Torque is what turns them into maraca's. Small or large tire?

 

I ran a 8.5 (a 2002 - 8.625") 10 bolt housing with 8.5" aftermarket gears, an aftermarket clutch-type Eaton Posi, and 30 spline axles in my 500 hp S10 with Hoosier slicks and Caltracs. My housing/axles are identical to the ZR2 housing, but slightly narrower. Been fine for several years. Then busted the spider/side gears out of the posi center first time I used a newly installed trans brake. Now have a spool with c-clip eliminators, a forged pinion yoke, a 3" racing driveshaft with 1350 joints and a forged slip yoke, an aluminum cover with bearing supports, and all new ring/pinion/bearings. NO SPIDER GEARS!! I still have stock axles, but I am knocking on the 10-second door where I will need aftermarket axles per regs if I go any faster. So far all is living fine with the new trans brake.

You ask about axles..... but I think you meant axle as an entirety. If you meant the factory 30-spline axles themselves, they are actually pretty strong - significantly stronger than the 28 spline ones found in earlier years due to the larger diameter. Also did you know that the 8.625 rear housings have the same size (big) carrier bearings as a 12 bolt. Aftermarket center sections are made to fit inside these bearings, but factory 8.5 centers cannot be used as they require a smaller ID bearing. An adapter bearing (large ID) is made to put aftermarket carriers in old housings, but not vice versa. Adapter bearings are not needed if you start with a 8.625 housing - you just use 12 bolt carrier bearings. 8.5 pinion yokes fit in the 8.625, but you have to also use the 8.5" seal. If you set up a 8.625 housing, the carrier bearing shims that come with the bearing kit may not be correct diameter (mine weren't). Also if you use C-clip eliminators from strange with factory housing ends you need to have drum brakes installed. The backing plate register on the disk brake housings is ever so slightly larger than the old drum brake housings, so the center hole in the 1964-1972 backing plates need massaged in order to use them on the disk brake housing with a Strange Engineering c-clip eliminator kit and get them to seal.

Bottom line....... if you will be trying to use a factory 8.5" Gov-lock center with a serious hotrod where a tire might spin, throwing out it's cog to lock up can be death to it. An aftermarket 8.5" Eaton Posi center will hold your big block in a light vehicle on street tires, but gets pretty iffy if your car is heavy or if you have serious traction - spider gears are definitely a weak link, as I found out. I think mine with the spool and all the make stronger parts would now hold your 600 HP with aftermarket axles and slicks, but why do it. Just build a 9" Ford for it and be done. I would have, if I had known my final intent to only race the vehicle. But I had too much money in the existing rear/springs/shocks/Caltracs/driveshaft when it broke, to start from scratch again.