Difference in Early Year GMT400 Brake Rotor Heights

[honor first]

All, I am seeking help on what is surely a dumb question. I feel bad even making a post but I have searched on here and google, and it is hard to find info on this OEM parts question and not all the threads about upgrades, etc.

1991 Regular Cab Long Bed, WT, 2WD
I have the original window sticker and may have the build sheet, and I don't think it has any heavy duty brake options.

The parts store says there are two rotors available, the difference being height. Never even heard of such a measurement on brakes, height (and not diameter), but I'm assuming it is taking hat height.

Options are:

Brake Rotor And Hub Assembly Front; 5 Lug; With 4.185 Inch Overall Height
Material: Cast Iron
Number Of Studs: 5

Brake Rotor And Hub Assembly Front; 5 Lug; With 4.55 Inch Overall Height
Material: Cast Iron
Number Of Studs: 5

I am asking because once I get the rotors off, I can't bring them to the store with the truck disassembled and not drivable.

Any help is appreciated!

 

[someotherguy]

Edited for clarity.. sorry..

The 1988-1991 regular cab trucks (half ton) were standard with 1" thick rotors...extended cabs got HD brakes with 1.25" thick rotors. The spindle is different as well, to accommodate the rotor.

Your regular cab 1991 almost surely has the thinner rotors, but it could have been ordered with HD brakes. Not many were. For 1992-up, GM decided to give all of them HD brakes regardless of cab style.

If your SPID label (option code label in the glovebox) is still legible, search for the JBx codes on it. JB5 = heavy duty brakes (for a half ton.) Again though, most likely you do not have them and the shorter rotor is correct for your truck.

Richard

 

[DonYukon]

That’s interesting I know there’s differences in 1500 and 2500 brakes but not within the same chassis . do you have access to a digital meter to measure them without taking them off? Also have you asked the parts store if you can take both and return the ones that don’t fit? I have a RPO code De-coder on my computer but it’s not available to me currently

 

[Schurkey]

The 1500s could have the very light-duty brakes (JB3?) or the thicker, more-powerful JB5.

Regular-cabs got the JB(?)3, extended-cabs got JB5.

The JB5 got thicker but same-diameter rotors, bigger pistons in the calipers, bigger pistons in the master cylinder, and a more-powerful vacuum booster.

[EDIT] On the 4WD, the steering knuckles were the same, I guess the hubs are different. The 2WD steering knuckles are different--the spindle is longer on the JB5 [/EDIT]

Photos from my K1500

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After a certain model year, the JB(?)3 was discontinued.

Both the '3 and '5 got the same crappy rear brakes--the "10-inch" leading-trailing shoe design. The '6 got the '5 front brakes, but with proper 11.x duo-servo rear brakes.






Describing rotors based on "height" is nuts.

 

[HotWheelsBurban]

The 1500s could have the very light-duty brakes (JB3?) or the thicker, more-powerful JB5.

Regular-cabs got the JB(?)3, extended-cabs got JB5.

The JB5 got thicker but same-diameter rotors, bigger pistons in the calipers, bigger pistons in the master cylinder, and a more-powerful vacuum booster.

The steering knuckles were the same, I guess the hubs are different.

You must be registered for see images attach

You must be registered for see images attach

After a certain model year, the JB(?)3 was discontinued.

Both the '3 and '5 got the same crappy rear brakes--the "10-inch" leading-trailing shoe design. The '6 got the '5 front brakes, but with proper 11.x duo-servo rear brakes.






Describing rotors based on "height" is nuts.

That's a nice thing about Suburbans, they got decent brakes (JB6)from the start.

 

[df2x4]

Regular-cabs got the JB(?)3, extended-cabs got JB5.

Are we talking a certain year range here? My '97 C1500 standard cab got JB5.

 

[someotherguy]

The 1500s could have the very light-duty brakes (JB3?) or the thicker, more-powerful JB5.

Regular-cabs got the JB(?)3, extended-cabs got JB5.

The JB5 got thicker but same-diameter rotors, bigger pistons in the calipers, bigger pistons in the master cylinder, and a more-powerful vacuum booster.

The steering knuckles were the same, I guess the hubs are different.

The steering knuckle.. the spindle is longer for the HD brakes. You can't install the HD rotors on the non-HD spindles; they aren't long enough.

Are we talking a certain year range here? My '97 C1500 standard cab got JB5.

1992-up all cabs got HD brakes, no more 1" thick rotors on the 1500's.

Richard

 

[Schurkey]

The steering knuckle.. the spindle is longer for the HD brakes. You can't install the HD rotors on the non-HD spindles; they aren't long enough.

I don't understand what you're saying.

I was told that the '3 and '5/'6 steering knuckles were the same, but the hubs were different between '3 and '5/'6. So the thicker rotors would fit fine...if you also swapped hubs.

There is no "spindle" in the conventional sense, the closest these 4wd trucks have to that is the stub shaft from the outer CV joint that is held to the hub with the bigass nut. Are you saying the CV shafts are different?

When I converted from '3 to '5/'6 up front, I did swap steering knuckles and hubs, but in my case the whole swap was because I had a damaged steering knuckle, and by accident the donor truck happened to be an extended-cab. I didn't know I had bigger/better front brakes until I got the donor parts home and started comparing. Had to go back later and get the matching master cylinder and booster. The only parts I saved were the lower control arms, and that was because I couldn't get the torsion bars to slide out of the arms on my truck, or the donor truck. I had to replace control arm bushings and ball joints with the torsion bars locked to the arms.

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[Anubis]

I wish I had known this while going through my front end. I would have switched to the HD set up. My K1500 is a 1990. Other than rotors, calipers and pads what parts do I need?

 

[454cid]

I don't understand what you're saying....There is no "spindle" in the conventional sense, the closest these 4wd trucks have...

The OP has a 2wd. Sometimes it's easy to forget the other "wheel drive" exists