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#717477 12/02/17 6:35 pm
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Hello all you Triton builders, have any of you coupled a pre-unit Triumph engine to a Norton 4 speed gearbox using a belt drive. I would like to do this and still keep my alternator but the clearance seems fairly tight. Also to do this correctly the primary drive housing would have to be oil free. My crankcase is a 1958 pre-unit with no oil seal fitted to the primary side. I believe there are engineers out there who do this mod, can anyone put me in touch with one in the U.K. Alternatively have any of you ever fitted a rubber shielded MS11 main bearing to alleviate this problem? Also have any of you found a good way to allow the crankcase to breathe better and reduce the bottom end pressure.

Some good old beefy questions for you Tritoneeers th get your heads round eh!

Good hunting, Maddoman.

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You'll have to makes sure that your gearbox main shaft and crank drive shaft are absolutely parallel in the horizontal and vertical plane otherwise the belt will wear away on the front pulley keeper flanges. So your going to need the use of a milling machine and/or a surface table and toolmakers blocks.

Fitting a duplex or triplex chain would be simpler as the chain can tolerate a little misalignment. But even so the ideal is both shafts square to each other.

Crankcase breathing can be improved by fitting a reed valve. They're available on eBay for about £10.


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Keeping crankshaft and gearbox input shaft parallel, as Simon said, is indeed important, it is therefor recommanded to fit a gearbox adjuster on the LH side, as most Norton Commando owners who use beltdrives do.

Last edited by Peter R; 12/02/17 9:44 pm.

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Simon, most of what you stated is absolutely true and important.
BUT I disagree with you about the triplex chain. If the belt pulleys are out of line but parallel, the belt will centre between the flanges and find its own place on the wider clutch pulley. if the shafts are not parallel one side of the belt will wear prematurely.
If a triplex chain is misaligned in any way it will chew up the sprockets and likely break the gearbox shaft at the circlip groove


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I use a Triumph gearbox and primary case with a Tony Hayward belt. I didn’t use milling machine, blocks or paranoia: I just fitted the parts. I moved the gearbox adjuster to the drive side.

There is still no seal on the crankshaft, but not much oil comes through to the primary case. Not enough to cause a noticeable leak, although I eventually lined the clutch plates with cork to stop the slipping that happened whenever oil (mostly gear oil) reached the Surflex plates.


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It's the bottom mounting that determines if the main shaft is parallel to the crankshaft. Fitting a second adjuster can only alter misalignment in the plan view (looking down on the bike) and can only be done if there is clearance between the bottom mounting bolt, gearbox casing and engine plate holes where there should be no clearance. A second adjuster can not alter misalignment in the end view ( looking from the back of the bike).

This is why the crankcase, engine plates and gearbox case should be assembled and then using crankcase timing chest gasket face as a datum check that the gearbox inner gasket face is parallel over the entire surface to the crankcase datum. A surface table comes in useful for this. Any misalignment needs to be corrected on a milling machine where mounting holes can be drilled and reamed perpendicular to the crankcase datum. Gearbox casing can then be sleeved back to standard size and top hat bushes fitted to the engine plates. Or you can hope for the best which will work fine if you don't go any where on the bike.

P.S I doubt misalignment will break gearbox main shaft as the engine power output dosen't change.


Last edited by Simon Ratcliff; 12/03/17 8:06 am.

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As a Triton is mentioned custom engine and gearbox mounting plates would be needed anyway.

Maddoman, do you already have a Norton 4 speed or is it just an idea at this stage ?
I have a QPD belt drive for my own pre unit and it was going to do some form of crankshaft seal (the engine case is already a little banged up in that area)
I might have read (here at bb.com) of someone machining a seal holder and epoxying it in place.

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I don’t think a sealed bearing is sealed well enough to make much difference to oil escaping into the primary.

Enlarging the hole in the crankcase to hold an oil seal outboard of the bearing looks like a simple job for any machinist, but I don’t blame you for wanting a guy with old bike experience. What about Dresda?

For alignment, yes get the stud holes round and straight. Assemble engine and box in the bike and check alignment all the way from crank to back wheel sprocket. Then correct any issues.



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Hi, I am doing something similar on my Triton build. I am using a Bob Newby belt drive with an oil seal holder machined from aluminium plate. I can’t yet comment on the success of this as the bike is still under construction. If you are interested, there is a picture in my build thread (http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=491632&page=1). I have not yet fitted the seal holder and am considering options for how to attach this (epoxy, welding, rivets...).


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Originally Posted by triton thrasher


For alignment, yes get the stud holes round and straight. Assemble engine and box in the bike and check alignment all the way from crank to back wheel sprocket. Then correct any issues.



How do you go about checking for perpendicular and parallel in the horizontal and vertical plane?


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You could use a Theodolite which is used to measure frame accuracy. You need reference points on the engine and gearbox that define planes and position to do this correctly.
If belt sprockets are not parallel the belt will walk off the pulleys or run up against the flange and wear.

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Originally Posted by Simon Ratcliff


How do you go about checking for perpendicular and parallel in the horizontal and vertical plane?


Straight edge against pulleys and sprockets and test assembly.


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The advantage of a surface table is that it is a fixed horizontal plane from which accurate measurements can be taken.

