neilG

I recently stripped out a 5mm thread on a seatpost clamp, tapped to 6mm, installed a new bolt and tightened it to the original 5mm torque spec which seems adequate. Clearly a 6mm bolt can take more torque, but how does that apply to what's needed to provide proper clamping pressure? Hope that's not too vague a question.

N

dsaul

I'm far from an expert, but I put some values into an online bolt torque calculator. It seems that if the torque spec on the 5mm bolt was 5nm, You would need to increase to 6nm on the 6mm bolt to achieve roughly the same axial clamping force.

Troul 

Dry threads would require a slightly higher tq value. If you apply lube to the threads when torqueing the bolt, I'd use the initial tq value #.

sdmc530

I am far from an expert but I don’t think the bolt is the issue with torque it’s probably the frame that needs the limit to prevent damage. So I would use the original factory spec. And as already stated grease is a good thing too. 5nm +- 1mn is probably just fine but I would not go crazy. Good luck

HillRider

+1 The frame is the thing that limits how much clamping force it will tolerate and how much force it requires to keep the seat post from slipping.

dsbrantjr

Here are two bolt calculators which calculate that, given a constant clamping force, the 6mm screw will need 1.2x the torque of the 5mm. This is counter-intuitive but they agree exactly.
https://www.engineeringtoolbox.com/b...or-d_2065.html (set torque reduction due to lubrication to 0)
https://www.engineersedge.com/calcul...orque_calc.htm does not account for lubrication

neilG

Thanks!

Dan Burkhart 

The finer thread pitch of the 5mm (.8mm) bolt results in slightly more axial force for the torque applied vs a m6 x 1, but the difference is very small, and I would think within tolerance.

noglider 

I tighten the seat clamp only until I can no longer spin the seatpost in the frame and no longer. I don't even torque on the seat very hard to test.

neilG

Hi Tom,
Yeah, that's what I do now. I have no way of knowing if the torque shown is the max or what's recommended to secure the component. Always torquing to max doesn't make sense. The one I stripped was a clamp on a Madone seat cap. I was always careful to use a torque wrench on it, but it still stripped out. Not the best quality.
Neil

operator

This isn't a engine rebuild.

5nm or 6nm doesn't ******g matter.

Go ride your ******g bike.

neilG

Until you find out, how would anyone know that it really doesn't matter that much? Some of us are determined to break as few things as possible.

operator

Because I just told you.

10+ years shop experience, it isn't going to matter.

neilG

It was dsbrantjr who provided the proof. Your assertion that is doesn't matter means little in the face of real data. I have 25 years of shop experience. I was the one who had the knowledege, tools and ability to make the repair. At home. Why do you have to be confrontational when I was merely seeking some new knowledge?

WizardOfBoz

neilG, you indicated a curiosity, so.... here goes:

The point of a bolt is to hold things together. The point of torquing a bolt is to put it under tension so that it holds things together tightly. The point of torquing is not torque, it's tension. What we call torque wrenches, the Brits call tension wrenches. For maximum strength, you'd like to use the proper tension (torque) value.

If we were dealing with a steel tapped clamp and a steel bolt of similar steel, a rule of thumb is that a tapped hole of material similar to the bolt needs to be as deep as that bolt diameter to get maximum strength. That is, if the tapped hole is 1 diameter deep, adding more depth doesn't improve strength. It's a rule of thumb, but its often close. Put another way, a tapped hole or a nut will allow us to put the bolt in enough tension to hold things together in the strongest, most reliable way. If this were the case, you could use the torque spec for M6 bolts now installed, and not M5 and your setup would be stronger than original. Provided the other parts survived the extra tension.

But I suspect that the clamp is aluminum. If this is so, you probably don't want to use M6 steel bolt torque values. But even if the clamp were steel, it may not be thick enough to have enough threaded length to allow you to use M6 torque values. The threads might strip (again!). Further, even if the threaded section were steel and thick enough to allow sufficient threaded length, the M6 torqued tension might be high enough to break or bend the clamp or other parts of the bike.

So, let's assume we want the same tension force from the M6 as we got from the M5 so that we can hold the seat post and not break anything. I'm assuming standard and not fine M5 and M6. So the pitch is 0.8mm for M5 and 1mm for M6. The helix angle for each is arcsin(0.8/(pi * 5)) and arcsin (1/(pi*6)), or 2.91° and 3.04°. The mechanical advantages (force of torque to bolt tension) are the cotangents of these angles, or 19.61 and 18.82. For those of you playing the home game, I think that this means, to get the same clamping force (tension force) in the M6 you need to take the M5 torque and multiply it by 19.61/18.82, or 1.04 the M5 torque. So torquing it to the M5 spec gives you about the same tension. Math telling us that your initial inclination to torque to M5 values was spot on.

Now, the M6 bolt is gonna have 36/25ths more area than an M5 bolt. So the resulting stress in the M6 at this torque will be 1.04 * 25/36th which is about 28% lower than that in a torqued M5. In a heavy duty service (e.g. main bearing or con rod bolts on a nitro-fueled funny car) this would be a serious issue and would lead to failure. But it's a bike seatpost, so I think you're gonna be fine.

Going back to the point that if the tapped hole is less than 1 D you are not getting the full strength of the new bolt. I wouldn't torque the thing to M6 specs, but if you are Jonesin' to do that, or if you are a bit paranoid about the threads cut into the alloy seat post clamp, you might try putting a backing nut on a longer-than-usual M6 bolt. That is, install an M6 bolt that extends about a nut's width from the clamp. Put a nut of similar material as the bolt on the extension (use antiseize for Ti or Al or Stainless nuts and bolts). Start tightening up the bolt in the clamp, and nut on bolt alternately. Sneak up on the torque spec.

I kind of went through this because 1) you asked for knowledge, and 2) it amused me. But as others have pointed out, there's an awful lot of seat posts that were put together by inexperienced pimply faced kids who used a "torque by touch", otherwise known as a combination, wrench. The great majority don't fail.

Good luck.