OK, this is very likely a stoopid question, but my seat keeps slipping down to a certain point (which is too low) even though I'm tightening the quick release hex screw tight enough so it's not quick release any more. So I'm figuring the seat post has become narrowed over the last couple of years. So time to replace.

I'm assuming you buy seat posts according to the external diameter of the post, but you seem to be able to buy them with a variance of a fraction of a millimeter EXAMPLE – 26.4 400mm – the first measurement I assume is the diameter. To me it measure about 27 mm diameter. How accurate does this need to be? is there a standard size?

Also, for the quick release collar, EXAMPLE – I'm assuming you should measure the external diameter of the frame? Again, there seems to be lots of different sizes. Not as standard as I'd hoped.

yep. it needs to be right. they normally come in .2 increments, so your 27 maybe 26.8,27,27.2 etc

get it wrong and it won't work right

The most common seat post diameter in that ball park would be 27.2mm. 30.9mm and 31.8mm are also fairly common. What frame (make, model, year) do you have?

Just google the frame & seat post size and it'll tell you what size you need.

BTW there's tens of different diameters.

Thanks good idea. I'll google the frame and try and find out what the spec says.

It's a Scott Aspect 20 – 2009

Also, due to manufacturing tolerances, you may find that the "correct size" post may not be a perfect fit, we've sometimes cured a slipping 27.2 post by fitting a 27.4 instead, the larger size being a nice sliding fit, pretty much as the original should have been.

You could try reading the size that is printed on the side of the post.

If all else fails, try using some of the carbon mounting grease whether your post is carbon or not. It has special grit (I'm sure there is a better description, but..) in it that supposedly means less slip and/or less clamping force (and it's cheaper than a new seat post)

You could try reading the size that is printed on the side of the post

I could do, but there is no measurement printed anywhere on the post. They may have been once but if there was it's rubbed off now. Thanks for the contribution though.

Some seatpost tolerances aren't particularly good either, so it could be an undersized post.

what frame is it, and is the standard post and collar?

You sure there's no size marked on it? It should be stamped on the post near the bottom, it generally can't rub off.

Aluminium Scott MTBs are normally 31.6 (not 31.8, that's usually a seatclamp/front mech size), I'm not aware of them doing an 27.0 frames, that was traditionally a Marin size, so my guess would be a 27.2, pop to your LBS, and ask to try a different post in there, just take one out of one of their bikes, virtually all road bikes will be 27.2.

Even if it has the size on it I wouldn't rush out to to replace it. It's slipping so it could be the wrong size.

It could be a crap clamp, rather than the post, mind.

Or a broken clamp, I've just replaced mine as the little plastic washer under the QR had cracked, and no amount of tightening would hold the post properly. Bought a Salsa one, and that seems to have done the job.