Caliper bolt + torque question

You're not going to find out what happened until you bite the bullet, remove the bolt, and look at its condition and the condition of the caliper bracket it threads into. Until you do that you are basing your safety on humming loudly and hoping the problem magically goes away in summer.

You may end up needing a new bolt, and possibly the caliper bracket (which I think is maybe a hundred bucks from the dealer, not cheap, but not outrageous for tuition).

One thing that can have happened is that there was some piece of grot in the threads and the bolt bound up on it before it was fully bottomed, was hard to turn for a moment, then got easy again until it was fully bottomed and you tightened it for real.

That's a thing that can happen (though when it happens to me, I like to take the bolt back out and clean up both sets of threads, then make sure it doesn't happen on the next try).

You just won't know until you find out.

 

I would remove the bolts and bracket and inspect the condition of the threads. Compare the two bolts as well, you may have stretched the bolt in question. If the threads in the bracket are damaged, you can repair them using a timesert kit.

 

Probably a stretched bolt? Hopefully will come out in one piece. Think it's this:


2013 Toyota Prius Two HYBRID Disc Brake Caliper Bracket Mounting Bolt (Front). Bolt, Flange. Bolt - 91552C1250 - Genuine Toyota Part

 

That is consistent with the available facts and would cost the OP $0.00, so hopefully that's what it is.

But I agree that it's worth taking apart to be sure.

 

Worse case scenario, you will have to get a new bolt and timesert the bracket.

 

Great description. And what you did - taking the easier step of going by feel, after having torque wrenched 3 identical bolts, is exactly what almost anyone would have done, including me.

Having it "turn freely by hand" after you broke it? Well that's definitely not good, and should be fixed right away. I'd think that a steel bolt into an aluminum caliper bracket would fail by having the aluminum threads strip out - so they now stay within the bolt threads. Just based on strength of materials, you'd expect the aluminum to fail first. On the other hand, it depends on how many threads are involved. The cylindrical area of the caliper bracket that would need to shear is greater than the circular area of the bolt. But the threads are most stressed at the top, and as you get deeper into the hole, they contribute less. So ... you don't know for sure how it will fail.

Assuming the engineers did their job, neither failure is especially weak and likely to happen. But it's generally cheaper and easier to make the hole a little deeper (more threads engaged, stronger "nut") than it is to make the bolt a little wider / stronger, so in practice it is usually the bolt that is the weaker of the two options.

When "nut" threads strip like that, the closest, highest stress one fails first, and then the rest all fail sequentially since they're now carrying the load the first thread let loose, etc. So it can seem very sudden.

But if that happened, rotating the bolt would cause the bolt + the now-embedded aluminum thread spring to rotate past the newly created hole in the caliper bracket where the threads used to be. So that would probably not turn freely by hand. Rough surfaces grinding past eachother.

If the bolt itself were to just shear near the surface of the caliper bracket, that might turn pretty freely.

I'd get a replacement bolt and plan for some down time.

Timesert would be a great repair if it is the caliper bracket that failed - will be at least as strong as the original caliper bracket. Very easy to do, but you need to buy the kit and wait for it to arrive.

Drilling out a snapped off bolt requires some skill and experience, but should still be easy. Since it is a through hole (nothing on the other side [right?]), I would try to simply drill it out with as big a drill bit you can confidently keep centered in the hole - so you don't damage any threads in the caliper bracket. Often you'd use left handed drill bits to drill out a broken bolt. The concept is that once enough of the bolt is drilled out that it can comply a little, the CCW torque from the drill bit will spin it up and out, not requiring a broken bolt extractor. That concept works, even better and easier if it is a through hole - a right handed drill bit will screw it out the bottom at some point. And if not, you can then use the broken bolt extractor.

 

If the threaded hole stripped, it is simply in the caliper mounting bracket, which Toyota will sell you separately from the caliper (or you can get one from a recycler for less). Simple replacement, no opening the hydraulic line or bleeding, just attaches to the knuckle by two bolts and the caliper by two slide pins.

If you buy an aftermarket caliper, the bracket is usually included (and the whole aftermarket caliper might be less money than the bracket from a dealer). You could still use just the bracket and not fuss with the caliper.

The threads in the bracket could be time-serted, but if it were me, not already having a drawer of time-serts at the ready, I would just replace the bracket and call it a day.

 

That bracket has a suggested retail of $145 USD. Couldn't find it for sale at McGeorge, but I'd suspect the discounted retail around $120?

Huh, a lot cheaper at amayama:

Genuine Toyota 47721-47060 (4772147060) MOUNTING, FRONT DISC BRAKE CYLINDER, RH - Amayama

 

He might be better off with a Timesert kit and a new bolt. The timesert kits are about $100, sometimes less. I do not remember the size of the bolt offhand but I can take mine out and measure it if needed. But it is probably M12x1.25.

