What is the "12 bolt" on a new rear end? What part of the rear end is being changed from 10 bolts to 12?

 

its the size of the housing. our 10 bolts have a 7.5" ring gear where a 12 bolt which is much larger has an 8.75"?? ring gear. the larger the parts, the longer they will last and hold up to abuse. when guys talk about a 9", it has a 9" gear for comparison.. its also about the most heavy duty rear you can get for our cars.

 

Number of bolts that secure the ring gear, not the number of bolts on the access cover. Generally, the greater the number bolts, the larger the diameter of the ring gear.
Doug