I recently bought a frame on craigslist that came with a seatpost already installed. Unfortunately, despite being 0.2mm too large, the seatpost had been rammed in there by the previous owner. I've sanded down a stem before so I'm game for that, but it's something like 350mm long...I'd like to cut it down so I only have to sand a smaller portion. In determining the new minimum insertion length could I simply look at other aluminium seatposts I have and use their min. insertion length as a proxy?

Also, if I decide I don't want to do this, what sort of shop would I go to to shave off a couple millimeters from a post?

Thanks.

I wouldn't, and you shouldn't, either. Unless this is some rare, valuable item, just buy a cheap seatpost.

A couple tenths of a millimeter it sounds like. Just buy a new post, turning down a aluminum seatpost is going to be more expensive than a replacement.

The minimum for posts nowadays is 4 inches or 100mm but that doesn't really tell the whole story since seattubes often extend above the toptube. I like 100mm but also making sure the bottom of the post is at least 50mm under the lowest welded joint at the seattube/toptube/seatstay junction.

My son's bike had a similar problem, but I discovered it was limited to the area around the top of the seat tube -- at some point the bike must have had an undersized post. So we spread it back out as best we could, smoothed out the top portion of the seat tube with a round file, and the correct post fit perfectly. I have an old fashioned inside caliper that I can use to check the size and consistency of seat tubes.

TL;DR: Make sure it's really the post. Maybe you've already checked the specs for the frame, in which case you can ignore this.

Very helpful responses, thanks! I'm going to cut it down with 100mm in mind. If anyone has any tips to make the sanding go faster I'm all ears, lol.

bottom end of seat post, below the bottom side of the top tube , where it joins the seat tube.
is a a good minimum..

I'll get it at least past the lug then.

Put a starnut or compression plug into the bottom, screw in a M5 screw, then using an electric drill with M4 bit, spin seatpost while holding sandpaper wrapped around. I've seen this done before in increments of a minute at a time. I'm not sure how long it took as I went home while they were still fiddlin' with it.

The rule of thumb for insertion depth is 2-1/2 times the diameter of the post.

Where can I buy a seatpost .2mm larger than needed?

You'd be better off all around if you just got one of the proper size.

...IME reducing the diameter of any alloy component like a stem or post goes pretty quickly, as long as you have already discarded the excess length on the bottom of the post. I woiuld be more like one of those "get the proper size" people on this one, though. I take a certain amount of comfort in having fouir or six inches of post down below the top tube juncture, because I'm a large person and the seat tube/post interface on my bikes takes a lot of weight bouncing around up there on top. I want a good, solid, slip fit for some distance along the post/frame tube interface. Not sure I can do that on something like a seatpost (turning it down accurately along the length of it.).

Only reason I'd ever cut a seat post is, if it hits the water bottle boss
penetrating the lower - middle end of the seat tube,
before being down far enough, For fit length, saddle height..


....

Suppose that's true, slimming down a seatpost by hand is a bit different than a stem in how they stay in place. Hmmm. I guess I ought to just buy a cheap new one.