Old 10-03-20, 03:02 PM
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That cinch slot is severely pinched. The correct size post is significantly larger, by about 0.5mm.

The inside of the steering column is severely corroded, which is fairly typical, given the age. However, you can see the base of the five ridges, suggesting that it's a Columbus steering column. Reynolds steering columns did not employ stiffening ridges. Prior to 1984, the only Columbus road tubesets that used this steering column were Columbus SL and Columbus SP. They used 27.2mm and 27.0mm posts respectively. If you remove the fork (you should overhaul the headset anyways), you should find the Columbus dove logo stamped into the outside of the steering column. Columbus SL/SP is what would normally be expected on frame of this nationality, era and feature level. Given the small frame size, Columbus SL would be typical.

Based on this, it is almost certainly imperial tubing with the seat post having a 28.6mm outer diameter. If you want, this can be verified with reasonable accuracy without a precision caliper.

1. Cut a strip of paper, approximately 1cm x 10cm.
2, Wrap it tightly around the outside of a clean, unmarred section on the frame's seat tube, so that it overlaps itself with the top and bottom edges aligned.
3. Mark where it overlaps itself with a very sharp pencil.
4. Remove the paper and measure the longer portion, from the end to the mark. This will be the outside circumference of the seat tube.
5. Repeat the process to ensure accuracy.

Using this method you should be able to easily measure the outside circumference to within 0.5mm of its actual value. Divide the circumference by pi (3.14) to calculate the outer diameter. The result should be accurate to within 0.16mm.

Last edited by T-Mar; 10-03-20 at 03:05 PM.
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