seatpost minimum insertion...?
 

The stock Campagnolo 27.2 alloy seatpost on my 1973 (or '74) Raleigh Professional is not marked for minimum insertion. Is there a general rule or way of figuring it out?

Thanks.

Steve

2.5 inches is safe enough

the post needs to extend at least 10 or 15 millimeters below the bottom of the top tube to seat tube intersection

It depends on the length of the seatpost and the construction method of the frame. For lugged steel and either the 130 or 180 mm seatpost that Campagnolo was then making, merlin55's suggestion of 1 cm below the level of lower side of the top tube will be sufficient. If you are using one of the modern 300 and plus mm lengths, you would need to insert much more deeply, otherwise the potential leverage of the long seatpost could destroy the frame. Mackerel's suggestion is likewise OK with the Campagnolo seatpost that you have (his recommendation is even deeper than merlin55's recommendation), but not necessarily if you use one of the superlong modern seatposts.