ill probably get booted off here,but what the he//.paid $1,200.00 for a new diff from dr. diff.nodular with 410 gears,had to send it back,backlash was way off.was told he was having hard time getting the 410 to set up correct???so i told him to go with the 391 gear.fast forward,i didnt check it and just put it in.it clunked bad.pulled it out and took it to a local guy.pinion was way too deep,you could hardly turn the yoke by hand,way too tight.backlash was incorrect.was told they were us gears,there was no name on the ring gear???dont know whats up with that.anyway he set it up,backlash is now correct and you can turn it easy.the pinion nut was all boogered up like the incorrect socked was used,we replaced with a new nut.i know a lot of people swear by cass,but i was dissapointed to say the least.cass was very friendly and helpfull,but i would have one set up local if i had to do it over.any way thats all i know,the car drives fine now with no clunk.i quess im venting over, $1,200.00 for a rear that had to be sent back once then sent out to have it re-set.