Gary K, I just finished rebuilding my SM420, using a rebuild kit from Novak and other parts I aquired from some folks on this site. The problem could very well be with the shift tower as 3B stated. There are three shift shafts in the top cover assembly that move the shift forks when you move the shifter. On my SM420 these shift shafts were rusty and did not move very smoothly because of this. The rebuild kits do not contain any of the parts for the shift tower assembly, therefore there's a good chance that the rebuilder did nothing in this area during their rebuild process. I totally tore down my shift tower assembly and ended up replacing one of my shifter shafts and cleaned up the remaining two with a wire wheel. These rods slide within the top cover though very close tolerance holes and a rusted shaft will make the shifting difficult. To dissasemble the shifter shafts you will need to drive out the expansion plugs in the cover and the taper pins holding the shifter forks on the shift shafts. When removing the shift shafts turn them 180 degress and then carefully remove them from the cover, being careful to catch the spring and ball that are used to locate the shift shafts. I also removed the spring loaded reverse shift pin to clean it up as well (mine was quite rusty). I was able to locate the taper pins at my local hardware store and the expansion plugs were found at a local engine rebuilding shop. The NOS shift shaft I obtained was from someone on these boards. Good luck.
Jim
