Sorry to say, but that rear hub is worn. It is worn either because a poor fitting sprocket has been mounted or the nut has unravelled. It takes only one occation of a loose nut to nearly ruin the hub! And the danger when the sprocket can move if only a very minute amount to and fro, is that the nut WILL unravel after a while and the sprocket will wear the hub more or worse, throw the chain at speed!

Repairing the hub is difficult, best repair is to cut off the old snout and weld on a new one. New snout with splines has been marketed somewhere. It it is really important to make both the rear and front sprocket to sit firmly on the splines and that goes for the rear brake drum as well!

One remedy is to fabricate a ring that utilise the remaining long splines at the backside, a ring (better be hardened) that is snug on the splines but slides over the painted portion of the hub, and weld that ring to the sprocket. 

Really the sprockets should sit snug on the splines hard enough that it is needed to lightly tap them on with a rawhide hammer!  There is no cushions anywhere from the crankshaft to the tire like in modern bikes, so the forces are tremendous when the chain slaps going through potholes, lugging the engine or retarding!

Another thing noticed is the small sprocket is worn more on one side than the other, the chain line should be inspected, one problem can be that the distance slugs at the end of the hub are put on the wrong side. In the 101 R&O manual, George made an instruction on how the chain line can be corrected. The frame rear should be measured up, the rear fork should be in center of the frame but with unknown history can be bent slightly over to one side as well...

Ensure that the small sprocket is hardened! The example below shows a soft sprocket with ruined splines after just a few miles of running. Even if the nut did not loosen up firsthand, the forces deformed the splines. Mount a soft sprocket is absolutely useless and can be dangerous!

A unravelled sprocket nut will spin the sprocket on the fine threads on the final gear and that is catastrophical. So prevent that nut from spin off completely is very important. A nut locking device can't be enough emphasized!

In the example picture, the soft washer bent up against the flat was compromised by the forces of the sprocket rocking motion. However the screw head, placed like that did positively prevent the nut from coming off. A hex washer as on the crankshaft nuts, might have broken off and that would not have prevented the nut from undoing.

Still, I always use Locktite on all threads all over the bike. I'd use red, hard on those large nuts. Locktite is always possible to loosen with a bit of heat. A professional hair dryer is often hot enough and does not discolour steel or aluminium.

Make or buy proper good fitting tools for the special nuts. Making them takes a while but it is very rewarding.

Need a realy good welder to take the remedial steps that Carl-Erik suggests, hubs are I believe a cast malleable similar to frame lugs. Brazing and remachining the slots is possible, or indeed as Harry suggests, carefull filing, but neither are easy. It is difficult not to damage the threads, which are very fine.

I think George used a silicon bronze here, which is slighly harder than a normal low fuming bronze, but I recall even then that wear was noticeable again after a couple of thousand miles. As a result, I have never tried it, as for the time and effort required, and the necessity of delacing the hub for major repairs, it doesn't seem cost effective to reuse the old hub, when Randy sells replacements of hi quality now. Thats me talking from a commercial perspective tho, and a decent home workshop repair is on my bucket list of interesting fixes to try out on my own personal bike.

Would be great to hear how anyone else has managed it.

Let us know !

Tim

Thanks for all the help.  I have a two part plan.  I figure the hub is toast so I have nothing to lose in trying to repair it.  I'm going to try filling in the missing metal with the welder and file it down to shape.  We have a four day trip down to death valley coming up in March.  If the welding is beyond my abilities, or I see any wear at all on the trip to death valley I'll order up a new hub, bearings and races. 

I've got lots of time before sept to make a good decision!

Thanks everyone I'll let you guys know how it comes out.  

Dana

I spent the weekend working on it.  It took all weekend and I'm not sure it was worth saving $350 but here's what I did.

First I milled a small piece of steel as my measuring block. The exact size of the slot should be. 

Then I layed down a bead of MIG over the missing steel.

Then I used a dremel tool to do a rough cut to remove most of the extra material.

Then I used a large hand file to do the big stuff and then a small file to get the measuring block to fit.  I tried to keep it a hard tight fit that needed to be tapped in.

My first try was milling a cooper block to slide into the slot when I was welding but the first weld came out crappy and I had to add another bead to fill it in right. So that was a bust

Once I got really close I'd use the sprocket almost as a chisel because it was much harder then the weld.  

I had to use a small hand file on some of the threads followed by a thread file to clean up the threads near the bead.  The nut goes on smoothly all the way and the spocket fits tight and doesn't move.

Like I said I have nothing to lose trying it.  I'll log some measurements and do some long trips and see if anything changes, and then re-evalutate the situation.

I've attached pics of the result.

Hope this works,

Dana

They are a snug fit, any clearance in excess of this, is wear and tear which will deteriorate/disintegrate rapidly.  Without possessing any accredited engineering prowess, I most certainly suggest a replacement hub, followed by removing the wheel, tyre then spokes and relacing and truing the wheel, where ultimately, a securely and correctly aligned sprocket is attained.