Here are some additional pictures of the timesert repair that I did on a STI caliper. The process sounds intimidating but is actually quite simple.

First the damaged threads are drilled out using the included bit:


Next you make a counterbore for the lip of the insert to sit. Tool is included.


Then you tap new threads into the hole. I prefer to use a tap guide to make sure the tap is started straight:


Result of the tapped threads:


Lastly you use the included install tool to thread in the new insert. The tool expands the last few threads of the insert which allows the insert to lock in place.


If you decide to go the timesert route, be sure to measure the depth of the threads. The inserts come in various lengths.

 

Even at full list price, one time, for what one does not expect to be a recurring expense ... doesn't sound like the end of the world.

A cheap one from a recycler (or a $50 autozone reman caliper, use the bracket, toss the rest), even less so.

 

The threads are in the caliper bracket. The knuckle act as a spacer.

 

I believe this to be true.

The timesert kit you'd want, assuming those are M12-1.25 bolts (probably are, but measuring would be good to confirm) is #1212 (BTW, they typically do the first two digits as the M## and the 3+4th digits as the first two digits of the thread pitch)

I have that kit, so if you want to pick it up here in CA ... Or if you have any questions about it, I can look.

I don't know if this is feasible, and would not want to be responsible for recommending it, but for repairs in non-critical areas, it might be possible to think about making a temporary repair by using a longer bolt that goes through the caliper bracket (drilling it out if needed) and using a nut on the end of the bolt.

Also, OP, I'm surprised that a coil of aluminum threads did not come out, on the bolt. Maybe you'll see them once you disassemble further.

 

Your photo looks worse than it might really be, as the caliper bracket shifted a bit when you took the bolt out, and your photo shows the holes not lined up.

When you get the tire off, I would just remove the caliper (the two smaller bolts up higher, that go into the caliper slide pins, no more than 25 ft lb on those, use some string or wire to tie the caliper to the suspension spring so it doesn't dangle by the rubber hose. Take the bracket out (just the one remaining bolt holding it in there), get out into good light and see what you've got.

 

That bolt (the one on the left) seems very odd. Something not adding up??

 

So you know we're talking about the same thing here, the bolts in question here have 17mm hex heads, and the threads have a major diameter of about 12 mm, with a pitch (distance from one thread to the next) of 1.25 mm. In the JIS standard, that's an M12 bolt, or M12-1.25. So whether you refer to it by the hex head size or the thread size, we are talking about the same bolts.

I did not follow everything you said there, but looking at the photo, I'm even more confused.


First, as I expected, and mentioned in my posts above, when you strip out the female aluminum threads with a steel bolt, they will often remain embedded in the bolt. That's exactly what I see there in the photo of the bolt on the left. That aluminum came out of the bracket, so it needs to be repaired or replaced. Maybe it still has some threads left in it, that may appear to work with a good new bolt, but many of its threads are now gone.

Sounds like you don't want to do the TimeSert repair, so I'd plan on a new bracket rather than waiting to see if the new bolt will hold.

Also, mainly for curiosity, it looks to me like the threads of the bolt on the left side show that the bolt has stretched. The thread pitch on the left bolt seems bigger than that on the right one. An easy way to check is to hold the two bolts so their threads are interlocking - you can measure pretty accurately doing that. Here is a photo where I compared an old, purposely stretched cylinder head bolt with the new one I was replacing it with. These are Class 10.9 M10-1.5 head bolts.


Without stretch, the threads would line up, and for example two new bolts will line up with virtually no gap. But here, you can see pretty easily that one bolt has stretched by about a third of a thread pitch, = 0.5 mm over the length of the bolt.

If the bolt has stretched, and especially if it has the aluminum souvenir spiral from the caliper bracket stripping still in it, it will have problems threading into a good threaded hole. That may explain some of what you noticed. So you do have a bolt problem, but not the only problem, as I see it.

Also, not so relevant, but on most cars, you don't need to touch the caliper bracket bolts for a simple pad change. Was that the case here?

 

Curiouser and curiouser.

Yes, my call is also that the 'coil' that fell out is really ex-threads, and what is still stuck on the other bolt is also ex-threads, and the bracket is done for.

One of the bolts stretched? Or is held nearer the camera? (I can't really tell.) Or somebody was in there before and mixed up a fine-thread and a coarse-thread bolt?

Is the bracket even supposed to be aluminum? I know the caliper is, and the knuckle is, so I guess they wouldn't have used a dissimilar metal for the bracket. They were ferrous on my Gen 1, probably remembering that.