Checking two shafts for parallel in this case is done by assembling the crankcases and gearbox shell to the engine plates and then setting the crankcase timing chest gasket surface parallel to the surface table. Using a vernier height gauge (for example) the gearbox inner gasket surface can be checked at several points to determine if the surface is parallel to the crankcase. As the crankcase and gearbox bearing housings are machined square to the gasket surfaces you are determining if the shafts are parallel to each other in the plan and end views.

A straight edge can only check for parallel along a narrow edge. Great if the parts your checking are say â…›" wide.

I don't know how a theodolite could replace a surface table in this instance.

Last edited by Simon Ratcliff; 12/03/17 6:50 pm.

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That does not check parallel of the shafts.

All you do is mount the belt drive pulleys, check the front pulley for run out, check the rear shaft for run out and then check the pulley and basket mounted with a machinists level (0.0005"/ FT ) on the horizontal.

Sure you can try and get things perfect but if captivated by plates (even if ground) and then the unit is bolted into some ancient frame by its mounting points there is a very good possibility of distortion and all those poofteeths will go out the window, even more silly when in the case of a Commando it is mounted in rubber isolators.

The only way to check if the shafts are in workable condition to mount a drive (chain or belt) is with the unit in the final resting place, not on some fancy slab of kitchen bench surmising the gasket surfaces are perfectly perpendicular to any bores for bearings.

There are other ways to check using dial indicators but you need access to a lathe and mill to machine adapters.





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Originally Posted by Simon Ratcliff
The advantage of a surface table is that it is a fixed horizontal plane from which accurate measurements can be taken.

Checking two shafts for parallel in this case is done by assembling the crankcases and gearbox shell to the engine plates and then setting the crankcase timing chest gasket surface parallel to the surface table. Using a vernier height gauge (for example) the gearbox inner gasket surface can be checked at several points to determine if the surface is parallel to the crankcase. As the crankcase and gearbox bearing housings are machined square to the gasket surfaces you are determining if the shafts are parallel to each other in the plan and end views.

A straight edge can only check for parallel along a narrow edge. Great if the parts your checking are say â…›" wide.

I don't know how a theodolite could replace a surface table in this instance.




There is no point aligning things more accurately than the engine plates and frame can hold them.

It’s a bit like the lonely furrow you ploughed, about T100 rebores to a 5th of a thou.


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A Theodolite can measure points in 3D space. We used it to measure twist and deflection of 500lb bomb wing kits to be sure they were in specification before sending them off to Boeing. Probably a more critical measurement than a belt drive.
You can measure four points on the engine and clutch sprocket and determine the angularity and offset of one to the other (assuming the sprockets are square to the shaft which you could check by rotating the shafts 90 degrees and measuring again).

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An Engineers surface table is flat enough to accurately determine perpendicular of a shaft in relation to a machined surface. Mine is flat to within 1.6 millionths of a meter. The surface table is a horizontal plane from which measurements are taken in relation to a parallel surface. Toolmakers blocks provide a vertical datum.

Surface tables are limited in use by their surface area, as is a straight edge. So it's impractical to use one to check an aircraft wing.I see no advantage of using a theodolite over a surface table to check a motorcycle frame or to determine parallel of shafts.


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Originally Posted by Simon
Mine is flat to within 1.6 millionths of a meter.


Millionths shmillionths!- This has got to be dead-on. We’re talking about a Pre-unit special here, with an odd gearbox.


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If you are fabricating engine plates for a Triton using a pre unit motor with a belt drive you do not need gear box adjusters at all. Set the gearbox in proper place and drill round hoes for the top mount not slots. Once belt tension is correct adjustment is not necessary just like a unit motor does use an adjuster when using a belt drive.

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Don't drill the top mounting hole till you have the belt in place and aligned to proper tension! Then, mark & drill it.


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Originally Posted by triton thrasher
Originally Posted by Simon
Mine is flat to within 1.6 millionths of a meter.


Millionths shmillionths!- This has got to be dead-on. We’re talking about a Pre-unit special here, with an odd gearbox.


Absolutely....I bet once it's all adjusted you can push down on the belt or chain with your finger and observe the clutch asembly move slightly ...


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And when the engine is running and the bike is accelerating everything will move all over the place!

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Useful or not, I don't know, but I soon gave up on trying to squeeze an alternator into the chaincase, with a belt drive.



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You could bolt it together by eye, straight edge and a tightline, run it for a while and then check for witness marks.

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Originally Posted by Les P
That does not check parallel of the shafts.

All you do is mount the belt drive pulleys, check the front pulley for run out, check the rear shaft for run out and then check the pulley and basket mounted with a machinists level (0.0005"/ FT ) on the horizontal.

The only way to check if the shafts are in workable condition to mount a drive (chain or belt) is with the unit in the final resting place, not on some fancy slab of kitchen bench surmising the gasket surfaces are perfectly perpendicular to any bores for bearings.


Checking for run-out only tells you if a shaft is bent, it won't tell you if it's parallel to another shaft. To do that you need an Engineers grade surface table and toolmakers blocks, or even " some fancy slab of kitchen bench" is literally better than nothing.

Crankcase and gearbox bearing housings will be machined during the same stage on a milling machine ensuring they are perpendicular to each other.

I'm curious exactly why you'd use a spirit level on "the pulley and basket" ???? That's funny!


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