I'm in the middle of replacing my left rear caliper and ran into a snag. Once I got the caliper removed I noticed that the mounting bolt threads were pretty worn, even a bit eroded up near the head. So, I decided that I'd just replace the bolts with good new ones instead of taking a chance on the old ones. Can't be too safe with brakes.

So looking at the vendors, I notice some have kits with bolts that are threaded all the way up the shank and others are only partially threaded with a shoulder up at the head. Given that the caliper body is not threaded (the bolts just pass through) I would think the shoulder would be necessary to keep the caliper centered with the mounting holes in the bracket.

Corvette Central Part#: 192109 (no link, sorry)
Ecklers Part# 35967

The bolts I pulled from my car look more like the Ecklers set, with the full threading and the little tick-marks on the head. The Corvette Central set shows a 'double circle' headstamp, which some other vendors also made effort to point out and I get the impression that this is the period correct headstamp needed for a restoration. I'm not doing a factory-correct resto, so the headstamp is immaterial to me, but I am curious about the shoulder.

I did a little searching first and couldn't find an answer. Can someone suggest to me which of these 2 kits I should get?

 

I never had to replace a caliper bolt. They're pretty hearty and a few threads being worn is nothing to worry about. I worry more about chinese made bolts that will break more than my old AMERICAN bolts failing.

 

I personally take my old bolts and go by Lowes, Tractor supply or other store with bolts. Then I buy the identical item in grade 8 for about 1/2 the price of those kits if not 1/4 the cost.

 

Do you have the Parts and Illustrations Catalogue. It lists fastener sizes. I'm sure the bolts are grade 8 and any well stocked hardware store is likely to carry them. Take one of yours with you when you go.

 

I'll have to see if I can take a good picture of them when I get home tonight. I think after seeing them your suggestion may change. Though I can agree about using good American parts instead of repros of unknown source.
I would certainly do the same, but I do like to check the vendors and AIM first to make sure that the part I have is in fact correct. For example my seats were fastened down with machine screws and nuts instead of proper bolts. I've learned not to trust anything I find on this car. Verify Everything.
I actually do have that catalog: I'll look when I get home tonight. Hopefully it can clear up the shoulder question. Paragon describes it as a 7/16-20x1-1/8" bolt, but it's the shoulder I see in some of the bolt kits that has me questioning if I have the right ones. I have no idea what all has been done to this car before I got it, which again is why I don't trust anything I find.

Interestingly, Paragon lists the GM Part# as 3843384. I've searched this on the GM Parts Wiki and come up with nothing.

 

Do not go to Lowes or TS for these bolts. As mentioned they are Grade 8. Quality bolts will have the manufactures mark on them. No manufacture mark, I wouldn't use them on my brakes.

 

Okay, so my memory was slightly flawed. My bolts do have the shoulder, so that is what I will purchase to replace these. That is, unless you fine folks think these are okay to reuse...

Bolt 1


Bolt 2


These are clean, by the way. I scrubbed all the dirt, grease and junk off of them. See how eroded the threads are up toward the shoulder? I could try a 7/16-20 die to recut them but I can't imagine it doing much good.

(sorry for the picture size: I don't know how to make them smaller)

 

I would NOT re-use those bolts, I would also NOT buy bolts from a Corvette parts vendor. I get bolts from McMaster-Carr, John Deere, Ace hardware & True Value, you can also probably trust Fastenal quality.

 

Yeah, hadn't really planned on re-using them. That was kind of a smart-alek comment on my part. I went ahead and ran them through a die to clean up the threads, just for giggles: it helped a bit, but they're just too far gone.

I'll probably get an order placed for these and a few other things sometime in the next few days.

 

From a machinist's perspective you want as much shoulder as possible to fit into the unthreaded hole in the caliper without the shoulder extending past the caliper face....this supports the caliper better and prevents threads from wearing the caliper hole larger.

It is NOT a must-do.....it is more of a rust-deterrent/aviation safety factor.

 

I remember that from the machining class I took in college, DG. Always had a fun time in that class.

Another question that crept into my mind this morning: when I get the new bolts and get set to install the caliper, should I use threadlocker or anti-seize on the threads, or nothing at all? I can imagine threadlocker would be a good idea in order to keep the bolts from backing out, but then there IS a lock-washer that I presume is to do that job. After how rusted these things are and how difficult they were to get out I would think anti-seize would be a good idea: rebuilding the trailing arms is on the list to be done in a year or two. On the other hand, I did a little searching and found several instances of others putting threadlocker on these bolts before installation.

