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Torque on 2 piece Rotor / Hub Assembly

6K views 26 replies 11 participants last post by  dreamweaver 
#1 ·
69 Camaro with 2-piece assembly - Rotor to hub

I took the hub apart form the rotor to get the rotors turned, paint them, etc. Apparently this is not how I should have done it, but here I am. I am trying to re-attach the hubs to the rotors.

Looking in the assembly manual it says to use 120 ft lbs of torque on the 5 bolts that attach the hub to the rotor. 2 problems:

1. Since the rotor is off the car it is a magic act for two of us to get 120 ft lbs of torque

2. When I crank on it the lock-nut washer just spreads out, even at 100 lbs.

Now what?
 
#2 ·
#4 ·
I guess I didn't know a 3/8" bolt could withstand 120 pounds of t??

At least I think they are 3/8" bolts?? What's t for a 3/8" grade- UNF guys??

Mount them back on the car and then set them down on a block of wood and have at it is the only way I see to do it??

pdq67
 
#6 ·
The bolts are 5/8" and they mount from the inside... would have to be "Rubber Man" to get to them. Here is some additional info:

The "120 ft lbs" requirement I am basing my supposition upon is from the Chevrolet Chassis Manual for 1969 year models. I am looking in the rear of the book under "Specifications 2" under the heading of "Bolt Torques". Since the Assembly Manual does not identify what the bolt is called, I was assuming that it was the "Brake anchor Bolt - Disc Brakes" - for that bolt it requires 120 ft. lbs. Have I identified the bolt correctly? I can't fathom how that much torque can be required for the 2 reasons I mentioned in my original post.
Steve
 
#8 ·
So, if I torque them to 80# di I run any safety risks? The last thing I want to have happen is for the wheels to come apart while I'm driving it (if I ever get to than point :) ). I just can't believe that I'm reading the manual correctly. When we went to take them off it didn't seem as if there was nearly as much strength required to get the bolts off as it is taking to get them on at 100+ ft. lbs. But I want to make sure I am safe on the road (wherever that is).
 
#9 ·
Boy, that kind of looks like a lousy design from GM. But it's cheap. But I'm still wondering why the torque value is so high.

That sort of torque would require a 1/2" UNC grade 8 bolt, or 9/16" in grade 5 with NO lubricant. If you use antisieze, you will need to reduce the torque value by 30% to get approximately the same bolt stretch. With a 5/8", you should be just fine.
 
#10 ·
I think the 'anchor bolt' reference may be to the large bolt that secures the top of the caliper bracket-the top mounting boss on the spindle. At least, that is what that bolt is called in a drum brake setup, as it acts as an 'anchor' for the drum brake springs.

Secondly, I thought the two piece rotor design went away in '69? I'm not up to speed on these kinds of trivia though. In any case, the reason I understand this design went away was because a) one piece design was cheaper to manufacture?
b) More importantly, a one piece rotor is safer. If those mounting bolts work loose, the disc is no longer connected to the hub & no more brakes! Today's two piece rotor desings have the rotor mounted from the front of the hub, over the wheel studs.

Back to your problem, if you cannot find the appropriate spec in the manual, I would go with the known torque values for that size fastner. But the 'size' of the fastner is determined by the shank diameter, not the bolt head size. I have a pair of disc hubs on my workbech at home, and I don't remember those bolts being quite that large.
 
#12 ·
The bolts have a 5/8" head - we took them off with a 5/8" socket and there was no play in them. As far as the shank size I'm not sure, but they are definately thinner... probably 3/8" or 1/2". So, here is what I am reduced to:
  1. I could just go get new one piece rotors and be done with it. I would lose a bit of originality, but might gain some piece of mind. The real drawback is that I would have been defeated by a simple brake job that I pooched when I didn't have to.
  2. I could try to torque the bolts to a different torque specification. This is my preference but I can't figure out what the proper spec is.
I want to proceed with the mission but am willing to wait to "do the right thing"...
  1. Do you think that if I called GM they would be able to help determine what the torque should be?
  2. Do you think I would be "safe" by torquing them to 80 lbs. and calling it good?
  3. Is there another manual ohter than the one I have that might give me the information I am looking for?
Thanks for the help everyone,
Steve

P.S. - Fro anyone who knows my "history" since being a member of TC you know how impatient I am. Well, I've gotten so impatient with the whole process of trying to figure out how to get this car on the road that I decided to get another one. So,today, I did... 69 stroker, 4sp driver. I'm finally drivin' a 69..... What fun
 
#15 ·
I checked my hubs last night and the 'disc retaining bolts' are 7/16-14. I would go with whatever is 'typical' for that size fastner. Hopefully someone can chime in on what that value is. Worst case, use the torque value for the lug nuts since that is the same size??

