Hi, i just got a miyata 414 from the 80's with triple butted spline tubing. i need a new seatpost for it and currently have no way of measuring it. anyone know what size i need?

the Miyata 310 is 26.8 , not sure if that is your size, but I'm sure someone will need the info some time.

so is the 912.

My Miyata Pista is also 26.8.

Also, if you take out your seatpost, the size should be stamped on the bottom of the post. You may have to clean it up a bit and hold it up to a light to see it.

Take it to your LBS. They should have either a set of calipers (nice ones) or a stepped seat-tube measuring... thing. Either way, your LBS should be able to tell you what kind you need. While you're there, cough up the 15 bucks for a cheap Kalloy post. They come in just able any size you'll need, are light enough (esp for a conversion), adjust fine, and are cheap. All you need for a conversion.