8 lug hub differences? --PICS ADDED!!--

[kd7kmp]

I know it's a bit long, but please bear with me.

I recently purchased 3/4 ton axles to put in my 1990 GMC Suburban. The axles supposedly came out of a 1979 K20. The rear 14 bolt FF went in without a hitch...I only have the parking brake cable to work out. The front 10 bolt (I'm sure it's a 10 bolt) installed easily enough as well. The problem I'm experiencing is with the front 8 lug hubs.

When I purchased the front axle the backing plates and hubs were not installed on the axle. They were in a box. I replaced the inner and outer bearings as well as the wheel seals in both hubs. I mounted the backing plates and hubs and then went to install the brake calipers. Image my surprise when the calipers wouldn't slide over the rotor with the outboard pad installed. I looked at the position of the rotor and noticed that it seemed to be about 0.25" too far out from the center line of the truck. I thought maybe I didn't get the hub pushed all the way back on the spindle so I double checked the spindle nuts...they were good and the hub was as far back as it would go. I then pulled off the hubs and pulled the bearing to make sure I got the right ones. They slide off and on as they are supposed to. The inner bearing is the later design with a 2" ID and the spindle matches that. Then I looked more closely at the back side of the hub. I noticed that the back of the hub is only about 1/8" farther out from the vehicle center line than the rotor. I looked at the hub on my 1/2 ton axle and noticed it is about 3/8" farther out from the vehicle center line than the rotor. The difference between the two is the 1/4" I need to get the caliper to mount. At this point I put the 8 lug hub on my 1/2 ton axle only to find the caliper wouldn't fit for the same reason just mentioned.

I thought, and have been told, the only difference is the backing plates and hubs while the spindle are the same. Thinking I got some funky hubs I went the salvage yard and pulled some 8 lug hubs from a 1980 K20. I got them home and still have the same problem! Can anyone tell me what I am missing? Is there really a difference in the spindle that I don't know about? The 'Burb has been on jack stands for a couple weeks now. I really want to get it done. I really appreciate any help you can provide. Thanks.

Kevin

 

[thedrip]

switch up the order you're installing the backing plate and spindle. if I remember right, its knuckle then backing plate, then spindle. if thats what you're doing, then reverse it, install the spindle and THEN the backing plate.

 

[sweetk30]

knuckle - spindle - backing plate.

 

[thatK30guy]

knuckle - spindle - backing plate.

X's 2...

 

[kd7kmp]

switch up the order you're installing the backing plate and spindle. if I remember right, its knuckle then backing plate, then spindle. if thats what you're doing, then reverse it, install the spindle and THEN the backing plate.

I appreciate your response...but I'm certain it's knuckle, spindle, backing plate. I've looked at several fronts the last couple weeks and that's the order they all follow. Also, the manual supports that. Cheers.

Kevin

 

[chulisohombre]

did you put it together in the right order.spindle first,then backing plate,then inner bearing and rear seal.hub over that then outer bearing,nut and the spacer then outer nut.and then the hub lockout and brake caliper.sounds like you seem to know how it went together.if you switch the backing plate with the spindle it will be about 1/4 inch off.

 

[kd7kmp]

chulisohombre,

I put it together the exact way you mentioned. I am starting to wonder if the guy sold me the wrong backing plates...he did have a lot of spare parts in his garage when I got the axles. Hmmm...

To everyone:

I know the spindles are correct because I took one off my axle and compared it to a spindle at the salvage yard. They were the same.

What's the difference between the 1/2 ton and 3/4 backing plates? Anyone got a set they can look at to compare? All the salvage yards around here are closed until Monday at the earliest.

