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Problem description:
Aluminium seat post in aluminium frame (27.2). The clamp is nice and tight. But There is a certain length of the seat post that won't stay in the position for long - it always slides down to a height which is just below my preferred height and then it can stay there forever (it won't slide down any lower).
Is it an issue of the seat post or seat tube itself? 😕
If it's Thomson then it's the post...othersie you'll need to measure it with a micrometer.
I suspect the frame is reamed internally so the post is just hitting the internal step where the reaming ends and stopping.
You could try carbon assembly paste - it really does help.
based on what you have said- worn seat post.
New post or a KCNC clamp that grips the post and the frame seperately
I had the same on my CX bike with a Thomson post.
In the end i used a KCNC post grip and it solved the problem:
http://www.xcracer.com/shop/viewproduct.php?productid=351
Actually... I think I had an old BBB seat post somewhere on the loft! I'll give it a go, thanks! 8)
So, is it a Thomson?
Haha, no it isn't! 😉 It's a stock Merida one.