[Closed] Seat tube and seat post fit - a tale of tolerance...

I have a Ti road frame (Van Nicholas Yukon) with a 27.2 mm seat tube. I've been using a Thomson post in it for a few years without issue, and another alloy post before that. In my wisdom (!), at the start of December I decided to order a Van Nicholas Ti seat post. It arrived, eventually, but that's another story.

Anyway, this morning I go to fit the new post into the frame and it won't go in beyond 7 cm, at which point it's stuck. It took some considerable effort to get the post back out and left the bottom of the post scored. I tried again in vain with the same result.

So, after much swearing I picked up a set of Vernier Calipers this afternoon. They were only £20 so not super-accurate but I thought they may be useful. I set about measuring both the Thomson and VN posts.

Neither post was perfectly round, with both displaying ovalisation of up to 0.1mm. The Thomson post was under-sized by up to 0.1 mm with measurements taken along the length and across 2 axes. Similarly, the VN post was generally over-sized with measurements up to 0.1 mm over the 27.2 mm stated diameter.

The Thomson post has never slipped during use so seems to be a good fit.

I tried rubbing the inside of the seat tube with wet and dry to remove any contaminants but this didn't help. As I don't want to get the seat tube reamed, I guess I'll just have to stick with the Thomson post... I'm fairly sure that VN won't take the post back as it's now heavily marked.

This got me thinking that maybe, as the Thomson post is undersized, the tightening of the seat clamp has pinched the top of the seat tube a little thus preventing the VN post from fitting. I would have thought though that this would only affect the top of the seat tube? As stated, the VN post won't go in beyond 7 cm, with the first 5 cm going in relatively easily.

Confused. 😕

I await a response from VN.