Bicycle Mechanics - Seat Post Widths: Need Advice

I'm replacing the seat post on an old Atala Grand Prix.

When I put the calipers across the seat tube hole, it reads [avg.] 26.175mm. I'm looking at some Nashbar seat posts I like that come in 26.0 mm or 26.2 mm. Which one should I get?

Put another way, and remembering always I'm a rank amateur. . .

How would I ream the hole bigger for the 26.2mm post? or

How would I shim the 26.0mm post to fit?

Thanks again. . .you folks are great!

No takers on my questions?

Do you have the seatpost that you are replacing? Most are marked with the width, though often you may have to do some cleaning and polishing to find it. Beyond that, I'm not much help and don't want to say what I would do in my uneducated opinion and lead you down the wrong path.

Do you have the seatpost that you are replacing? Most are marked with the width, though often you may have to do some cleaning and polishing to find it. Beyond that, I'm not much help and don't want to say what I would do in my uneducated opinion and lead you down the wrong path.

Or measure the old seat post. If you don't have the post, I'd probably go with the 26.0 since it's easier to add a shim then to ream the seat tube. The binder bolt might take up the 0.2 mm difference but I doubt it. You could be lucky and I could be wrong but...

You could get the 26.0 and use a beer can shim. That's what I did for a 26.2 Peugeot frame I have.

Here's a pic.