Hi All,
I am working out what size of spacer I need for the end float on the main gearbox shaft in my C15.
According to Rupert Ratio the smallest size available is 0.093" (40-3020).
I've worked out that I need 0.047", although I could factor in a gasket (0.016) and make this up to
Oddly the existing spacer is 0.074", maybe a B washer was used instead of an A washer.
So.. What is the best way to deal with this? File down the washer? Add multiple gaskets?
Could I get a custom shim spacer made up? Or just run without one at all?
I am wondering if this might just be the original cause of some of the issues I have had.
Hi,
I have just done this and because of the excessive end float that was there the splines were slightly worn so I needed quite a lot more and ended up with two shims.
As there is nothing holding the shaft to one side or the other they also seem to as thrust washers. Also the clutch is on the one end and it will move in and out by the amount of end float available so barely feelable is what I went for.
I would think that washer/spacers that can be filed may be a bit soft. I found some proper gearbox shims in my box of bits and spent many hours reducing it on emery paper laid on the base of a pillar drill to keep it as flat a possible. I did not use a gasket.
Dave.
Thanks Dave!
I am also wondering if I need to replace the 'B' spacer but Rupert doesn't seem to do this. Does anyone know how to seperate the gears on the mainshaft in order to get at the B spacer? They are locked together but I am not seeing a circlip or anything like this.
If I found that a mainshaft thrust washer only .047 was needed, I'd be looking for the reason elsewhere! I've never found one to be needed less than the standard alternative thicknesses in the position (as you say, thinnest original option .093") and more often a thicker than standard washer is needed.
Replacing the B spacer isn't usually needed as it doesn't wear much, unless you are fitting a new gear or shaft, then the correct thickness spacer is needed.
First thing I'd check is that the mainshaft 1st gear is pushed fully home on its splines.
Thanks Rupert!
( Are you the Rupert of the book?)
I double checked. The shaft is properly seated. Around 0.140" back from the side of the gearbox.
The bronze bearing protrudes by about 0.090" Could this be the issue? This bearing came, ready-fitted to the replacement side cover which I bought. The original cover was cracked and the first replacement cracked similarly.
I slept on it this afternoon, and woke up with a thought in my head. In order to get a good measurement, does the clutch nut need to be done up on the main GB shaft? I have been taking measurements with the shaft in place but not fastened. I mean, we are only talking tiny measurements, might it make a significant difference?
Clutch doesn't need to be fitted. Best way to measure mainshaft endfloat is to fit it alone, fully fit the inner timing cover and put a dti on the end of the mainshaft.
The mainshaft should easily slide side to side, the distance it moves being measured by the dti.Thats the thickness of thrust washer it needs minus about 4 thou to ensure it doesn't bind.
Agreed, the thickness of the flange on the bush at the kickstart end of the mainshaft is the other factor affecting mainshaft endfloat. I'll measure an original one and get back to you.
Original mainshaft bush flange thickness .090"
Near as dammit the same as mine at 0.093"
BTW If I do it the alternate way and refit the side cover and measure the float with a DTI, Is this possible with the engine in the bike? DTIs are magnetic but the engine is largely aluminium.
If so I will order a DTI.
I might just have to get my wife to wobble the shaft while I hold the DTI in place.
I only ask 'cos in the video I saw, the engine was out of the bike and on its side.
Lacking a DTI I measured the end float using the butt end of my digital calipers, with the help of my dear wife. Several attempts later the float looks to be around 0.070 to 0.072". The spacer is 0.074".
As previously discussed it looks like I have a B spacer rather than the 0.090" A spacer which I should have.
I am tempted to fit a gasket (around 0.013") and maybe order the next size down B spacer (0.070") to stop it from binding. But I am bothered that this is a bodge to fix a bodge.
I haven't found any issues elsewhere.
Is it possible that someone has fitted a non-correct part from another model and messed up the float? Just a thought.
As ever I would be grateful for any ideas.
I repeated the experiment with the spacer in place. As soon as I tightened up the machine screws on the end cover the shaft locked up. SMOKING GUN
Fitting a paper gasket (rubbish quality and a bad fit!) was enough to unbind the shaft and give a float of around 0.002 to 0.003". I'll try another gasket.
