Hi all...I have been asked to restore an M20 gearbox for a friend of a friend. The dogs on each of the gears and the dog clutches show significant wear & tear. It would cost heaps to replace the gears, or maybe we might be able to find some second hand replacements in better shape. I thought there might be another option - Can the dogs be grounded or filed to shape them up?

Where abouts are you?
You should be able to pick up good use gears and sliding dogs quite easily and cheaply.

I'm in Australia. I got the guy to get another gearbox. Between the two we should get a good box.

Russells still have a healthy stock of NOS, I bought some new ones last year, they are not expensive.

And if you are in a hurry Mike Riely will have most in stock as the same box was fitted to M24 Gold Stars
Bill Green motorcycles is a M20 specialist and has most tings in stock although some of his bits from India could be better made , they are good enough for those 13 throbbing horses.
Modak also have a good selection of innards, no cases.
Bundy Biker also does a lot of M20 stuff from time to time

email (option): bsansw1@tpg.com.au

Is it true that BSA changed the shape of the dogs somewhere late forties ?

And what exactly is the effect if your dogs are worn ? (mine are, and sometimes i'm in a false nutral - could that be it ?)

The tooth form was altered on the gearbox dogs to aid engagement..Wartime examples are flat on the top face of the dog whereas the later type have a slight angle machined onto them that slopes in both directions from 'centreline' of the top face of the dogs..

The most common wear point on both types is the corners which can become excessively rounded...

Wear in the slot where the selector fork runs can also be an issue if the box has been run incorrectly assembled or set...Burring of the edge of the groove, a rough surface finish in the groove and heat discoloration are all good indicators...Plus the same on the forks..The fork should be a reasonably close fit in the groove...

There are various reasons you may not select a gear correctly resulting in a 'false neutral'...Wear in the various parts, gearbox shimming, gearbox setting or something as simple as not allowing the box enough time to change. These boxes have large gears, dogs etc. and due to inertia require a slow change to allow the various components to slow down and 'do their thing'...

Gear changes that are too swift more or less guarantee false neutrals...Ian

email (option): ian@wright52.plus.com

P.S...I forgot to mention, the early and late type dogs are interchangeable..
Ian

email (option): ian@wright52.plus.com

And you ommitted the biggest sin.
Filling the box with grease.
Followed by the second biggest sin
Running the box with no lube.

False neutrals are usually due to excessive wear n the forks or a badly adjusted detent.

email (option): bsansw1@tpg.com.au