Seat post tolerance in a frame

[Woz]

So the 27.4 trial post arrived today and it is very tight. Fits in the top fine but the further it goes the tighter it gets, I have to push pretty hard to get it down, and I was worried it would get stuck. It could nearly be ridden without the clamp being tightened, not had a post this tight in a frame.

Maybe this is why there is conflicting info on the correct size post online, as many claim it takes a 27.2 post, while others claim 27.4.

Heres a pic of the 27.4 post fitted.

The build requires a nice black post so its going to be a real pain to find one by the looks of it.

The carbon post i tried fitted better than another 27.2 post, I do have some others so maybe I'll find one that is really 27.3 ish?

Anyone found a post that does fit aside from the oe one, as I don't have that.

So, what is the precise measurement of this trial post?

 

[ishaw]

That's an interesting option, thanks.

I have measured the posts tried so far and the 27.4 is a true 27.4, the alloy 27.2 varied between 27.1 and 2, the 27.2 carbon post was 27.2/3.

Basically I think this means I need a seat tube size that was never made/sold.

My vernier only have 1 decimal place so can't tell the exact size.

 

[FluffyChicken]

I just stumbled across a picture of mine when I last did a seatpost fitting

You can but a specialist rod with the sizes machined in, or you can use random shims you've not notice you have amassed over the year on Use seatpost.

 

[d8mok]

Buy a pair of £5 digital calipers and measure the posts.

even a smidge over 27.4 won’t fit. That’s what I’ve been saying since the start. It needs to be dead on 27.4 to fit assuming it’s not got the shim that some lightnings had in which case it’s 27.2.

 

[captaincowelly]

Is that top one (in the pic) a Mt. Zefal pump?Great weight weenie option if they do one in 27.4!Early dropper.

 

[2manyoranges]

Hmm you have said clearly that the true 27.4 binds lower in the tube. That suggests that there may be some heat distortion from welding, but which GT didn’t much care about, and whacked a post in. Not unfamiliar. As much as you hate USE, that may be the best approach without molesting the frame. The current 27.4 shims are deep and well machined. But it may be worth talking to Enigma or another Ti fabricator (Stanton?) and get their take....

 

[integerspin]

It's a long time since I measured a post, all the ones I measured were something like .08 under.
I measured a few frames and they were all oval, averaged around the nominal diam[well a
marin frame I measured was almost a .1mm larger than expected].
That is measuring where the stays and top tube are welded to the seat tube.

 

[d8mok]

Hmm you have said clearly that the true 27.4 binds lower in the tube. That suggests that there may be some heat distortion from welding, but which GT didn’t much care about, and whacked a post in. Not unfamiliar. As much as you hate USE, that may be the best approach without molesting the frame. The current 27.4 shims are deep and well machined. But it may be worth talking to Enigma or another Ti fabricator (Stanton?) and get their take....

I read it as his calipers don’t read past one decimal point. So the post could be 27.45 for example. For mine it had to be 27.40 on the nose.

 

[ishaw]

Yes, only have 1 decimal place on the callipers so could be over 27.4. Might have to invest in some better ones, plus contemplate a use post/shim affair if I can't find a nice black or ti post that fits.

 

[Jesper]

I have had many problems with fitting posts into frames after original posts have been swapped out for possibly the incorrect post size (too small), or a post that has a slightly smaller O.D. than spec'd either due to wear (multiple insertions/removals), or intentionally to fit a seat tube that may have had the clamp dimension at the point of insertion changed due to use of a smaller post being used and misshaping the clamping area causing the specified post size to fit too tightly. Usually after getting the clamp back into a more circular shape the specified post size fits as it should. I have used shims for fitting the next size smaller post without having any issues. Grease on shim and post, no slippage when properly tightened, but not being able to tighten to a point where the clamp slot appears to get narrower at the top indicating that the post and/or shim is not of sufficient girth to fit without potentially affecting the integrity of the frame/seat cluster. I have a 27.4mm Campagnolo Record post, but I cannot recall why I have it since the biggest seat post I.D. for my steel frames is 27.2mm; possibly needed for a reamed out tube. It would not be a cheap if someone desired it, but it certainly would not be over 75 pounds; I would have to look at what the market is asking since it was bought many years ago. PM me if you are interested.