Opinions?

 

With a lock washer I don't use thread locker. Anti seize i use on stainless steel hardware but I wouldn't use stainless on a cast iron caliper. If you plan on driving your car and keep it in the garage, rust shouldn't be a problem anymore. Every couple of years you'll be doing breaks so the bolts getting locked on as they were before won't happen agian IMHO

 

Go to the Chevy dealer. They still carry the correct Grade 8 shoulder bolts. They will have to pull out the old thick greasy parts book under the counter. They are not in the computer. I bought these a couple months ago as well as the shoulder bolts that hold the rear driveshaft Ubolts on, in stock. Find the old guy with the worn out looking Chevrolet work shirt in the parts department.

 

Alright, update:

I placed an order for these bolts and a few other parts earlier this week, and they arrived today. The bolts I received are threaded all the way up the shank, rather than having a shoulder as the picture on the vendor's site showed. Note: the vendor I bought from is not either of the 2 in the OP. I was really hoping to get my rear brakes finished this weekend since Saturday is going to be the only nice day in the forecast, but now I'm looking at maybe having to return these bolts.

I'd rather not have to return these bolts and then wait over a week for a new order (with a different vendor) to be placed, shipped and received. Doorgunner seems to indicate that it's not a huge issue, and in my impatience to get this done I am inclined to agree. The fact that most vendors seem to only sell non-shouldered bolts further implies that it's not a big deal. However, I don't want to cause preventable wear and tear on my car.

How critical is having the shoulder on these bolts?

 

It's not a big deal, there's probably as many running around without shouldered bolt as with. The important thing is that they are tight and don't come loose. How many marks on the head of the new bolts ?

 

I always try to replace these bolts with grade 8 shoulder bolts and new washers.

Torque to 65 ft lbs with a drop of loctite.

 

6 tick-marks, with what looks like a lower-case 'n' mark if held so the mark is at the 6 o-clock position. The bolts and washers are a metallic golden color; perhaps zinc-chromate plated for corrosion resistance?

I called the vendor about the discrepancy between what they show and what they shipped, just to see if maybe I was shipped the wrong part by mistake. They said these were the only ones they carried, apologized for the confusion, and said they would refund the price of the bolts since shipping them back would cost more than they are worth. I was rather impressed by that.

 

Agree. And I would definitely use a shoulder bolt like you surmised.
To get the shoulder length that you require, you may have to buy a longer bolt and cut to length, depending on the availability in your area.
Generally, 3 hash marks on the head is a grade 5, and 6 or more is a grade 8. My local Napa also has a good selection of bolts.
The "n" is probably a brand logo.

 

I was reading this again, and looking back over my photos above, and noticed a few things. First, look at the picture with the lock washer: the washer takes up almost all of the shoulder by itself. Hardly any of that shoulder is supporting the unthreaded mounting hole on the caliper. Next, look at the threaded area just down from the shoulder: it's worn/eroded, perhaps like something has been sitting on it? Perhaps, maybe.... the caliper? Finally, the very end of the bolt, the threads are still solid and clean: that's the part that was threaded into the mounting bracket.

So, logically, GM used the wrong bolts to start with. Ideally only the last half-inch or so of the bolt would be threaded: the rest would be unthreaded shoulder to support the caliper.

That's what I figured regarding length, Sayfoo. I just wonder why GM didn't use the correct bolt during production.

I assume the bolts that I removed from the car, pictured above, are the originals. They look like the ones in the AIM, at least. Different headstamp than the ones CorvetteCentral and Paragon offer (they call it a double-circle headstamp: mine just have tick-marks, not shown above), but otherwise the same. I figure if it took 39 years to wear the threads down that badly, it must not be too critical. I'll more than likely end up using these new bolts even though they don't have a shoulder, because to get a shoulder in the proper length would require custom-making the bolts, and I don't have the equipment for that.

 

How do you know that the bolts are original?
And even so, why not try to upgrade while you can.
It shouldn't be too hard to get the correct shoulder length and cut some threads off with a hacksaw and file to make pretty, if needed.
Anyway, either way should hold.