I read somewhere that many Chev/GMC trucks had 8.5" rear axles. Does anone know if this is true and if it is can the parts be used in a car. If this is true it would be a great source of lower ratio and posi diffs as ther are about a bizzillion of them out there. o No:
GM started using the 8.5 in their lighter duty 1/2 ton pickups in 1978. Heavy Halfs & Big10's received the 12 bolt pickup rear up through '83. At least until recently, all oem 8.5 truck posi rears used the POS Eaton Gov-locks. OK in a stock tired pickup to help get going in light mud (what the unit was somewhat designed for), but far from optimum in HP street or steet/strip apllications or if 4WD, when mud bogging with tall tires. Over the last two decades, have run across countless 8.5 & 12 bolt truck Gov-locks that have blown up, always taking out the ring & pinion, & often destroying the lh bearing cap & mating area. Many times to replace I've had to source another housing to build off of Try finding the 4 wheel drive 1/2 ton housings, they're tough find, usually only found in late model pickup salvages, & then they want to sell you a very expensive r/e, which is basically a core to build off of. Needless to say, I won't build any GM rear with a Gov-lock. In the last few years, GMC started putting regular Eaton posi units in all their 1/2ton offerings. If I'm not mistaken, they are now standard equipment. Unfortunately, these 30 spline latemodel Eaton posi carriers will not show up in self serve yards anytime soon. One can buy them new though, with a good deal under $350. Used GM pickup 8.5 ring & pinions... This is a buyer beware situation. Up through the late 90's, I pulled & set up a ton of used 3.42 & 3.73 8.5 ring & pinions. Many were handpicked & pulled from 1/2 ton pickups & vans. As the majority of these trucks had a lot of miles on them, & most were seldom serviced, it became more & more of a crap shoot, even with gears which had a tight pinion & reasonable pattern. Building & shipping rears across the country, due to warranty concerns & time constraints, I got away from installing USED 8.5 gears. Only exception is low mile GM 3.23's. Aftermarket 3.23's are only avail new from Richmond, & I will not install & warranty Richmonds due to noise. Getting away from installing used 8.5 gears may have cut my profit but was well worth it in the long run. No do-overs, very high CSI, & has allowed me to focus on higher performance builds & high end restoration work. :3gears: Roger
Roger - Did they start using the Eaton units in the early 90's? Or was it much later than that? Thanks :TU:
Scott, Hello. It's been in the last 5 model years. In 2000, I built up & a sold numbers & code matching 12 bolt to a lead engineer in the GMC group for install in his '72 GTO. He was SOLD on the conventional Eaton posi units, & as a coincidence, within a year, GMC was advertising the 30 spline 8.5 posi carriers as std in GMC's light trucks. If you have a friend in latemodel salvage/rebuilder biz, a burnup latemodel 1/2 ton GMC might yield a cheap 30 spline reg Eaton posi. Usually heat warped rears will go to the scrapper, as they cant be sold for regular $$$$. Just a lot of fun to beat the c-clip axles out of them, even after getting the c-clips to drop. :3gears: Roger
