I have a 26.4mm seat post and need one that's around 25.6mm. I know that sanding off .2mm is commonly done, but would it be possible to sand down .8mm? I realize that I could get another seat post for relatively cheap, but I still want to know if sanding down this post is possible and safe.

25.6 mm probably isn't right. Kalloy has 25.4 and 25.8 mm.

The straight ones without the clamp on top only sell for around $10.00. You're talking about a lot of work and still having to wonder if it's safe just to save $10.00. I wouldn't do it.

I have a seatpost from an old Schwinn mountain bike that is clearly marked 25.6mm, and that one seems to fit pretty well. I also have a 25.8mm that seems to tight. And like I said before, I realize that a new seatpost can be purchased rather cheaply- i just wanted to know if sanding down that much could be done, and if anyone had done it.

I made a seat post for a Sport III dumpster rescue with a piece of steel pipe and a bench grinder. Roll pipe on grinder, test fit and repete till it fits.....took less than 5 minutes. Follow with a shot of rattle can black, then dig through parts crate for a seat. Smile every time i walk by the old girl. One cool thing is the post will never drop all the way down into the tube if you don"t reduce the entire length.

I would say that sanding down 0.8 mm is a bit excessive. Measure the seat post wall thinness and see how much of that is 0.4 mm.
How are you planning to do that, anyway? If sandpaper, then by all means, try it! You will lose interest in a couple of minutes.
In any case, the structural integrity will be reduced. However, this may be not a problem, since critical failures will not cause too much damage.

On a lathe, it would take a few minutes. By hand, you'll be there for days and you'll wind up with a wavy, uneven post that may not even clamp properly in the frame. Just buy the right size post (https://www.niagaracycle.com/product_...ducts_id=15842) or use a 25.4mm post and shim it to fit.

Here you go! https://www.yellowjersey.org/frogstem.html
all the intermediate sizes 25.4mm
25.6mm
25.8mm
26.0mm
26.2mm
26.4mm
26.6mm
26.8mm
27.0mm
besides 27.2
and all the modern diameters
and a few vintage posts

I turned a seatpost down using a lathe once. I don't recommend the process. Simply not worth the trouble. Sanding one down by hand would be futile, especially taking that much material off. The end result is going to be any thing but round or strong.

1. How thin is the seatpost wall now, how much do you weigh, and how much seat post extension do you need? From an engineering standpoint, if you are lightweight, don't have much seat post extension, and the 0.4mm that you take off isn't too big a percentage of the seat post wall thickness then you might be able to get away with it.
2. If you do this of course you are on your own. Also from an engineering standpoint- if you sand it down you should use very fine sandpaper for the final bit of sanding. You don't want any deep scratches in the surface of the seat post when you are finished. The finish should be as smooth or smoother than what the original seat post had. If you have any significant scratches they can act as stress risers and aggravate the initiation of fatigue cracks. In college I tested large steel bolts in fatigue and different patterns of threads made a big difference in the fatigue life of a bolt. The guys that build racing engines pay similar attention to parts such as connecting rods and crankshafts.

Sudden failure of a seatpost can be very critical if it catches the rider by surprise. It can result in a hard landing on the broken remains of the post or on the rapidly moving rear tire. Either one can have painful and severe consequences:
https://forums.roadbikereview.com/showthread.php?t=72813

Complete waste of time.

I agree. Just get the right size seatpost. Lordy!

Removing .4mm is not going to significantly weaken the post. I use a Porter-Cable pad sander to speed the process. Constantly turn the post to keep it round. I finish with finer emery paper, then 0000 steel wool and then buff on a wheel with tripoli for a mirror finish. The entire project should take about an hour. It's up to you to decide if it's worth it.

I've seen hand-sanded posts come out like Dali works of art with not a straight side anywhere. I've used a lathe numerous times and it comes out A LOT better. You can really tell the difference when inserting a dry post into the bike. You'll find that the hand-sanded post barely makes any contact, primarily 3-5 spots and that's it (from the old-grease). The spun one will be perfectly even.

Thanks for all the great info. I still haven't decided if I'm going to sand the post down, but your opinions are interesting- although varied enough that it hasn't made my choice any easier

You can judge the number of contact points just by sliding the sliding the post in? I'm impressed!

My sanded posts are not lumpy.

Sure can, the old grease and dirt inside the seat-tube sticks to the newly sanded post and shows the uneven surface easily. Most people who sand down their post don't bother getting the surface better than an AR 100 finish, so you can't see waves. I've placed those posts on my mill and barely skim the surface. What do you know? A couple of high-spots shows right up and the rest of the post is complete untouched.

Besides, I takes me less than 5 minutes to create a CAMworks batch file and chuck the post into the lathe, so I don't bother with sanding:

How much money do you make per hour?