adkein

Hi all. Pretty sure this issue isn't addressed elsewhere: the seatpost bolt (the screw that secures the seatpost in the frame) broke on a Casati Ellisse I acquired a year ago. A local bike shop replaced it with a too-large bolt with a washer tucked in to make up the difference. But however hard I tighten it, the seat gradually lowers over the course of a 20' ride. I suspect that what I need is the proper size bolt, but I have been unsuccessful finding what that size is, let alone finding such a bolt. And the shops I've taken this problem to also have not been of help in getting the right bolt. Any suggestions? I'll be grateful for any input.

FBinNY 

I suspect that your real problem isn't the bolt itself, but what caused the other to break in the first place.

Odds are the post doesn't fit properly and the bolt needs to be over-tightened to hold the post. By now you may have a worse problem in that over tightening may have bent the ears so they now touch in the back, limiting how tight the tube can close.

Start by taking a look at the ears to see if they touch, and also the slot to see how far it closes. Most seat tube slots are about 3mm or 1/8" wide before clamping. If they close so the width at the top is less than half the width at the bottom than odds are the post is too small. If the slot width looks OK but the ears touch, then check for cracking where the ears join the tube, or for bending. Ultimately these problems would need to be fixed before any bolt can solve the slippage problem.

In the meantime, especially if the problem is only marginal, you can improve the grip with lower clamping force by using some coarse lapping compound on the post in lieu of grease. Lapping compound is basically abrasive grit in a grease base, and when compressed between two parts will bite into each creating a strong mechanical interlock that only needs minimal pressure to keep from slipping.

adkein

Thanks so much FBinNY. Indeed the ears do touch when the bolt is tightened, but they are nearly parallel when the bolt is loosened. Might this indicate that the damage is not irreversible? That the seat post might not be the right size is certainly possible, because I bought the bike used and other parts of it have since proved to be mismatched. I suppose that the next step is to go get the proper size seat post.

HillRider

+1 Absolutely a must-do.

FBinNY 

The tipoff of an undersized post is the width difference between the top of the slot and the bottom (which doesn't close when the post is tightened). If the top of the slot closes more than 1/16" (or about half way) then odds are the post is too small.

This also tells you if it's a post size question, or that the ears are bending, or breaking away from the tube. Ears touching before the slot almost closes is an ear issue, and may not resolve even if you use a larger post. Sometimes if you need only a bit more you can file between the ears, so they have more room to close, but all to often this just make it possible to break open a crack that you didn't see forming.

Once you confirm the right size post (the biggest that will slide more than 3" into the seat tube is the right one) You can get a better hold with less stress on the pinch bolt and ears by using coarse lapping compound. The grit in lapping compound bites into both the post and tube locking together with very low compression force.

ultraman6970

That bike uses a 27.2 seatpost. Some bikes can get away with 27, other ones dont, it depends. In your case i agree the problem might be the wrong seatpost, did you measure it using a caliper??? If the seatpost is wrong stress finally made the bolt to snap apparently. If the ears are almost touching probably you have a 26.8, in that case i cant even imagine how is that the clamp is still graving it.

As for the bolt, do you have a picture of the lug to see what it is?? maybe is a regular campagnolo design?? (seatpost binder)... maybe the threads are at the other side of the lug? if thats the case for sure is an M5, if the shop coudlnt figure it out how to dremel and cut a regular M5 to fit in there I would advice you to learn to do the stuff yourself and dont ever comeback there, sloppy work you know.

Good luck.

whatbrakes

I'm surprised the shop didn't point out the wrong size post. Not a very helpful bunch I gather?

adkein

So, it took me a long while, but I investigated the seat post size. ultraman6970 is correct that 27.2 is the right size and in fact that is the size of the current seat post, ruling out incorrect seat post size as the problem. Here is a photo of the lug, as requested. Obviously the ears are close to touching. In my non-expert opinion, I'm looking at the touching ears as a likely culprit for the post slippage at this point. Happy to hear any other opinions, or suggestions for how, if possible, to repair such bent ears. I will probably end up trying the coarse lapping compound suggested by FBinNY (thanks!), though in the long run I would prefer a more permanent solution. In any event, thanks everyone for your help so far. Now that the summer is here I'm looking forward to resolving this and getting back on the road!

FBinNY 

When the ears are close to touching with the correct post, it means that the ears are collapsing from over-tightening or, on Tig welded frames , that the non-reinforced seat tube is cracking where the ears attach.

As long as the ears don't actually touch, you can still clamp the post, but a process has likely started and the ears (or tube) don't have far to go before breaking. You'll need to be extra cautious about clamping force, so the coarse lapping compound may be the only way you'll get decent hold without finishing off the ears.

If the ears still look passable but touch, you can buy yourself a bit more room to tighten by carefully filing between them to improve clearance where they touch.

Ears collapsing was fairly common with over-tightening on many steel frames, and the remedy was to form them back to the original shape, and fill or reinforce them with braze material.

On modern tig frames where the seat tube extends 1/2" above the top of the weld fillet, I suggest sawing off the ears, cleaning up the last vestiges down to the tube with a file, and using a collar. Note that this requires a usable 1/2" (or very close to that) of clean circular seat tube above the weld. Do not saw off the ears if you don't have that.