a simple rearward extension.

Don't remember about speedo calibration.  You'll likely need a new cable though

Nick

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Hello Nick

Thanks for the reply, I am planning on fitting a OD prop shaft, rear support and chassis bracket. Hopefully that will resolve the issue around length difference. The bell housing mod is the interesting part, initial thoughts are to drill through the original housing and then have a plate fitted/ welded on the inside if room. Thinking of this route as I have never 're built a gear box before and time.

The diff I am changing as my current one is a non OD ratio one, so will change to an OD ratio one.

Nick you did not mention the main shaft so I assume this is not a problem?

Thanks again for the advice.

Graham

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4 minutes ago, Graham C said:

The diff I am changing as my current one is a non OD ratio one, so will change to an OD ratio one.

Are you sure? I had a recollection that the "different ratio for OD" was discontinued for Mk3. Either way, a 3.89 (OD spec) diff is a little short for a GT6 and the 3.27 with OD combination makes for much nicer cruising. Mine has a 3.63 (Spitfire 1500) mostly due to the poor availability of 3.27 ones, which is generally felt to be a good compromise.

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47 minutes ago, NonMember said:

Are you sure? I had a recollection that the "different ratio for OD" was discontinued for Mk3. Either way, a 3.89 (OD spec) diff is a little short for a GT6 and the 3.27 with OD combination makes for much nicer cruising. Mine has a 3.63 (Spitfire 1500) mostly due to the poor availability of 3.27 ones, which is generally felt to be a good compromise.

Easy to check diff (as long as its standard of course): serial number non-OD is KC***** = 3.27 and overdrive KD***** = 3.89

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You will need (as Nick says) a special prop. The OD gt6 prop will be too long …..

I know a couple of  cars that have been converted, (spit to gitfires) using the 1850 box. I will be seeing the chappie who did them at the weekend, so can ask him for specifics.

But Diff, as above, 3.27 for non-od and 3.89 for non-od is factory, both giving the same speed/100rpm in top/top od.

A good compromise ratio is the 3.63 from a spit 1500, stronger too if you get the right one. Or just use the 3.27 and have a lazy motorway rpm....depends what you want from the OD really.

Clutch bearing the same between the boxes. And cover, just need the fine-spline clutch plate but best, again as Nick says, if it matches the cover. So a dolly 1850 clutch kit is ideal.

If the speedo is out you can alter the speedo drive in the OD. It should match the diff, so do little research as even ODs have been swapped and are often fitted with the wrong drive gear.

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The single rail 1850 box will not fit onto the GT6 bellhousing due to the single rail remote extending into the bellhousing. Not to sure if you can modify the GT6 bellhousing to take this remote.

John Kipping/Mike Padworth used to convert the 1850 single rail to three rail to get around the problem.

The 1850 bellhousing has the starter motor on the wrong side so a non-starter!

I agree the 3.63 diff is the best to use with O/D.

Dave

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As I see it the GT6 belhouse will bolt onto the 1850 casing ok but the gearchange rod needs to be able to move forward and also be sealed. However it cant be too difficult to remove a section of the aluminium belhouse to leave a flat surface onto which some sort of seal block can be bolted. Perhaps this could then be sealed to the iron casing with a recessed o-ring to allow for any misalignment......

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24 minutes ago, johny said:

As I see it the GT6 belhouse will bolt onto the 1850 casing ok but the gearchange rod needs to be able to move forward and also be sealed. However it cant be too difficult to remove a section of the aluminium belhouse to leave a flat surface onto which some sort of seal block can be bolted. Perhaps this could then be sealed to the iron casing with a recessed o-ring to allow for any misalignment......

This is basically true and has been done at least a couple of times before, but I think quite difficult to do nicely while maintaining the strength of the bellhousing, especially if you want to avoid aluminium welding. You may be lucky enough to have someone who’s actually done it pop up to comment. I seem to remember quite large quantities of JB Weld featuring in one solution.........

I wasn’t sure what you meant by “main shaft” as this is buried inside the gearbox, but if you mean the tip of the input shaft that goes into the bush in the back of the crank, that is one area that will work exactly as before.

Re the diff, my suggestion, even if you have the 3.27 diff fitted now is to try it before making any changes. There is very little repeat work involved with approach. If you find it too high geared (though really doubt you will), then I would also suggest getting a 3.63. I have them both in my Vitesse, which is well proven and works very well in spite of still being 2L powered and having a .79 5th gear which is taller than OD, and also in my GT6. The GT6 is untried so far, but being both lighter and lower drag than the Vitesse, I have no worries.

As I said before and others have confirmed, the single rail gearbox is 1” longer than the three rail so standard GT6 OD propshaft and chassis bracket are not going to work without modification.

