My SM420 was hard to shift, gears would grind. I pulled the trans, disassembled and found the PO/Hack mechanic had installed the wrong scynro in the third to fourth position. First picture is what I found. Second picture is of the correct Made in the USA part. She will now shift like new.

1953 Chevy 3100
261 and SM420
53Chevy 3100
1953 Mack Firetruck
1972 Porsche 911
1986 Honda Goldwing