If you have had a GM 10 bolt spider gear failure, or a G80 (Gov. Lock) Failure now is a great time to upgrade to this Nitro - GM 10 Bolt 8.5/8.6 Posi (Positraction) aftermarket differential. The Nitro posi features a billet hat and forged internals. It is a long lasting excellent positraction unit. One of the major weaknesses of the GM 10 bolt 8.5 inch and 8.6 inch diff is the carrier. This posi unit will eliminate that weakness. If you are regearing your truck and have a Gov lock, we highly recomend replacing it as we have seen on many instances the factory limited slip blowing apart and damaging your new components. This Nitro positraction unit fits GM 10 bolt 8.6 inch differentials in stock form, but requires updated carrier bearings for 10 bolt 8.5 inch differentials. If ordering a master install kit (GM8.5) with posi select the heavy duty (HD) kit to get the correct bearings to complete your install. NITRO POSI"S HAVE CARBON FIBER CLUTCHES!!!!