In reassembly of my gearbox I've run into a "situation" with the mainshaft.  I've got new bearings with correct dimensions (Walker supplied) but I'm using the same mainshaft, sprocket drive gear, seal retainer, chain sprocket, and nut for the chain sprocket.  If I install the smallest thrust washer (.060) on the shaft on the sprocket driver end then the shaft doesn't protrude far enough out beyond the chain sprocket and nut to expose the shoulder on the shaft for the cupped spring washer to ride on.

If I reduce the dimension of the spacer collar behind the chain sprocket then that will make it work, OR if don't put in a thrust washer next to the sprocket driver gear then that will give me the clearance I need.

The bearing is fully seated, the spacer collar is .57 and I believe that's correct.

I don't know what changed, and I did not pay attention when I took it apart last year!  Any ideas?  Thanks!

The diagram is showing the older gearbox with the small clutch and those don't have a thrust washer at the sprocket side, the newer gearboxes with the large clutch and clutch side bearing mounted from the inside, has different thickness thrust washers at both sides, the thicker one at the clutch end.

There's something strange going on with your gearbox as the drive gear threads doesn't reach all the way to the nut end. The threads should reach the full thickness of the nut. Perhaps the drive gear doesn't seat against the bearing completely or have you put a shim between the bearing and the drive gear? Is the distance ring made too tall? 

Wait a minute with making the collar shorter, check the gear dimensions first, as you have an additional problem with fitting the thrust washers.

Most important is that the nut has full thread cover. Don't overtighten as the threads are fine thread, thus clamps hard with less tightening force compared with coarse thread.

Be absolute certain that the sprocket is correctly hardened, a soft sprocket might destroy the drive gear splines irreparably as the sprocket will start to distort at the splines and loosen up the nut.

Even a hardened sprocket can loosen up the nut and then the sprocket wanders out on the thread. When that happens the sprocket will spin and destroy the edges of the splines and also destroy the fine thread on the gear. 

Absolute make a security screw that secures the nut from falling off the threads. It might be difficult, almost impossible to cut threads in a hardened sprocket but one remedy is to drill through and spot weld at the backside, a threaded pin where a safety lip can be bolted to. 

The way I did it with a folding washer was not sufficient even if the screwhead prevented the nut from unravel totally, I was lucky there. The sprocket wasn't hardened.

The ruined gear is from another sample where a hardened sprocket destroyed the gear irreparably. There is tremendous forces at the sprocket splines with the jolts, tug and vibrations from the chain. In addition to the security lip, use hard locktite as extra safety.