For added piece of mind, you could loc-tite the bolts, but need to take that ino consideraion with the torque value used. You could also go as far as to safety wire the bolts if you really wanted to.
 
#17 ·
Thank you all for the replies. I have "stalled" the process until now but am ready to move forward. Seems like the 65-80 range is where I need to be. Two more questions:
  1. What correlation does the torque value have to whether or not or what kind of lubricant I use? I don't quite understand... if I use a lubricant, does that change the torque value, and should I use a lubricant?
  2. Straight-Line-69 said, "you'll need to have the rotors turned again as an assembly to remove the run-out...". I'm not quite sure what this means? Here's what I did:
  • I disassembled the rotors and had them turned
  • I had the hubs "cleaned" in a parts cleaner to remove all the grease - they're dry
  • I soaked both the rotors and the hubs in a rust-remover solution and degreased and dried them
  • I painted them with high-temp paint, carefully masking the rotor faces and the hub opwnings.
  • Got new berings and seals
What is "run out" and do I still need to turn the rotors?
 
#18 ·
Any supplied torque value is considering the bolt is installed dry, unless otherwise noted. By using a lubricant, even thread sealers and lok-tite type products will reduce the required torque spec.

As for the run out, I hadn't caught the fact that the rotors were turned while off the hubs. Runout is the amount of 'wobble' you get when the rotor is spun on the spindle. This tolerance is usually within .003"-.005" and needs to be measured with a dial indicator. The place that turned the rotors should be able to measure runout on their machine as well.

The rotor braking surface essentially needs to be arallel to the wheel mounting surface, and so turning te rotors withut the hubs could creat a problem with excessive runout. You also should not paint the hat part of the rotor or back of the hub where the rotor attaches.
 
#19 ·
For more info on bolt torque go to ARP's site. They do a better job of explaining it that I am about to.

When you torque bolts down you are stretching them so they will maintain clamping force and not come loose. Too tight and they will break.
Using a torque wrench is not the best way to check this stretch, since it is a reading of the resistance to turning the bolt, not the actual clamping force.
For instance, if some of the threads are damaged, or have debris in them you will have more resistance.
The same kind of thing happens with lubing the threads. If the lube is too good you will stretch the bolt and can snap it before you reach the torque spec.
 
#21 ·
Run-out,..you need to assemble your hubs to the rotors, then take the hub/rotor assembly to a brake shop (or some parts counters are now turning rotors) to have the rotors checked and turned (trued up) if needed. I can, with about a 90% confidence level, tell you that they won't be true after assembly,..the 35 year old crusty hubs with new rotors will have some wobble.

Good luck!
 
#22 ·
Thanks for all the help... I picked up some new lock-washers today.

So, if I put some lube on them (regular bering grease is OK???) I should be OK torquing them to 65 lbs???

I'll get the rotors checked and re-turned as soon as I put them back on.
Steve
 
#23 ·
If after assembling the rotor/hub it is found that there is excessive runout, you may try 're-clocking' the rotor to hub assembly to see if you get better numbers. You may be able to get within tolerances without further turning the rotors. You will obviously need a patient machinist to work with you if you go this method, since you could conceivably end up checking each rotor assembly five times for runout...
 
#24 ·
I think you're putting too much thought into the attachment and process and torque specs, etc. The rotor's not going anywhere,..it's sandwiched between the wheel and the hub.

C2 and C3 Vettes had two piece rotors that were revited to the hub, and in order to replace thin rotors, the revits have to be drilled out. The original rotors have 10 holes,..5 for the wheel studs and five for the revits. The replacement rotors only have 5 holes, for the wheel studs. Yes, they're not permanently attached to the hubs. Not necessary. But again, they have to be turned (surfaced and trued) as a unit (lugs nuts hold the rotor to the hubs while surfacing) and since they're not attached, you need to make sure you install them on your car with the same "clocking" or "phasing" as when they were turned.

Good luck!
 
#25 ·
Guy's,

I'm not totally up to speed here but I will say that I have two-piece hubs with '88, 'Vette 13" front rotors on my car now!!

Sandwiched between my wheels and the hubs fine.

I think the deal was that the early OEM 2-piece jobber's bolted onto the back and weren't sandwiched like the newer cars do it is all just to get the needed "J" spacing is all.

pdq67
 
#26 ·
My chassis service manual says nothing about torque or repalceing just the rortor. It says if the front rotors are out of spec they should be replaced with a new one.
 
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