3 speed gearbox shimming | Scotts Owners Club

This topic has 8 replies, 3 voices, and was last updated 10 years ago by gammakeith.

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30th May 2013 at 6:37 pm #3346

gammakeith
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Dear all,

Trial assembly of my gearbox indicates a satisfactory 11 thou of end float on the layshaft. The mainshaft is, however, not so good. I knew it was going to be bad from a quick previous check prior to disassembly but I have just measured the grand sum of 85 thou of endfloat!! I’m suspecting that the wrecked main roller bearing, worn boss bush and the excessive endfloat will together nicely account for the horribly crunchy gear change and difficulty getting into first.

There is a slight but clear mismatch between the sleeve gear and the corresponding layshaft gear (hopefully clear on first pic). Thus, I assume that I should not use a shim between the gearbox casing and the sleeve gear as this mismatch will be preserved. A shim on the mainshaft itself will move the sleeve gear outwards and into better alignment with the layshaft. However, a shim can be placed either side of the selector dog. If I shim on the ‘cover’ side then the selector dogs don’t completely seat in first gear. On the other hand, if I place the (70 thou) copper shim on the clutch side then all appears well so I am assuming this is the way forward (second pic). Assembling it all with the selector mechanism confirms that the gears align nicely so I’m quite happy. Inspecting the sleeve gear itself reveals what appears to be a region worn by the mainshaft (third pic). The brass bush is slightly recessed below the gear face so there will be a steel to steel contact at this point except when the shafts are locked together in 3rd gear.

Firstly, can anyone confirm that my logic above is correct re shimming? Secondly, what should I use for the shim? The copper washer is OK regarding dimensions but I’m guessing thats probably too soft and I should machine something from a brass bar?

Thanks, Keith

1st June 2013 at 8:44 am #11005

scott596
Participant

Keith I would refrain from doing anything that would restrict full engagement of the dogs, I personally would not worry about the mismatch of the gears, however as you have a built up layshaft you can either grind the spindle or the gear to improve alignment. I have recently refurbished a vintage gearbox that required all new bushes, and as the sleeve gear was badly worn and pitted I took the opportunaty to grind the diameter and backface of the gear this required an undersize bush with a thicker shoulder to take up all the end float, the only problem with this is the need to mill part of the new bush away when fitted in order to clear the layshaft gear when you have a vintage solid layshaft. Good luck. Alan Noakes.

2nd June 2013 at 8:09 am #11006

gammakeith
Participant

Thanks for the feedback. I ultimately went for a steel version of the copper washer that is just visible in the middle picture. Despite appearances, its sized so the sliding dog slides over it. Thus, all dogs can engage fully and I have ended up with a final 15 thou end float on the mainshaft. I reason that its easy to pull the end cover with the gearbox on the bike so in the event of any problems I can easily investigate. The box is now fully assembled and adjusted and selects nicely so all appears well for now. I’m hoping to coax the clutch and gearbox back into the bike today. I have some other jobs to do but should be able to road test next weekend 😀

Keith

2nd June 2013 at 5:30 pm #11007

scott596
Participant

Keith, Scott gearboxes are very agricultural and yours will probably be perfectly OK for ever and a day but my point is that your sleeve gear seems to be .070″ away from its correct position and the sliding dogs will only stop in their indexed position and won`t go further into full engagement which would be the case if the .070″ was added to the bush shoulder. Alan.

3rd June 2013 at 6:35 am #11008

gammakeith
Participant

I think I now understand what you are saying. The whole thing is making my brain hurt though 😆 The selector dog is fixed in the engagement positions by the indexing mechanism. Thus, you are saying that by correcting the misalignment by shimming the sleeve gear outwards relative to the selector dog, I am reducing the depth of dog engagement in the third gear position. Rather, I should reduce the length of the layshaft and then shim both shafts at the clutch side. That now makes sense.

However, doesn’t that depend on where the misalignment came from in the first place? If its from wear (ie the recess in the gear that I have assumed is wear in my third pic) then I should be OK as I am bringing the box back to the state it was originally. On the other hand, if the misalignment is due to the layshaft being too long due (ie tolerances stacking up when it was built) then potentially I have made things worse……

I thing I’ll stick with running the bike and see how it is. I will take your concerns on board and if its not 100% ok then will pull it down again and try shimming it as you describe. Even if it appears OK then I’ll pull it over the winter and carefully check wear patterns.

Thanks Alan,

Keith

3rd June 2013 at 8:45 am #11009

BRIAN MARSHALL
Participant

It might well be worth your while and sanity to compare your built-up type layshaft with a one-piece type layshaft, in order to see if the gears are in the same positions, ie. is your layshaft top gear cog in the correct position on it’s splines ?……
I have some layshafts of both types that you could compare yours with.
Brian

3rd June 2013 at 9:39 am #11010

gammakeith
Participant

Thanks. I was fully intending on imposing on your hospitality to see if you could ‘see’ anything that looked odd. However, once I finally got the boss sorted I found myself keen to get the bike back on the road to see if I have fixed most/all off the faults. As you said before, the gearbox is actually remarkably simple to assemble especially given the availability of the articles you wrote for Technicalities Its certainly engaging a good way on the third gear dogs so, as Alan says, I should be fine for at least a good while. After all, the box didn’t explode before with nearly 10x the recommended end float and with 2 cream crackered bearings. I’ll revisit it all again in the winter as I still have the RG500 to prep for Mallory next month and my 996 Ducati engine to check/shim 😯

I assume there will be Notts meet on Sunday? If so I’ll try and get the Scott there. Assuming its not the death trap it was last time, if you fancy taking it for a spin I would value the feedback of an experienced Scott rider on how it all functions. 😀

Keith

3rd June 2013 at 11:29 am #11011

BRIAN MARSHALL
Participant

I’ll take my helmet and a leather jacket on Sunday then Keith ! Thanks for the offer of a test ride. Afterwards you can follow me back home, and we can have a rummage through my bits and pieces in the ‘playroom’ if you like….
Brian

3rd June 2013 at 1:07 pm #11012

gammakeith
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Sounds good! I’d love the opportunity to see that TT bike as well 😀

Keith
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