How to find seatpost size?

[phantasmagoriana]

The seatpost on my commuter/hack bike is looking decidedly manky (lots of surface rust - it's not seized though!) so I'd like to replace it with a nice alloy one. However it's not marked with a size, and I don't have any calipers. Is there an easy way of determining the correct size? I'm not sure whether measuring the old post with a ruler would be accurate enough. It's an old Raleigh Pioneer Elite, Reynolds 501 tubing (just like this, except smaller).

Thanks in advance!

 

[HLaB]

Not exactly scientific but you could try google if you know the year of manufacture.

 

[Gerry Attrick]

This site gives the size as 27.2mm.

 

[dellzeqq]

any decent bike shop will have a calibrated cone thingy.

 

[youngoldbloke]

You can always use a large adjustable spanner in the same way as calipers and then measure the gap with a ruler - not as accurate but will give you a good indication of the diameter, especially if you use a good steel rule.

 

[tyred]

The only sensible thing is to take the seatpost out and take it to a bike shop and ask for a replacement.

 

[Manonabike]

You can always use a large adjustable spanner in the same way as calipers and then measure the gap with a ruler - not as accurate but will give you a good indication of the diameter, especially if you use a good steel rule.

Not accurate but I like your idea.

Another idea, not accurate either but it will also give you an idea of what to look for:

Measure the circumference of the post and work out the diameter ( d = c / pi) --> d = circumference / 3.142

 

[phantasmagoriana]

Thanks for the suggestions - looks like the best thing to do is take it to the LBS. Knowing my luck, it's probably some obscure size!

 

[mattsccm]

I have a sneaking suspicion that it may not be 27.2 . 531 is but I think 501 has slighly thicker walls. 27.0 comes to mind and indeed somewhere on the web today I saw a post which confirmed that

 

[Gerry Attrick]

My old pub bike (Dawes Shadow) has a 26.6mm seatpost. You must measure to be sure.

 

[BikeCurious]

Or you could get a cheap set of digital callipers off eBay for about £10. Will come in handy next time you want to measure something. Plus you will probably end up saving money by not buying the wrong post that's 0.1mm out.

 

[Manonabike]

Or you could get a cheap set of digital callipers off eBay for about £10. Will come in handy next time you want to measure something. Plus you will probably end up saving money by not buying the wrong post that's 0.1mm out.

I read somewhere that you can be up to 0.2mm out.

 

[Zoiders]

It's fairly lumpy tube set so you will get away with it being a bit under, worst case scenario is a bit of coke can to shim it if it's truly too small.

501 is posh talk for bog standard 4130 chromoly that turns up in all sorts of weird non standards so it could be anything from 25-27.2, my money is it being between 26.6 - 27 though given the age.

You won't break the frame if it's .2 mm out, it's not that kind of frame, it's a hack and a beater with heavy duty welds not some hand built coke can thin race job in some fragile steel alloy.

Don't worry about it.