Is it possible to increase maximum seatpost insertion length by filing down a seat tube bottle cage mount boss? I have an aluminum MTB hardtail frame with a pair of standard bottle cage holes on the seat tube.
I want to insert a seatpost fairly deep into the frame sometimes, so that the saddle stays low. The limitation of my frame's maximum insertion depth is the top hole position. The bottle cage boss has a part of its fixture protruding into the tube. Thus, while theoretically there is about 20 or more centimeters of the seat tube left down to the bottom bracket, the post can only go so far.

With my current seatpost, I solved this problem by sawing off several centimeters of it. This way, I could have the saddle as low as I wanted it to be. I had to account for the change of minimum insertion depth of course, as the part of the post was not there anymore; I was OK with that. Yet, I want to keep the currently achieved minimum saddle height, or even decrease it a bit.

I am now waiting for a new seatpost to be delivered, and it is several centimeters longer than my current (sawed-off) one. Unfortunately, I cannot shorten it, because it is a dropper; apparently, the trick with a saw will not work on it.

I am thinking about approaching this issue from a different angle. I do not use that particular bottle cage mount. If I manage to remove it, or at least its internal protruding part, I will have enough additional centimeters of clear straight tube (until the second, lower bottle hole) to stick my new dropper post in.

This idea spawns a number of questions.


Is it possible to achieve at all?


Straightforward drilling of the down tube from the outside of the mount hole and making a bigger hole to get rid of the fixture sounds like an obvious but very dangerous option. The frame strength will suffer.
Instead, a careful filing of the mount point inside the tube sounds safer. But it can only be done by reaching from the top of the down tube (with a post removed) with a very long file, or a similar instrument. I do not know if such tools are available to bike shops.

Is it reasonable at all? Is that internal part of a bottle mount supposed to bear significant mechanical load?
I cannot imagine I am the first one to face this problem and to imagine such a solution, but has anyone ever tried it? Are there any documented attempts of such (un)successful operation? Maybe on a steel frame (but even I cannot dream up trying that on carbon stuff).
Drilling the seat tube is a bad idea, for the reasons you've indicated. Very rarely a bike shop will have a tool for reaming a seat tube (more usually framebuilders have those) that can to the task in a slightly safer way.

BUT from my experience with a welded steel frame where I didn't bother to back-purge (lesson learned!), even once you weld an extension onto the end of a half-round file, it's a tedious and difficult job to file down irregularities inside the seat tube. You're trying to apply pressure half a metre away from the part you're holding, and that's hard to do. You need a long, strong extension on that file. This is why framebuilders have seat tube reaming tools (which are expensive).

A reaming tool will probably not help you, because that boss is a single, hard obstacle and will likely damage the tool if you can even get it into position to cut the boss. If you do get it in there it's as likely to cut the opposite side of the seat tube away as cut the boss.

A better approach would be to drill out the boss. It's likely that the boss is actually a separate assemble riveted into the frame - they drill a hole, insert the boss, then compress it so it clamps into place. If you can drill out enough of the boss it will fall out, leaving you the original hole in your frame.

Which is the real problem with this approach - at best you'll end up with a hole in your seat tube, and the BB will fill up with water. You can out tape over the hole, and replace the tape when it starts to fail, but it's not ideal. Welding a cover plate on will reverse the heat treatment of the aluminium, leaving a soft spot in the tube. Soldering a patch on would be better, but soldering aluminium is hard (it's an excellent heat conductor!)

The extra stress from the end of the post should be minimal, most seat tubes are actually butted so that minor bends or dents to the tube don't lock the seatpost in place. You'll find the top 100mm or so is tight on the post, after that you have 0.1mm or so space around the post.