Just wondering if anyone has a idea what 27.2mm dropper post is the most suitable to have the external diameter reduced to 26.8mm, so basically which one has the greatest wall thickness to start and will therefore hopefully retain the most strength.

Unless you have a particular reason to go that route (warranty?), it’ll be easier to get the seat tube reamed out.

TBH I think I’d be happier potentially wrecking a seat post rather than a frame. Also it’s for the little fella so not getting used in massively gnarly way. Also intend to pick up something second hand.

I’d be looking at a quick release and normal seatpost rather than reducing the thickness of a dropper post.

As above, much easier, and probably cheaper to get the seat tube reamed out.

Turning down a post requires a lathe + operator (unless you’ve got a lathe), and I know most decent machinists are pretty busy.

Whereas a lot of decent bike shops will have a reamer, and it’s a pretty quick job to ream out 0.2mm from a ST.

If you do insist on going the seatpost route, then one of those very simple KS ones that you have to manually pull back up would probably be best.

I did the lathe thing on a cheapo £60 cable operated thing off ebay. RSP perhaps? Not snapped yet. It was, and still is, a cheap nasty clunky post and the bar lever is terrible.