BSAnever fitted a cover on the inner timing cover so I'd agree - fitting a bodge to solve a bodge.
Re the issue of the mainshaft spacer being thinner than expected, have you checked the sleeve gear is fully home in its bearing? If not the space for the mainshaft thrust washer will be reduced.
I would try to get a method of grinding the Spacing Shim to the correct thickness. One method which springs to mind, would be to put a round peg into a Vice and use it to hold the Spacer. Then lightly touch an angle grinder onto the Spacer and it should rotate around the peg, on top of the flat jaws of the Vice. The angle which the grinder touches the spacer will determine how fast it spins... Be prepared to have to find it if it flies off across the garage..!
Quote from: DAVE BRADY on 27 Jul, 2025, 10:30 Hi,
I have just done this and because of the excessive end float that was there the splines were slightly worn so I needed quite a lot more and ended up with two shims.
As there is nothing holding the shaft to one side or the other they also seem to as thrust washers. Also the clutch is on the one end and it will move in and out by the amount of end float available so barely feelable is what I went for.
I would think that washer/spacers that can be filed may be a bit soft. I found some proper gearbox shims in my box of bits and spent many hours reducing it on emery paper laid on the base of a pillar drill to keep it as flat a possible. I did not use a gasket.
Dave.
To avoid removing my fingerprints and to keep the spacer flat I used a round flat magnet from an old tank bag. This reduced the need for too much pressure from fingers. Some WD40 also helps to get it moving.
Dave.
Quote from: scifi on 02 Aug, 2025, 23:28 I would try to get a method of grinding the Spacing Shim to the correct thickness. One method which springs to mind, would be to put a round peg into a Vice and use it to hold the Spacer. Then lightly touch an angle grinder onto the Spacer and it should rotate around the peg, on top of the flat jaws of the Vice. The angle which the grinder touches the spacer will determine how fast it spins... Be prepared to have to find it if it flies off across the garage..!
When grinding Ducati valve shims I lay a sheet of wet & dry onto a sheet of plate glass (I saved it many years go from the front of a 1950's TV). Then, with a squirt of oil on the wet & dry, rub the face of the shim on the wet & dry using a figure eight motion, this helps grind the shim evenly. A 120 grade wet and dry works fine. I check the thickness of the shim after about every minute of grinding, it can take a while if you need to take of more than a couple of thou. I'm sure this method could used to grind your gear box shim safely without it flying off into space.
My plate glass is an old fridge shelf!
Same technique otherwise.
Okay, right now the innermost gear (sleeve gear?) is still in place being attached to the sprocket, so I will once more gird my loins and remove this in order to check that it is correctly fitted.
(I am ending up with a pile of parts... I do hope that they will all go back together!)
Assuming I don't find anything wrong I will try fitting the 0.070" shim washer which I ordered to replace the 0.074" washer, and see if this makes the difference.
If it doesn't give the required play I will take a look around for a piece of plate glass and start grinding.
The idea of attaching the washer to a magnet is a good one. I have a hefty rare earth magnet, but it is super important to use this with a glass plate, not a steel datum table for obvious reasons (Whack! Ouch!)
Expect an update next weekend.
BR,
Ian
As an update, I trial fitted the 0.070" spacer, but the shaft was still locked up. Hence I have ordered a deep socket to remove the 1.5" nut which holds the sprocket and the sleeve gear in. If I had thought ahead I could have ordered this a week ago! DOH!
Under the nut is a big washer with the edge folded up to lock the nut. Does anyone know if these are available? I guess I need to replace it with new.
Hi Guys, let's just see what the purpose of the thrust washer really is...
When the gearbox is in gear, as the gears are straight cut, there should be no axial thrust on the washer. The only time it takes up slack is when the clutch lever is pulled and the clutch plates are under tension. This tends to pull the mainshaft out towards the clutch assembly. The amount of clearance, say 10 thou, is subtracted from the amount of movement of the clutch friction plates.
If the clearance is more than this, you could run into gear selection problems, as each gear only moves about 3/8 inch, from one gear ratio to another.
Am I right in saying the far end of the mainshaft is located in a phosphour bronze bush..? Maybe this needs some adjustment, as well.
