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Laydown gearbox, 16h

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Laydown gearbox, 16h

Posted by kelvin_potter at April 20. 2010

Hi i have recently removed the engine from my 16H, its having a bit of a 8000 mile birthday. Seems it had broke the piston rings, there was also 15 thou crank end float but thats another story. While stripping the bike i had a spin on the clutch and have found that my gearbox also has a few problems, the complete clutch would move in and out in the bearbox, theres end float and then theres end float, i have stripped the gearbox and found the main bearing (the big one) has been spinning in the gearbox case, it has hairline cracks around the bearing housing, i have another housing luckily, i am trying to find out where this bearing fits in the case should there  be a metal shim put in the gearbox first, then the bearing pressed in, then another bigger shim, then the shaft that then fits in (the one the sprocket fits on) should this be a press fit into the bearing, as mine is a little loose on the inner bearing, it does thighten to the bearing when the sprocket nut is done up, its not realy slack it can just be pushed into the bearing by hand. I was realy just wanting to make sure i dont have the same problem again, so if this shaft needs to be replaced i will do so. I dont know if there is also any way or need to set a end float on the main shaft, as it has i thrust bearing in between the shaft and no other way to adjust it.

If anyone could please give me some advice i would be very greatful.


Kelvin Potter (Isle of Man)

Re: Laydown gearbox, 16h

Posted by robert_davis1 at May 06. 2010

Hi Kelvin,

I've done a couple of these gearboxes over the years. Some have had the shims you mention in them and some havn't. Product of time I guess. The bearings I buy always have seals in them. I pull the seals out of one side of the bearing only. Leave the seal in the outside, so the oil from the box gets into the bearing but not right through and out the other side leaking all over the floor. I've never bothered with the shims. The bearings should be an interference fit into the cases. You should need to heat the cases to get the bearings out and get them back in again. The shafts should be a very neat sliding fit into the bearing. The short shaft that holds the sprocket is held fixed in place with the sprocket nut as you said. Having said that about the fits of the bearing in the cases and on the shafts, you will find any combination when you pull your box apart including cracks as you have found. Sometimes the shaft is frozen into the bearing with rust & grime and both bearing & shaft will come out of the gearbox together and you have to seperate them later. There are loctite compunds available to help hold the bearing in the case and stop it from spinning if the fit is not tight. You don't want your bearing turning in the cases. Use a roller bearing on the layshaft its an NJ 203 (I think!). There should be a thrust washer against the large gear on the end of the sprocket shaft. This is where I take up end float. It's one of those things where you need to assemble & disassemble the gearbox a few times to get the thickness of the thrust washer right. I know Russel Motors had these thrust washers, but I make them myself to suit. I'm not sure if those large shim type washers were indeed shims or just some method of restricting oil leakage to the outside before bearings had their own seals. I also leave the outside seal in the bearing on the other end of the mainshaft. Helps stop gearbox oil seeping down the clutch pushrod tunnel through the mainshaft and getting into and contaminating the clutch. I've done this on my Commando box, which is basically the same and with a pushrod seal in the clutch I very rarely have any of the typical clutch sticking problems so often associated with these gearboxes.

I have a 1950 / 51 Big 4 with laydown box. It probably has about 15 thou crank end float as well. I don't worry about it. It goes like a jet.......



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