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Disc Brake mount bolts - spec?

Old 11-02-20, 04:43 PM
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aggiegrads
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Disc Brake mount bolts - spec?

I always seem to find myself rummaging through bolt bins looking for the M6 bolts that are used for disk brake mounts.

I am going to put in an order at McMaster to stock up on M5 stainless bolts and washers, so I thought I would get some M6s too, to have in my toolbox, but wasn't sure if anyone knew the specs. They seem to always be black oxide, which tells me that they are a higher carbon content (and higher strength) alloy.

I wouldn't mind spending some extra coin on stainless, if I knew that the strength of the material was close to that of what is normally supplied with calipers and brake adaptors.
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Old 11-02-20, 06:00 PM
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The only common 6 mm bolt is M6x1.0 mm and you specify the length. Most stainless bolts are 304 stainless or 316 stainless which are very corrosion resistant but not particularly strong compared to high strength carbon steel bolts.
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Old 11-02-20, 06:30 PM
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Originally Posted by HillRider View Post
Most stainless bolts are 304 stainless or 316 stainless which are very corrosion resistant but not particularly strong compared to high strength carbon steel bolts.
I agree, but not all carbon steel bolts are high strength. If the ones that normally come from Shimano, Hayes, etc. are grade 12.9, then I will stick with carbon steel. If they are only grade 5, I will go with stainless, as they have 70% of the strength of grade 5.

Some people use Ti bolts for rotors, which is less than half the strength of grade 5, and a third of grade 8. Knowing what the engineers specified would help inform if stainless was safe or not.

I also see SRAM sells adaptors with stainless hardware and flat mount bolts of Ti. Maybe that answers my own question and I will be fine with a stainless bolt.

Last edited by aggiegrads; 11-02-20 at 06:37 PM.
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Old 11-02-20, 06:44 PM
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I just looked at the metric socket head screws in McMaster Carr. Seems like you can get very high strength Stainless Steel screws, but I don't think you need stainless fasteners.

Alloy Class 12.9 = 170,000 psi (tensile)
SST 18-8 and 316 = 70,000 psi
SST 316 High Strength = 110,000 psi
Stainless A286 = 160,000 psi

I assume the real answer that's need is what is the minimum fastener strength needed? The bolts provide a certain clamping force which generates friction that resists the braking action. Some argue bolts aren't meant to take shear loads, just tensile (clamping, in this case) loads.
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Old 11-02-20, 06:59 PM
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Originally Posted by aggiegrads View Post
Some people use Ti bolts for rotors, which is less than half the strength of grade 5, and a third of grade 8. Knowing what the engineers specified would help inform if stainless was safe or not
IMO a lot of people misapply Ti fasteners on bikes. Fasteners are one part of a system and substituting them willy-nilly can get one into trouble, especially as regards safety-of-life systems like brakes. Yes, Ti has some attractive attributes but it also has its shortcomings.
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Old 11-02-20, 07:01 PM
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Originally Posted by MudPie View Post
I just looked at the metric socket head screws in McMaster Carr. Seems like you can get very high strength Stainless Steel screws, but I don't think you need stainless fasteners.

Alloy Class 12.9 = 170,000 psi (tensile)
SST 18-8 and 316 = 70,000 psi
SST 316 High Strength = 110,000 psi
Stainless A286 = 160,000 psi

I assume the real answer that's need is what is the minimum fastener strength needed? The bolts provide a certain clamping force which generates friction that resists the braking action. Some argue bolts aren't meant to take shear loads, just tensile (clamping, in this case) loads.
I looked at those numbers as well, but you may have forgotten to look at price. a bag of 50 18-8 M6x25mm screws is $9.93. The high strength 316 in the same size are $5.79 each. The A286 are $6.73 each.
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Old 11-02-20, 07:06 PM
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I doubt a nicely rolled bolt will fail, of any of the materials mentioned, for brake caliper mounting use. The stresses these fasteners see is not a lot (although somewhat "significant"). What I I do see, uncommonly, is the usual threading issues. Crossed threads, rounded out hex wrench heads, corroded in place bolts, wrong bolt threading used, too long or too short a bolt length you know the usual things. But I have to say I don't remember any of the many hundreds of disk brakes that I've worked on ever having a broken bolt. Andy
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Old 11-02-20, 07:07 PM
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My bike came with ti-bolts for the drop-out sliders (for IGH) I couldn't for the life of me figure out why it seemed like the rear hub was torqued and the (Rohloff) slipped in the gears. Switched it to same dimension stainless bolts (huge bolts) and haven't had a problem since. I can torque the stainless better to prevent slipping. I'm not switching my brake bolts to either stainless and certainly not titanium. If for nothing else than the fact that with stainless, I'd be worried about crevice corrosion, and with titanium I'd worry about the aluminium corroding. And the heads of titanium bolts/screws are not as rugged as steel when the time comes to remove them.

Tl;Dr: I think the bolts already in your brakes is probably the best compromise of them all.
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Old 11-02-20, 07:09 PM
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Originally Posted by aggiegrads View Post
I looked at those numbers as well, but you may have forgotten to look at price. a bag of 50 18-8 M6x25mm screws is $9.93. The high strength 316 in the same size are $5.79 each. The A286 are $6.73 each.
Actually, I didn't look at the price. I figure that was your department!

Wow, $5.79 each! That's a big decision maker. But maybe you can argue "you can't put a price on safety".
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Old 11-02-20, 07:31 PM
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It would be of interest to know the material specs for these and other fasteners. I know the manufacturers specify (much like any industry) to their suppliers, but that info doesn't trickle down to the consumer.
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Old 11-03-20, 11:07 AM
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The limiting factor is likely going to be the Aluminum threads in the mounting piece, not the strength of the bolt (Ti excluded).
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Old 11-03-20, 12:08 PM
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Originally Posted by gsa103 View Post
The limiting factor is likely going to be the Aluminum threads in the mounting piece, not the strength of the bolt (Ti excluded).
I can see how this would be true for the caliper mounting, but bolts that mount the adapter to the fork/chainstay mount would be in pure shear.
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Old 11-03-20, 07:40 PM
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My post-mount calipers have been attached with the same no-name, China-sourced M6 Ti bolts for over 40,000 miles. On two bikes, no less. Frame cracked on Bike #1 , I just moved the brakes over to the new frame, used the same bolts.

Every rotor I've used on both of those bikes has been attached with the same, no-name, China-sourced M5 Ti bolts, removed and put back on easily a dozen times.

This is perhaps not a location where the strength rating of the bolts is paramount. The ability to resist corrosion is, at least in my mind, perhaps more critical. Hence why I don't have black oxide on any bike, anywhere.
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