Yes there is a bronze bush. As per previous discussions the depth of the flange on the bronze bush looks to be correct. I could rub off a few thou, but it might be tricky what with the bush being pressed into the side cover.
Hi,
From recently doing the same job the best way is to get the shim right.
Taking the sleeve gear out will not make any difference.
It does not take much fore then shaft to be tight. Without the gasket try turning the shaft. Does it turn at all? If it can be turned then a few thou could make all the difference. Just perceptible clearance is what I aimed for.
Consider that as the clutch moves outwards the4 primary goes out of line by the same amount.
Dave.
The clutch isn't fitted right now. Just to reiterate, it had a 74 thou spacer fitted, which should have been more like 90 thou. With the new side cover fitted, and the hex bolts tightened, the main shaft was locked solid. Fitting the gasket gave enough clearance to free it up. I think the gasket was about 13 thou. Fitting a 71 thou spacer, without the gasket resulted in the shaft being locked up again.
SO, I could just fit the gasket, or I could file down the shim and the bronze bearing, but, whichever way you look at it something is plain wrong.
The side cover is a replacement for the two previous ones which were cracked, probably due to the main shaft being bound.
It's been a couple of weeks, as I have been away on holiday, but I am back to looking at the bike.
Removing the sleeve gear revealed nothing obvious. I am wondering if the ball race may be non-standard and slightly wider than the original. Hence I am reassembling.
Question: What is the best way to do up the sprocket nut to the required 100 ft lbs? I'd swear it wasn't done up any where near that tight before, as it came off easily. When I start to tighten it up I just spin the back wheel, even with chocks. I was just using a pile of planks, so I might have to make up a proper chock with wood screws.
I could screw the chock to the floor of my cabin, but I will save that as a last ditch option. I don't really want screw holes in the floor.
BR, Ian
I managed it! I chocked up the wheel and stood on the brake pedal (conveniently on the left on a brit bike!) , and did up the nut with my torque wrench. I just about managed the whole operation single handed without having to shout for help!
The saga continues:
I refitted the clutch and checked the alignment against the crank sprocket. They were both nicely aligned until I refitted the engine cover on the nearside and it pushed the clutch sprocket out of alignment by maybe 2mm.
I have ordered a new shim kit and oil seal for the primary sprocket, but I am wondering if the main gearbox shaft is just too long. Is that possible?
BR, Ian
Hi,
2mm is a huge amount. When you say nearside engine cover, do you mean the primary cover or the timing side? The primary cover does not touch the clutch and if it did it would push it inwards.
With the timing side inner fitted there should only be the end float that you have already achieved by fitting a 0.071 spacer and the gasket with a resulting end float of - . Fitting the clutch should not make any difference with the end float remaining the same.
I see from your first posts re 'wiring loom' that the bike was running so it is very unlikely that the shaft is too long.
Dave.
Actually there are some marks where the engine sprocket and chain have been catching on the casing, so I think that maybe a previous owner made a mistake in setting up the alignment. Maybe they shimmed it with the timing side cover removed.
Hi,
Can you post some pictures of the primary side to show the marks. The engine sprocket should not be able to touch casing whereas a very loose chain could. Is there a primary chain tensioner fitted?
Forgive me for asking a stupid question but just trying to visualise what you are describing. Did you fit the spacer on the end of the shaft that goes through to the clutch rather than at the other end?
Dave.
I'm trying to figure out how to attach a photo, I am sure I have done it before. So annoying!
Anyway, in the meantime, yes, it is possible that the chain was loose leading to the scrapes, but, as I have removed the tensioner I can never be sure.
I figured it out. You can't attach files to a Quick Reply. Only a slow Reply. Here is the snap.
Hi,
From what I can see the seal is knackered hence you have ordered a new one. The mark could have been caused by a very slack chain jumping off the sprocket. Once the spacer is fitted on the crank, up against the bearing, the sprocket cannot move in towards the case.
Dave.
Thanks Dave, I'll do that, Ian
I think there are several different widths of chain, with 1/16 inch between them. Check that you can't move the chain sideways on the rear sprocket.
List of sizes.... https://www.ebay.co.uk/itm/13530