Dommie 99 gearbox problem

Most of the internals can be accessed without the box removal. Its possible that someone has replaced the layshaft driveside ball  bearing with a roller bearing  and shimmed the  KS end  (with Commando Isos shims ) and not left clearance . Easy to do  (done that!) . The shims need  the ID increased to fit properly. Will not be shown in parts book. Try just taking one out , A bit of end float is normal.

There are also a few other possibilities , I expect others will chime in soon.

Hi Terry. I rebuilt my Dommie 99’s gearbox and found that the drive  sprocket was ‘difficult’ to turn. After a lot of measuring and fiddling with seals etc, I found that with the (new) seal in place but WITHOUT the metal spacer ring fitted into it, the gearbox sprocket  turned very easily. When I put the metal spacer ring back into the seal, the sprocket was once again a lot more difficult to turn. So it seems that the problem (for my gearbox) was very tight interference clearance between the lip of the seal and the edge of the spacer ring. I removed two thou off the outside of the spacer ring and it helped make things easier to turn. I’m sure it would be even better if I removed more metal but I don’t want to risk making an easy path for an oil leak past the spacer ring.

Regards

Tony

Before attacking the silentbloc swing arm bearings, I suggest you search this site for comments, warnings etc. When you say "they don't move", what exactly were you looking for? The rubber seems to have nearly indefinite life in that application. They only "move" by twisting the rubber between the outer sleeve, fixed in the swing arm, and the inner sleeve which should be fixed to the frame by tightening the spindle bolt.  Lots of blood, sweat and tears have been shed by owners trying and eventually succeeding on changing them, for no very good reason.

Has anyone here any reports of improvement in anything after replacing them?

Hi Terry,

I, like you, thought that the silentblocs were, from an engineering standpoint a poor choice for a swinging arm pivot and mine were over 30 years old when I was resurrecting my bike, so I designed and made some bronze bushes, sleeves with seals and added an oil supply to them.

It works very well, but having  completed it, I found it made absolutely no difference to the way the rear suspension behaves and is simply another maintenance task to keep up with. Really wish I hadn't bothered, and if/when they wear out I would go back to silentblocs in a heartbeat.

When you do the sums, with standard shocks there is less than 4° radial movement up and down for the swinging arm from fully compressed to full extended - well inside the elastic limits for the silentblocs. As for any sideways 'flex', with the swinging arm width/length on a featherbed that too is minimal - certainly a lot less than the tyre flex you get with modern tyres.

Lesson Learnt for me? Unlike lots of things with a Dommi, this is one of those 'If it isn't broke, don't try to fix it!' items. Proper bushes or needle rollers make sense on a modern bike with maybe 6" travel suspension and long swinging arms but on our old clunkers? Still, you are on the way there now so good luck. 

Hope that you get the gearbox sorted.

Regards,

George