Excessive Gearbox Layshaft Endfloat?

I had the same problem when I installed  the roller bearing on my 1960 box. I read about using modified iso shims  at the cover end of the box. The logic of this was lost on me. If taking out a bearing at the drive end and replacing it with something that left excess clearance ,then surely the shim needs to be between the  new roller bearing shoulder and the  4th gear. This would not move all the gears on that shaft to a new position.  I worked out what the clearance was and what shim was needed to give a minimal clearance and fitted it. Unfortunately I under estimated the clearance needed as things do close up and according to the box expert too much clearance is not a real  problem.  The bike was going to the IOM and I did not have time so I fitted what I had at the cover end and it has worked ok .When I fit a new KS bush I will get the shim sized for the roller end.  I will go for 10 thou  end float. These old bikes do keep the brain cells exercised, and I need that.

10 thou, do the maths. If you shim then 20 thou would be the lowest to go. You are shimming out across more than one tolerance part to another, that is why 20 thou would be the lowest. 

The Atlas appears to have no end float  and has a very good change ,but I can't remember what bearing it has. It may still have the ballrace  which tends to hold  things still. It also has the later KS spring detail which is much more positive and could mask any tightness I suppose. I would go for an uprated ball race if I get in there and not a roller.

Hi Ashley,  I suspect we are talking at cross purposes , I am referring  to a  min 10 thou end float on the kickstart lever after all assembled and tightened  up.  Certainly using a dial gauge and mike and calculations was unreliable   and to get a final 10 thou would probably mean shooting for 20.  To me the idea of ISO shims is just because its easier than pulling the gears and shaft out. But not correct.

If you can find pen steel washers of the correct diameter, there can't be any harm in using more than one?

The accepted way is to buy Commando  ISO shims  of a few thicknesses  and find one that works.  From memory you have to  file/grind out the bore a little and they are a little fiddly to fit  ,think you have to remove pawl . (senior memory moment!).   I had to swap them over as the clearance closed up  from what I thought it would be.  Front and rear iso shims  have different bore sizes check RGM site .

No crossed purposes, I have the drawing for MK3 gearbox assembly and it clearly says 0.020'' minimum. What you are physically measuring is either the end float in the kickstart shaft in relation to the 1st gear or the layshaft to kickstart bush. Shimming out you do two things, reduce the clearance to the 1st gear and reduce the clearance to the shoulder on the layshaft that the bush in the kickstart shaft faces. Too fully understand you would need either use shim gauge like pastigauge, sectioned gearbox or do as Andrew needs to do below, if you don't then 0.020'' would be a safe place. 

Andrew, you have correctly measured the layshaft endfloat to kickstart shaft, but not kickstart shaft to 1st gear layshaft - fit it and repeat, if you shim from you existing measurements you could jam you kickstart shaft against the 1st gear, as the kickstart shaft is shimmed towards the 1st gear. 

Hope this helps

Just looking up what other people do, so I'll add my thoughts related to using a roller bearing on the layshaft drive end.

The endfloat measured at the kickstart (w/o spring) is the amount the layshaft can move axially, limited by the drive-side bearing on one end and the interface between 1st gear and the bush inside the kickstart shaft on the other. SKF specify a maximum axial displacement for the NJ203ECP roller bearing of 1mm,
see https://www.skf.com/group/products/rolling-bearings/roller-bearings/cylindrical-roller-bearings/single-row-cylindrical-roller-bearings/productid-NJ%20203%20ECP

1mm = 0.040", so somewhere around 20 thou (0.5mm) seems a good target to shoot for. Maybe 10 thou (0.25mm) if replacing kickstart with ball bearing. Remember to allow for the thickness of a gasket if checking "dry" (or better still, don't use one). And fully tighten the inner cover nuts. While checking clearances is a good idea anyway, they definitely need to be checked everytime the gearbox is reassembled if shimmed to closer tolerances.

NJ203ECP is roller bearing equivalent of the original 6203 with axial location in one direction only, It has double the dynamic load rating of a 6203. Install so that it is free to withdraw the shaft from the shell complete with inner race.

When I replaced the layshaft support bush in the kickstart spindle with a needle roller bearing it occurred to me that, as a result, I was lacking the effect of the flange on the bush that I had discarded. So I cut down the redundant bush such that all that remained was the flange plus about .015" of the smaller diameter to work as a locator in the hole now occupied by a needle roller. I thought this went together rather well and was consistent with the original designers expectations in terms of relationship between the kickstart spindle and the layshaft 1st gear pinion.

I then fitted a .025" spacer between the kickstart spindle and the gearbox inner cover to  bring the total kickstart endfloat down to between .005" and .010". This has also worked well over 20 k miles. 

The mystery to me is where this .005 - .010 tolerance came from? There is no mention of it in either my pukka "Norton Triumph" ringbound workshop manual or the Haynes version? I obviously know of it and have used it before, but can't remember who told me? There are a couple of videos of gearbox rebuilds currently on Youtube, one of which refers to this tolerance (between .005 and .010)  and the other doesn't. Having an actual Mk 3 gearbox drawing which shows .020 clearance is the nearest thing to a definitive statement I have heard and I intend to go with that unless anybody on here knows better?

I suspect that 5 to 10 thou is just one of those rules of thumb that gearbox designers use - enough but not too much. BMW airhead boxes, for example, use 6 to 12 thou for clearances of free-running gears.

The axial clearance we're talking about here is 1st gear, between layshaft shoulder and kickstart shaft inner bush. With a ball bearing, the layshaft is located in the box so packing out the kickstart shaft to give 10 thou running clearance on 1st gear is fine. At least until bushes start to wear. So I suggest that the 20 thou figure you have adds a little leeway.

If the kickstart bush is modified then you first need to address the clearance between 1st gear and the kickstart - enough to allow the gear to spin freely without contact while giving maximum engagement of the kickstart pawl.

I think you will have more issues with tight clearances than loose. MH said not to bother with shimming ,and he was not a fan of the roller bearing .