robo

After noticing some seatpost slippage and weird binding while trying to tighten my seatpost binder over the last year or so, I discovered last night that the threads on the bolt were... smushed? Not actually stripped, but deformed, as if the force on the threads had squished them. The bolt no longer threads smoothly into the nut, but requires some torque just to move, on account of the deformed threads. Overall, this is a problem and the bolt (and probably nut) needs replacement.



BTW, this seatpost binder is built into the seatpost lug, with a hex nut inserted into a hexagonal recess on the other side. Rivendell proudly statues that this system will never bend, ever. Unfortunately, it seems to have gotten the better of the bolt they used.

Image of the same lug, not my photo:


I know the bolt is M6 threaded, and has a 'T' stamped inside the socket. I haven't been able to find anything about what the 'T' means, but I know that bolts do come in different grades. Question is, does this matter for this use case? Obviously, this particular bolt failed after 10 years, but it could have been defective..

Should I be looking for a particular grade or material? Is stainless steel OK, or should I go for galvanized for better strength? I figure this bolt will be subject to pretty much zero shearing or sudden forces... it just has to maintain constant tension. So is one of the more brittle but strong bolt grades appropriate?

Or am I overthinking this and should I just buy whatever the local hardware store has in stock?

Thanks,
Robo

Edit: update... Just noticed that the nut (not the bolt) has the letters 'A2'. I guess that means it's the lower grade of stainless fastener (A4 seems to be the other?) Neither bolt not nut are magnetic, supporting the idea that they are stainless. So, FWIW, looks like the existing setup is A2 stainless.

02Giant 

I would replace it with another A2 unless you subject the bike to extreme conditions warranting better corrosion protection.

sweeks

FWIW, I replaced the seatpost binding bolt (and nut) with titanium equivalents. No more corrosion. There is, technically, the possibility of microscopic "spot-welding" between these parts, but I lubricate the threads with a copper-based paste and haven't had a problem. You could replace just the bolt and use the original SS nut.
Oh, yes... the link! Titanium Bolts, Titanium Bolt, Titanium Fasteners, Ti Bolts
Steve

HillRider

Can you contact Rivendell for a replacement? If it's non-magnetic stainless steel, it can't be heat treated for better strength so you will either have to get a direct replacement or find a stronger Cr-Mo or a 400-series stainless steel bolt that has the same head and nut dimensions.

nfmisso

1st measure what you have.

I would then look for a class 10 nut that matches; hopefully here: https://www.mcmaster.com/#hex-nuts/=18bqag5

And a bolt here: https://www.mcmaster.com/#standard-s...crews/=18bqb8q

Optional: clean with alcohol and paint the visible parts to suit.

coat with Tri-Flo or similar before assembly.

Arthur Peabody

I replaced my seatpost bolt with what I had in my parts' jars some 10 years ago. It's still holding. I'm sure it's the weakest steel. If the hardware store doesn't have a strong-enough bolt the auto store will.

Troul 

since it is clamping and not subject to a direct load, grade 10 SS, Ti, or treated and coated steel is suggested. You want more torque strength than the wrench is capable of in this case.

Marcus_Ti

Well....not exactly. That is how you can damage the frame's integrated seat collar. A bolt at risk of failing is a safety feature-as it prevents people from killing their expensive frameset. The bolt fails before the frame clamp does.


Seeing as no torque specification has been cited by the OP....I'll presume he's been flying blind and tightening it sans torque wrench or spec. Which is how broke fasteners and equipment happen.

FBinNY 

Stick with stainless, and lubricate the threads well. There are stronger alternatives, but the carefree, rustfree benefits of stainless are probably outweigh the benefit of a bit more strength. You might buy 2 so you have a spare in case there's a next time.

My advice would be the opposite if, for example, the bolt snapped before you could get it tight enough to hold.

Troul 

If Rivendell backs its design as the OP posted, then I'd not worry much about the hardware cinching that collar around the seat post in this case. If it were the typical manufacturer, I'd side fully with your logic.

sweeks

BingBingBing!
We have a winner!
Steve

robo

Hey, thanks for the responses.

As some people guessed, no, I don't use a torque wrench on this bolt. I do in most other locations but I guess with the seatpost bolt it was partly a matter of "I've been tightening seatpost bolts for 25 years, I can do it by feel" and also the fact that this is one bolt that is often adjusted on the road, when I only have a minitool.

I think the most likely explanation is that I overtightened the bolt and deformed the threads at some point. As Marcus_Ti pointed out, better to damage the bolt than any other part, even if Riv's claims of indestructibility of the clamp are true

I guess I could contact Rivendell but I need this bike to be operational ASAP as it's my daily commuter, so I'll probably see what the hardware store has to offer today. My local family owned hardware store has a pretty decent bolt selection. I would prefer stainless if possible just because of not having to worry about corrosion (as a commuter, this bike does see rain semi-regularly).

FBinNY 

A few points.

While more and more people are using torque wrenches, they should be considered an adjunct to skill and common sense, not a replacement.

Proper tightening of a seat post binder isn't very amenable to torque specs. because there are too many variables. The right torque is whatever it takes to achieve the purpose, ie. hold the post fast so the saddle doesn't twist or slip, and no more than that. On my various bikes, the torque needed for that varies by more than 2:1, and on bikes I've worked on it varies from near impossible with the design and hardware, to amazingly little.

My point is that it's not your fault, and you probably did nothing wrong, except possibly, using a system designed for one time or occasional use on a semi regular basis. The builder's assumption is that once a road saddle height is set, it won't be moved often if at all. That assumption drives the design and choice of hardware.

