Caliper Bolts and Loctite

[JusticePete]

Originally Posted by Spurshot

First, I'm not a metallurgy expert nor a structures engineer. But, I have stayed in a Holiday Inn Express and I am an engineer with test, failure, and accident experience over the decades I've worked in aerospace.

Our caliper bolts have a good portion, maybe most, of their load in shear. In other words: the caliper is trying to slide sideways (rotate with the rotor) across the bracket. This is probably the biggest reason we don't see failures of bolts in tension. There is some twisting force on the caliper, since it's hanging out off of the backside where the bracket attaches, and that causes tension force on at least one bolt.

I don't want to over complicate or over simplify, but understanding the way steel behaves as it is stressed (loaded with force, in our case tension). In the stress strain curve below for simple steels, it shows how force (stress) is applied and how the steel stretches (strain) and eventually breaks. Initially, there is a region the steel can stretch like elastic and return to original dimensions. This is called the elastic region. On that chart, it is the steep slope that is a straight line up to the yield strength. Most bolts and other objects made of steel are used within the elastic region below the yield strength. If you are ever on a steel trussed bridge and it bounces around, this is the steel flexing within the elastic region. Regular bolts are used within this region. You can tighten and loosen bolts within their elastic region a lot of times before you run into issues of fatigue. (but fatigue is another science project altogether)

On a Torque To Yield (TTY) bolt, when you torque it as the maker intended, the bolt will stretch beyond the yield strength mark on that chart. The yielding of the steel will more than likely be around the ultimate strength or left of that a bit, if torqued to maker's spec. But it will be permanently stretched. If you stretch it using the same forces (strain) again, it will push it further to the right on that curve shown, and closer to failure. That's why I would not recommend torqueing a previously torqued bolt to the same 30lbs + 90 degrees value that GM recommends for a new bolt. That goes for putting on a new bolt, torqueing to spec, then removing and reinstalling/retorqueing to spec again. I'd avoid retorqueing a bolt to the yield spec twice, regardless if you just installed it and had to remove it for some reason.

A simple test might be to measure the length of a new bolt accurately (+/- .001") , then torque per GM specs, remove it and measure the length. It should have stretched noticeably. Record that length, torque it to specs for a normal grade 5 bolt of that size, remove and measure. It should not be longer than the prior measurement. Torque it again to the TTY GM spec, remove and measure. It should be longer than the last measurement. That means it's moving along that stress strain chart curve to the right, closer toward failure, eventually.

So in the most simplistic terms, the bolt is big enough to withstand a second use, but don't push your luck and use new ones?

[Jrizo]

This is a great thread going on please gentlemen continue

[blewbyyou]

I recently replaced my pads and roters. I purchased 8 new bolts I'd rather be safe than sorry. But only the front bolts had a factory coating on them I guess it's like a lock tight product and that is why it is suggested that they be replaced. They were a little bit more difficult to install as compared to the back ones. I did use blue lock tight on the rears.

[lawman007]

Originally Posted by EarlyApex

I know I had read on the net that they are "torque to yield" bolts but looking at them they do not look like any TTY bolts I have seen. Typically they are designed with a specific cross section that will be the area that is to yield. That area will have a smaller diameter than the rest of the bolt. You also would not want the yield calculations to be in a threaded area as that would be much harder to get a consistent preload value compared to a smooth section. The caliper bolts look like a standard design bolt with a very short non-threaded section. The tightening process sounds like the "turn of the nut" method to spec the bolt preload. This is a more accurate method to create the desired bolt preload as bolt twist during torqueing is not a factor. I have only used this method when the correct bolt preload value is critical to the joint design. Usually I have used this method when the torque values are very high as a special torque wrench is not required, just a long cheater bar.

One way to test this is to measure the length of a new bolt and then install using the correct "30 lb and then 90deg". Then remove the bolt and measure the length. If it changes (longer) then the bolt yielded, no change in length confirms it did not yield.

What is 30 lbs then 90 degrees? I have used torque wrenches for a long time now in work and at home, the dial and needle kinds so I know how to use torque wrenches, what is the 90 drgrees part?
Troy

[theragtopguy]

If you merely punch the pins out to remove the pads from the top, how do you push the piston in the caliper back in?

