Seat post tolerance in a frame

captaincowelly

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ishaw

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So, have a 27.4 post on the way, it isn't the post I want to use but should at least help determine the size.

In the meantime I found a nice carbon post that might make the build. It seems to fit ok, slight wobble when the clamp isn't tightened but seems to fit ok once it is clamped. A few pics below of the slit in the seat tube pre and post clamp tightening.

Does this look ok?
 

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2manyoranges

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Hmmm....that still seems to be pinching the top of the tube together a bit more than I would normally expect. What diameter is marked on the carbon one?
 

d8mok

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It takes a 27.4

Thats the size the frame requires. There isn’t any question about that.

some posts don’t measure exactly 27.4 hence why my Thomson didn’t fit. The frame size shouldn’t be your issue it’s finding a post that’s a true 27.4

I gave up and stuck with the OE one
 

captaincowelly

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Agreed.Have had the odd seatpost that didn’t seem a great fit but that 0.2 makes a big difference.Think the coke can shim May have been mentioned?Worth a dabble.Few coats of lacquer on the post worked well for me on a recent carbon build,now a nice,snug fit.Bit of grip paste for added security.Don’t know if lacquer would make enough difference here.
Other option is having either a shim or an ally post machined to fit 27.4.Wouldn’t want to take any metal from a weight weenie post but something with a bit of meat on the bones wouldn’t be so bad (common sense applied).If Coke Can does the job then that and a dab of grip paste or machining a shim to 27.4 would be my first choice.If there’s no original posts about.
 

ishaw

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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.
 

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2manyoranges

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So.......one option is a USE black 25.4 post with a USE 27.4 shim. 27.4 metal shims currently on ebay - beautiful bit of machining. That would give you the ‘tight at the top loose at the bottom’ requirement.....oh oh another thought. Get a 27.2 silver post anodised black, or hard enamel coated.
 

ishaw

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Use post/shim I'm not keen on for this build, always found the result weedy looking.

The other route might be an option though if I can't find something that fits out if the box.
 

pete_mcc

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Thompson posts are well engineered and usually match their size exactly. Carbon posts are often undersized so a 27.4 carbon will likely be smaller than a 27.4 alloy.

To avoid all this you could get the frame reamed to an exact size. Enigma will ream a ti frame for a very reasonable price or....you could do what I did when I realised that my Steve Potts titanium frame needed a slight reaming and do it yourself. I used a 29mm flex-hone which takes off a tiny amount of material as long as you go slow and gradual. Not suitable for alloy frames but is great in titanium and steel and is really good for getting rid of burrs in frames. Just attach to a power drill and hone up and down the seat tube and it will polish out the tube and take a small amount of material (I should point out that Steve was happy for me to do this and it was recommended by various people on the frame building forums so it wasn‘t strictly a bodge)
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