Stainless steel hardware is prone to deformation and galling, It's fine for one time use at high load or repeated use at low load, but doesn't handle repeated high load use. So, depending on how often you expect to open this, ie. regularly to remove the post to fit bike in car trunk, you might want to consider reworking the design to fit a QR. Otherwise, stay with stainless, watch for deformation and replace as needed. That's preferable to going to an alloy bolt and risking rust issues, which can be far more of a problem.

Meanwhile, if you can't find stainless, use anything so you can ride the bike while you source what you need.

robo

The local hardware store came through. They had the exact M6x20 bolt (well, almost.. the original bolt had an integrated washer, but I don't think that's important), in either A2-70 stainless or 12.9 grade alloy steel (which should be very strong), as well as in some lower grade materials. I went with the stainless for now, but if there are any more issues, I might try the alloy steel.

I figured I'd try using a torque wrench to tighten just to see where I got. I ended up tightening to about 70 in/lb (7.9 Nm) to get the freshly greased seatpost secure. I've heard of people using anti-seize compound instead of grease, which is slightly gritty and provides some friction? Anyway, we'll see how this holds...

hokiefyd 

I don't think an integrated washer is critical, but I would sure have some sort of washer on it. A washer will prevent the bolt head from galling the clamping surface of the collar and ensure a smooth application of torque from the bolt. If you have a washer on it, then please accept my apologies -- I may have misread your post.

Crankycrank

I'd agree with this^^. It won't kill your bike not to have the washer but it will help to keep from galling anything (and ruining your feel for bolt tightness). You have one of those non-standard small OD washers which are almost impossible to find so either try to work the old one off or you can drill out another washer with the same O.D. I know this is a lot of fuss for such a simple little bolt thing but it's worth it. Details, details.

Trevtassie

M6 is pretty small for a seat post bolt!

WizardOfBoz

Plated 12.9 bolts are much stronger than A2-70. In fact, you can easily tell: the 12 in 12.9 stands for MPa tensile strength. The 70 in A2-70 stands for tenths of an MPa. The the 12.9 is 12/7 stronger in tensile. And as for yield strength (amount of stress before the bolt stretches permanently) the difference is even higher - about 11 vs 4.5. If a plated 12.9 socket head cap screw (which is what you've got) is adequate in the corrosion resistance department, or if you strip another A2-70, you might consider 12.9 bolts.

One caveat about these high-tech fasteners though, is that they are designed for the bearing surface (under the bolt head and nut) to be perfectly perpendicular to the bolt axis. In high performance auto and airplane use, those surfaces are spot faced to get this perpendicularity. And one uses hardened, ground washers to prevent the hard bolt from digging into the softer frame. To do this in your bike seatpost application, the airplane guys would use a self-aligning washer. Point is, I think the added strength provided by the 12.9 would not be realized unless you did some careful fastener engineering and got the right washers and nuts. If the OEM used A2-70, you should be good with A2-70 or 80. And I agree with hokiefyd: a washer of some kind beneath the bolt head and the nut would be critical. Doesn't have to be captive.

You are right to observe that the lubricant/anti-seize/threadlok/anti-gall compound used affects the tension realized in the torqued bolt. There's a product call ARP Ultra Torque that is supposed to give you the best accuracy and repeatability in torqueing stuff. That would be ok for the 12.9 alloy bolt. But I'd probably use CopaSlip or some other anti-seize/anti-gall for stainless (and certainly for ti).

The washer I refer to is below:

Retro Grouch 

Don't over think this.

It's a bolt. Stick on a new one. If you are afraid of having your replacement bolt break, buy two and you'll have a spare.

HillRider

Not at all. M6 is pretty much standard for both integral clamp bolts and seat collar bolts.

robo

Regarding the washer. I thought about this and actually purchased an M5 stainless washer along with the bolt. Problem is that the inner diameter is too small to fit on the bolt. I'd have to painstakingly file it out. I bought it anyway but haven't done that yet. I'd sort of assumed the washer was overkill, but maybe not.

M6 washer wasn't an option as it was wider than the bolt head, and the old bolt's washer looks like it was installed before the threads were created... it's not coming off, and probably won't fit on the new bolt either

robo

Ah, thank you - now I know what the things on my brake pads are called

ThermionicScott 

@robo, this is kinda left-field, but are you sure your seatpost is the correct size? An undersized one might result in more slipping, and so need more tightening at the seatpost collar.

(OTOH, if you're using the stock one, it's probably the right size. )

FBinNY 

The whole washer issue is being overblown for this application. The bolt head pockets into a recess, so it would need a nonstandard washer, with a smaller than standard OD.

Cap screws are used without washers all the time. One reason is that they can't mar the surface as they turn the way a hex head bolt, or the wench turning it might.

By the same token, disregard the galling references. SS isn't likely to gall when turned against a dissimilar metal anyway.

So, just use some grease under the head, which you'd need to anyway to prevent rust, and don't fret over what doesn't matter.

I'm amazed, (but not surprised) by the hyper-technical references to needing a torque wrench, bolt material issues, washer, and so on. It might be appropriate for an aircraft, but this is a bicycle, and there's plenty of room to be less than theoretically perfect, yet still 100% well suited for the application.

Retro Grouch said it perfectly --- don't overthink this. I'll add, don't let others cause you to think you're doing less than may be necessary.

FBinNY 

Not suited or beneficial for this application. The bolt head is pocketed in a relatively tight counterbore, and so constrained with very minimal room to float to alignment.