HD Mainshaft Bearing Thrust Plate, Type 1 & 3 1961 to 1977 eliminates movement of the main shaft bearing. 

 

This aids in maintaining the alignment of the gears on the main and pinion shifts to give the gears FULL CONTACT, which reduces the chance of failure. 

It also allows the use of 2 bolt nose cones on a 3 bolt (Late Model) style transmission.

There is a lot of discussion (ARGUMENT) on how to install this. We feel that you should simply remove .040" off the gasket surface of the nose cone. A lot of guys don't do this and it creates 2 problems. First, it moves your engine back 1mm. Secondly, now the backup light switch is 1mm farther from the reverse shift rod, so it MAY not connect every time you are in reverse. So skimming the nose cone is a wise move. It only takes 20 minutes or so, with some sand paper and a sheet of glass to keep it flat.

This product is absolutely necessary with the 9 tooth mainshafts, but not always needed if you have the 10 tooth mainshafts, because the 10 tooth mainshaft has a wide bearing fast that rarely mounts into the nose cone, but the 9 tooth mainshaft bearing is very narrow and pounds in almost every time.