Hello Mums49,
May not be helpful but I had a problem with our SM420 hanging up between 3rd and 4th gear. I was lucky enough to have a retired transmission mechanic living near us. He had a great amount of years of experience with these transmissions. He went through my transmission and rebuilt it completely. When I went to pick up our transmission he had all the replaced parts on his workbench. He had me look them over to see if I could find the problem. Everything looked fine to me. He picked up the narrow 3rd to 4th brass synchronizer gear and said,"see that even groove around the middle of the gear - it's not suppose to be there". It was occasionally catching on that groove and hanging up. That was 8 years ago, with no problems since. Just thought may be of help or to others.
Chuck52
