The process of shimming requires installing a main drive gear with a dial indicator attached that indicates in the rear bearing bore.
Then shims of varying thickness are installed between the trans and bellhousing to change the angle of the trans so the front and rear shafts are inline.
Over time the bellhousings warp putting the trans out of alignment making the Main drive gear and main shaft run at a slight angle to each other. This angle will walk the synchro drum off the main drive gear.