Kevin

 

[4X4HIGH]

The difference between a 1/2 ton 6 lug brake bracket and a 3/4 ton 8 lug brake bracket is the fact that the 3/4 ton bracket moves the caliper away from the center of the rotor about .429". A 1/2 ton rotor is 11 41/64" diameter and a 3/4 ton rotor is 12 1/2" diameter. So half the difference between the two is how much further the 3/4 ton brake bracket moves the caliper outboard which is .429"

 

[kd7kmp]

4x4high,

I understand what you are saying, but the rotors on the 8 lug hubs are the same diameter as my stock 6 lug hubs. The issue I'm having is that the rotor is about 1/4" too far out from the center of the truck for the caliper to fit with the outboard pad installed. Does the 3/4 ton backing plate account for this as well?

Kevin

 

[4X4HIGH]

Then you have an inner bearing race that isn't seated all the way in the hub. That is the ONLY thing that will make the rotor sit farther outboard that it should be. ALL the offset dimensions are the same between 1/2 and 3/4 ton hubs.

 

[Fordum]

You do have the caliper compressed all the way, right?

 

[kd7kmp]

4x4high,

I thought of the outer race for the inner bearing. I double checked to make sure it's seated all the way. It's all the way in. Plus, the 8 lug hubs I got from the salvage yard today mount to the same position...

Fordum,

The caliper piston is compressed all the way in.

Kevin

 

[4X4HIGH]

The only other way this could have happened is if you installed the caliper bracket BEFORE the spindle onto the knuckle.

 

[sweetk30]

well we are to this point now. so might as well post it.

 

[Fordum]

well we are to this point now. so might as well post it.

I thought that about the second post, but when I checked, I didn't see the "member" logo under his name, and realized that he could not post pics.

Maybe he could use MS Paint and draw some drawings.....

No, don't think he could post those either........

 

[4X4HIGH]

This would be the time to pony up the $25.00 membership and be able to post pics and have his issue solved pretty quick after that, would be the best $25.00 he spent recently.

 

[kd7kmp]

To all,

I have been a paying member in the past. I simply didn't renew when the time came as I haven't done anything to my 'Burb in a long time. I will pay the membership fee soon and post some pics. Thanks for the help and push to get on it!

Kevin

 

[kd7kmp]

Okay, here are some pics...

Pic 1: front spindle and backing plate on 'new' axle


Pic 2: 8 lug hubs. The one on the left (picked up at salvage yard) has the 12.5" rotor and the one on the right (came with the axle) has the 11 7/8" rotor


Pic 3: this shows the 8 lug hub offset. It's about 1/8". The stock 6 lug hubs have about 3/8" offset.


Pic 4: this shows the rotor alignment with the 6 lug hub. The 8 lug hub has the rotor offset about 1/4" further out from the backing plate. This is where the problem lies...the caliper won't fit with the outboard pad installed.


Pic 5: My 1990 Suburban waiting to be finished! It cries out to me all the time!


I'm starting to wonder if the backing plates that came with the new axle are the incorrect parts. Is there any difference in the lateral mounting position of the caliper between the 1/2 ton and 3/4 ton backing plates?

Hope this helps.

Kevin

 

[Fordum]

That helps a lot.
OK, I think I have a handle on this now.
But, I need to make sure so my advice makes sense.

If you lay a straight edge across the back side of the rotor over the seal, on the 6 and 8 lug hub, there is a difference between the two in the clearance between the straightedge and the back side of the hub in the 6 and 8 lug rotors, right?

I think that makes sense.

If that is correct, assuming you can understand what the heck I was trying to say, then my best guess is that there is a spacer that is supposed to go between the brake plate and the hub.

You said that you pulled the rotors off a junkyard unit, you might want to go back and see if there is a spacer behind the brake plate.

Of course, I run Fords, so I don't know for sure, but its either that or different brake plates, and a spacer would be easier for GM to make than two different plates.

 

[thatK30guy]

Backing plates aren't the problem. They're interchangable with all years.

Why not buy some new rotors from your local Pep Girls and install them onto the bearing hubs? I can get these from O'Reilly for like $30 a pop. Getting the year specific should guarantee they'll fit now.

I've never heard of different size of rotors on 3/4 tons before. Which makes me wonder if your used ones are correct GM parts or if one could be a Dodge.

If you have an extra $75-ish to spend, get some new rotors.