54 Belair - What you have is GM 3688585 , Main Shaft with 22 teeth (11/22/10) for SM420, 1948-1949 before 6-1-49 when they made an improvement and switched to an 11 tooth design ( 11/11/10) and also changed the 1st and reverse gear on the mainshaft. Part number 3688585 superceded to GM 3692608 Main Shaft and that is the correct part number you will need for 1949- 1967 (unless you have a 65-67 1 1/2 or 2 ton truck trans) You can also use Borg-Warner part number WT250-2A. Hope that helps, pm me if you have questions or need help finding the part you need. I may be able to help.
Last edited by Dusty53; 10/20/2015 8:06 PM.
