cqlink 

I picked up an old Jetter bike for $3.00 because I liked the paint. Trying to find a seat tube.

Seat tube OD = 32.9m or 1.295"
Seat tube ID = 25.4mm or 1.0"

What diameter seat post am I chasing?

Thanks!

zukahn1 

If it's an older high ten frame set the seat post size is mostly likely 25.4mm which was the most common size on these. This seems to match your measurements if there's a coop or friendly bike shop in your area you can get them to verify the size and pic up a basic post for $5-10.

T-Mar

I wouldn't even consider that hi-tensile steel. 3.75mm is an incredibly thick tubing wall, unless the OP is advertently measuring the lug outer diameter, as opposed to the seat tube outer diameter. Normally, there would be about 0.2mm diamterical clearance between the tube and post,suggesting a 25.2mm post. However, that would be rare. More likely, it it uses the very common 25.4mm and the seat tube is measauring slightly under it's true nominal diameter due to distortion, pinching, corrosion, etc.

cqlink 

Thanks!