Nick

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As I say I have no experience of this mod but Im surprised only 1" would be enough to need a special prop making up. They come as standard with a bit of flexibility on length (the sliding joint) which has always surprised me given that the diff and engine are both pretty well static anyway. In fact I believe some early props didnt even come with a sliding joint or u/j but a flexible disc coupling which looks like it only allowed very little movement and to me always seemed a better design (unless it breaks of course😣).

Unfortunately the various prop lengths that Ive seen online never specify whether they are the compressed or extended figures but I would have thought that with a bit of 'engine postion adjustment' the standard GT6 OD prop might fit.......

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Well, I can tell you that that approach definitely didn’t work for me with 1” longer boxes in Herald or Vitesse. Herald could possibly have been bodged with spacers. Vitesse fan would have been in the rad.

Sliding length is about 1”with the running point midway. Canleys have a list of prop lengths in their tech archive though I don’t remember if this non-standard option is shown

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Hello All

Thank you for the great feed back, given me plenty to think about. Sorry some of my terms may have been incorrect but I believe you have understood.

So basically:

Prop shaft has to be shorten. My replacement one is a sliding one so this will be interesting to see if Johny is correct.

1850 clutch kit will fit.

Support bracket will need altering.

Bell housing needs to be alter to accept single rail remote. Now fortunately I have the 1850 bell housing fitted to the replacement gearbox box, another thought was to have that part of the 1850 grafted/ welded into the GT6 bell housing. Both aluminium. I assume the critical thing is to ensuring the selector shaft fits and has full movement. I assume the closed hole is to stop oil reaching the clutch  I assume no seal is fitted.

The above will allow the gearbox to be fitted, 

The diff is another interesting subject, did not realise the GT 6 did come with that option. So that will drive the car and see.

Thanks again for the advice, any more will be greatly welcome.

Graham

 

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2 hours ago, johny said:

As I say I have no experience of this mod but Im surprised only 1" would be enough to need a special prop making up. They come as standard with a bit of flexibility on length (the sliding joint) which has always surprised me given that the diff and engine are both pretty well static anyway. In fact I believe some early props didnt even come with a sliding joint or u/j but a flexible disc coupling which looks like it only allowed very little movement and to me always seemed a better design (unless it breaks of course😣).

Unfortunately the various prop lengths that Ive seen online never specify whether they are the compressed or extended figures but I would have thought that with a bit of 'engine postion adjustment' the standard GT6 OD prop might fit.......

Heralds came with solid props (just a UJ at each end) and some had the dreadful "strap drive" props.

AFAIK GT6 and Vitesse always had sliding joint, though vitesse 1600????

I will ask about closing the selector rod holes. I think he may have used a cap or suchlike...but can confirm after the weekend.

Re prop, not a bad idea to get a brand new one made up. An iffy prop can cause untold misery.

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Spoke to Pete. He drilled the bellhousing, and fitted a tube with a blocked off end. Used some sort of adhesive/sealant to fit it, not a stressed joint. But wants to be a good, tight fit with sealant to complete the job. I wonder if he used a bearing lock compound? I can check later. Off to assist a local with him today, should see a Vitesse back on the road by lunchtime.

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Hello Clive,

Thank you very much had not thought of that. I assume the sealed tube is to stop oil escaping from the gearbox so the size of tube is not critical as long as the shaft is aligned and fits. If I use an aluminium tube then it can be welded in place. I am aiming to take the old gearbox out over Christmas and start trial fitting parts.

Graham

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  • 2 months later...

Tomorrow I am helping convert an early dolly non od (has the 3 rail box) to single rail overdrive. As we have not been able to source for sensible money a single rail bellhousing, I have a cap and will be fitting that.

I have borrowed a singloe rail gearbox casing so I can bolt the bellhousing to that, and use it as a guide to drill the  hole. The cap has a flat plate that will be attached to the bellhousing by 2 small screws (cheekily M5, I don't have any small imperial taps)

 

I will take pics as I go tomorrow. In between fixing an old Mamod steam engine.....

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9 minutes ago, Pete Lewis said:

ive got a mamod traction engine,, the side tank loco and trucks and a older  Gamages  stationary with hammers grinder, press and other 

belt drive accessories,   you cant beat rhe smell and fumes  of Meta solid meths 

pete

Beats glue sniffing I guess. Glues these days are "allegedly" healthier.

Meths, oh how I miss the good old days! 💀Now I have to make do with tobacco and alcohol 🍷

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I've an old Mamod from when I was a kid. Haven't started in since then. Imagine if the piston/bore, had proper compression (would probably be too quick, crashing into walls etc).

Maybe there is a scene where folk modify them and have 10 foot (no metres here) drag races.

Dave   

 

Edited by daverclasper
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17 minutes ago, Chris A said:

Meths, oh how I miss the good old days! 💀Now I have to make do with tobacco and alcohol 🍷

I recommend fondue, my kit has a meths burner and smells wonderful, normally I put cherry Kirsch in the fondue, but I suppose you could...……………………..

Doug

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