8.5 GM Axle strength

[Schurkey]

The 8.5" axle was originally intended for compact and intermediate cars. The 8.5 genuinely was an upgrade from the various 8.2" axles it replaced. The 8.5" axles were used on Camaro, Nova, Chevelle and some of their corporate cousins, but not on "full size" cars, and even so, the big-engined intermediate/compact cars got the 12-bolt until engine power dropped due to emissions control. The 8.5 axle assembly got an upgrade in '88--'89, the axle shafts were increased in diameter from 28- to 30-tooth splines, with matching side gears and axle bearings. I don't know what if any difference there is between the 8.5" axle and the 8.6" axle.

Far as I'm concerned, GM had no business putting an 8.5 under a full-size truck. As an additional insult, the 8.5 axle on the C/K trucks usually had the horrible leading/trailing shoe drum brake system. Friend of mine with an '89 K1500 shattered his 8.5" not long after I replaced mine with a 9.5". I traded my worn-out but usable 8.5" axle for a dorm fridge he dug out of a dumpster at the apartment complex he managed. When I say "shattered", I mean pieces blew out of the axle cover, and the truck skidded to a stop.

Upgrading to a 9.5" axle not only dramatically increases the strength of everything, you also get larger-diameter Duo-Servo brakes on drum-brake axle assemblies. Later trucks got rear discs, but I don't consider that much of an upgrade compared to big, Duo-Servo drums. Once you pull the 9.5 cover off, you'd think the 9.5 would be double the strength of the 8.5.

For the record, "horsepower" doesn't stress axles; "torque" does; especially when combined with shock-loading (manual transmission-dump the clutch, or harsh "shift-kit" automatic trans gear changes.)

 

[stutaeng]

I think I posted the torque ratings of the common GM axles in a thread on this subsection... Can't recall at the moment what it was called?

Edit: Here you go, post #22: https://www.gmt400.com/threads/suburban-rear-axle-into-c1500.52422/page-3

 

[Anubis]

Upgrading to a 9.5" axle not only dramatically increases the strength of everything, you also get larger-diameter Duo-Servo brakes on drum-brake axle assemblies. Later trucks got rear discs, but I don't consider that much of an upgrade compared to big, Duo-Servo drums. Once you pull the 9.5 cover off, you'd think the 9.5 would be double the strength of the 8.5.

thanks for your post. The big drums on the 9.5” rears concern me. I don’t necessarily need big brakes on the rear end especially if I can’t fit my factory rims. I don’t use the truck for towing so brakes are the least of my concern. I do not want aftermarket rims on my truck. I’m not into that donk look either. I would imagine I could swap the 8.5” brakes onto a 9.5” 6 lug rear if needed.

Please forgive the HP reference. I am an engineer by trade and fully understand the difference between torque and horsepower.

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[454cid]

thanks for your post. The big drums on the 9.5” rears concern me. I don’t necessarily need big brakes on the rear end especially if I can’t fit my factory rims. I don’t use the truck for towing so brakes are the least of my concern. I do not want aftermarket rims on my truck. I’m not into that donk look either. I would imagine I could swap the 8.5” brakes onto a 9.5” 6 lug rear if needed.

Please forgive the HP reference. I am an engineer by trade and fully understand the difference between torque and horsepower.

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I'm guessing your rims will fit fine over the larger brakes fine. My brakes dwarf the brakes in question here and I have stock 16" rims. The brakes on the 9.5 are big, as in bigger than the 8.5 brakes, but they aren't big as in huge, exotic, racing brakes.

 

[GMCTruck]

Your stock wheels will fit fine with the 9.5" semi float axle. In fact, many K1500 trucks came with the bigger axle as part of the RPO F44 option.

 

[Supercharged111]

thanks for your post. The big drums on the 9.5” rears concern me. I don’t necessarily need big brakes on the rear end especially if I can’t fit my factory rims. I don’t use the truck for towing so brakes are the least of my concern. I do not want aftermarket rims on my truck. I’m not into that donk look either. I would imagine I could swap the 8.5” brakes onto a 9.5” 6 lug rear if needed.

Please forgive the HP reference. I am an engineer by trade and fully understand the difference between torque and horsepower.

You must be registered for see images attach

9.5 brakes are far from overkill. 8.5 brakes flat suck.

 

[Anubis]

9.5 brakes are far from overkill. 8.5 brakes flat suck.

There’s nothing wrong with drum brakes in the rear. I’ll admit disc brakes are better in preventing brake fade but in my opinion there’s nothing wrong with the 8.5 set up for the rear end. Drums probably have more apply area than discs do. The front disk brakes however need to be addressed.

 

[Supercharged111]

There’s nothing wrong with drum brakes in the rear. I’ll admit disc brakes are better in preventing brake fade but in my opinion there’s nothing wrong with the 8.5 set up for the rear end. Drums probably have more apply area than discs do. The front disk brakes however need to be addressed.

You need to go back and read Schurkey's post. The design is inferior. To say the 8.5 brakes suck is an understatement. The 9.5 drum brakes are good. They're a noticeable improvement and they can actually hold an adjustment.

 

[Anubis]

Is anyone aware of a rear disc conversion that would allow me to use the stock 16” rims?

 

[454cid]

Is anyone aware of a rear disc conversion that would allow me to use the stock 16” rims?

There’s nothing wrong with drum brakes in the rear....

Wait, What???