The last runs were assembled without care, and you would probably be ahead by completely disassembling and starting over. A cheaper way is to get older transmissions- they are almost bulletproof. You can wear them out, but it takes a very, very long time, running 8 hours a day.
It has been over a decade since I pulled one apart, but I think I had to make a special tool. The wheels had to come off too, I think, and re-assembly required a clamp, hammering, and solder.
Early assemblies had heavy duty knurls, and were unlikely to ever come loose.
