26.4 vs 26.6 – how can you tell?

[danc]

This is probably a classic dumb question.

I need to get a seat post for a cheapo Fausto Coppi frame I’m putting together for the winter. With a ruler, it looks to my eyes like the seat tube is 26.5mm. I don’t have either a 26.4 or a 26.6 post to test it with. Does anyone here have a neat trick for getting a more accurate measurement? Equally, any views on which side it’s better to err?

D.

 

[ScillySuffolk]

Vernier calipers will give you a more accurate measurement, but 0.2mm is easy to misread and you could easily get both measurements on an old tube (in my experience).

The only way (I know of) to be sure is to actually fit a post.

I've just this evening taken the plunge with a 26.4mm Selcoff: if it doesn't fit then I'm happy to let you have it on a sale-or-return basis or even just a trial basis: I'll keep you posted.

 

[Robbied196]

You could use a piece of paper to measure the circumference although there's only 1/2mm difference so you'll have to be accurate. If you cut the length so that the two ends meet up perfectly then measure the length of paper. A 26.4mm post will be 82.93mm a 26.6mm will be 83.56mm.

 

[FluffyChicken]

50/50 chance there

BUT it'll only tighten up smaller, not larger*.
You need to measure inside, not the clamping part.
So I would err on 26.4mm



* assuming a too small post hasn't previously been used and the clamping part is now slightly bent, leading to a larger post being needed and/or two clamping marks down the seatpost .

 

[danc]

Thanks all - much appreciated.

 

[biggs682]

be carefull as trying to squeaze a 26.6 into a 26.4 could soon lead to a very stuck seat post , so try smaller size first .