Joined: Jan 2010
Posts: 9,809
Likes: 1,784
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
I've twice had Campag SR posts that were problematic.
One I had to switch to another brand (Avocet) to keep from slipping.
The other I went to the next larger size, sanded it lightly and honed out the seat tube to make it reasonably easy to install.
I just measured 5 new NR posts, in the 26.8 and 27.2mm sizes. The undersizing was as follows:
.05 .06 .07 .08 .12
The last dimension was for a new NR post that measures 27.08mm, or .12mm undersize.
I also measured a new-looking Chorus titanium post, it measured a very consistent 27.12mm, very round and straight just like the alloy ones.
I've measured many posts over the years, and the .08mm undersize seems to me to be about the average of all of them.