[hesster]

As most have stated here, best to replace for peace of mind. I have replaced mine every time rotors came off. So I took some measurements for yucks of old versus new to see if there was any stretch, from the base of the bolt head flange to the end of the bolt. Results show that in my case they had stretched, from 0.013" to 0.035".
So is up to 0.035" of stretch mean the bolt will fail if re-torqued? Got me, but I do know they had stretched, meaning they had moved somewhere past the yield point on the curve shown in this thread.

Vehicle damage from a bolt failure versus the cheap cost to replace is a no brainer.

Front New = 2.195", 2.200", 2.195", 2.200" (all 4 pretty consistent).
Front Used= 2.230", 2.230", 2.213", 2.213"

Rears are a smaller bolt with less torque, but results were about the same.

And per the previous (2) posts, I never had to push the pistons back in when I did any brake work, but a large channel lock will work.
A simple way to get the 90 degree rotation after the 30# of torque without a angle adapter on the wrench is to simply torque the bolt to 30#, make a mark with a with a sharpie somewhere on the bolt flange and immediately next to it on the caliper where you can see it, then make another mark 90 degrees CCW on the bolt flange from the previous mark, and crank the bolt until the new mark on the bolt flange lines up with mark on the caliper. I always torque stripe all my fasteners anyway.

[zlathim]

Originally Posted by lawman007

What is 30 lbs then 90 degrees? I have used torque wrenches for a long time now in work and at home, the dial and needle kinds so I know how to use torque wrenches, what is the 90 drgrees part?
Troy

Use the torque wrench to get the fastener to the desired torque (30 ft lbs in this case), then rotate the fastener another 90* past the torque specification. (1/4 turn)

[12blkgrayRS]

Ok, I can be an example here. I reused my front caliper bolts on a brake replacement on my old track car. about a month later on the hwy I heard a loud THUNK under my car. What tha!!??
by the time I got home I was starting to lose my front brake on the left side. the eventually, rolling up the driveway the caliper completely fell out. Fortunately I was in my driveway and not in traffic, but it could have been worse. The bolts were retorqued to spec. but the left calipers bolts(one left) were stretched. And worked their way out. yes, Blue locktite as when they come off with. So now I always, replace the bolts, and they don't cost that much. parts places like O'Reillys also carries them for much cheaper.

[CreeperRS]

Funny, how I’m about to replace my rotors and pads, and came across this thread, good info. I will replace my bolts, cheap insurance. Why not?

[SSEssence]

If 30 bucks can save major damage to your car, why not thanks for the info guys!

[Camaro LA]

Originally Posted by Deputy347k9

Bolt, front caliper, quantity needed 4, part number GM 11570788

Bolt, rear caliper, quantity needed 4, part number GM 11515781

Crush gaskets/washers (if removing calipers), quantity 8, GM 21012386

Thank you all for the information. I have a 2015 V6 2LS camaro, and when I look up these on www.gmpartsdirect.com or www.gmpartsgiant.com they come up as not fitting my car.

Can someone help me find the correct part numbers for the V6?

Cheers

[12blkgrayRS]

Yea, mine have actually fallen out once, so yea replace those bolts, the old bolts are stretched will not torque down the same, physics involved here somehow. Anyway, DO NOT REUSE!!

[12blkgrayRS]

Originally Posted by Camaro LA

Thank you all for the information. I have a 2015 V6 2LS camaro, and when I look up these on www.gmpartsdirect.com or www.gmpartsgiant.com they come up as not fitting my car.

Can someone help me find the correct part numbers for the V6?

Cheers

Call your local dealer part dept to get the right numbers. GM sometimes changes numbers for 2nd-3rd design parts. it wont cost you anything....well it shouldn't..

[Heliogordy]

Everywhere I look, I'm finding Disc Brake Caliper Mounting Bolts for the FRONT, but not the REAR...are they different parts or the same thread/pitch/etc ?

And do you replace the guide pins as well?