Hi ya,

I am looking for an extra seat post. The spec sheet for my Brodie Dos says 27.2 but turns out that it can only accept 26.4mm which the current seatpost is. (man that sounds like Yoda-speak). Anyway I tried jamming in a 27.2mm in because that's what Brodie spec listed dammit! Took quite some effort to push it in. The hole was a bit small despite lubing it. (man that sounds like...nevermind). 1" into the process my sensibility kicked in didn't want to go further.
I'm on the hunt for smaller diameter seatpost. I have a choice of either 26.8mm or 25.4mm. What's your advise?

If you can't get the correct diameter, use the largest post and smallest shim; combo available.

Would first measure what it is now, as if you've stuck the wrong size seatpost in, you may have done some damage to the frame.

Measuring is tricky, since an out-of-round seat tube may not fit a post whose diameter, when measured by calipers, should, theoretically fit. https://en.wikipedia.org/wiki/Curve_of_constant_width

There are sizing gauges available, like this one, which will give a more accurate measurement: Seat Post Sizing Gauge | Weldtite If you can find a local shop which has one this would be the best approach.

Remember, when all else fails, there's the Thomson Elite. Comes in 25.0, 25.4, 26.6, 26.8, 27.0, 27.2, 27.4, 28.6, 29.0, 29.4, 29.8, 30.0, 30.6, 30.8, 30.9, 31.4, 31.6, and 32.4mm. And it's a fantastic seatpost to boot.

This.

Don't cobble your bike. If you have a seat post that fits, check out the number stamped on it's bottom and replicate that precisely. The correct size is out there, you just have to find it.

seat post sizing shims use a very common 25.4 inner diameter seat post and outside diameters of all seat tubes in 0.2mm increments

your LBS will measure your frame and order the sizing shim that is appropriate..

next jump is a 27.2 seat post and sizing shims upward from there ..

I have 2 CC 'Thudbuster' seatposts in oversize seat tubes using sizing shims to fit them.

NB: a thicker tube wall will have a smaller ID, a 28.6/1.125" seat tube will be thinner tube wall on top , to accept a 27.2 seat post .
0.7mm to be specific .

If you require setback the smallest diameter available is 26.8. The only smaller quality option I know of is a 25.4 enve composites post with a shim to fit. Something about shimming an enve seatpost rubs me wrong though. If you just want a post, Giant sells a myriad of sizes. The post quality is fine, but they're not the lightest ever made.

if you dont want to spend up to Thomson levels , Kalloy makes seat posts with a set back head in All Diameters needed..

all distributors to LBS can supply them . >That includes 26.2mm..

Or, as I said, a 25.4 seat post and a 25.4 to 26.2 sizing shim..
that is a .4 mm shim thickness (1/2 the diameter difference of 0.8)

I suspect Brodie changed the seat tube spec, or you have a lesser model frame than you thought you did.

How far does the 27.2 go in? Past the slot? Is the 26.2 really loose?

I've got a bike that came without a seatpost. 27.2 was really tight. 27.0 just slid right in.

Anyway, if the 27.2 is close, I'd try either the 26.8 or 27.0.

Is it possible that a 26.2 diameter post was incorrectly installed in the bike? I tend to follow, not disregard, manufacturer's specifications until all possible alternative causes for a problem are explained.

If your bike did have a 26.2 diameter post incorrectly installed, then the top of the seat tube might be crushed. That would explain why a new 27.2 post won't go in. The very fact that you got it in 1" suggests that the bike was made for that size post. Either that or you hammered in the new post with such force that it spread the top of the seat tube. If that's the case, you've got more work to do.

Along those Lines .. Is the slot in the frame still having parallel sides? or is it narrower at the top than the bottom of the slot?
the latter would indicate it was gripping an Undersized seatPost.

wow, I didn't expect this to be so complex! Appreciate all the replies.
I am not sure why Brodie supplied the bike with a 26.4mm seatpost. It did come with a Brodie shim. The current post slides in effortlessly. The seat tube diameter is 27mm. Mfg variation? Who knows.
I'm gonna get 26.8mm seat post to try at the LBS. I think 25.4mm is too small.

Everyone is acting like this is a rare size. It isn't. Your local bike shop should be able to order a Kalloy for you or you can find several just by doing a Google search of "26.4mm seatpost". Neither is very expensive.

Get the right seatpost and don't try to jam one that is way too large into the seat tube.

IF you accurately measured the seat Tube Internal diameter to be 27.0mm then 26.8 will be too small By 0.2mm .

If you cannot accurately measure go to a Bike Shop and seek Help.

x2...I have several Kalloy seatposts lying around @ 27.2 and down in 0.2 increments and a 25.4. Great for the occasional odd size frustration tempting to shim or try to force fit.

I would contact Brodie and ask the question.

Brodie Bikes