So it's the official opinion of the list membership that size does matter ;
The Motobecane Grand Jubilee I recently acquired from my local thrift had a seat pin that had been in it solidly for 25 or so years. It showe d some rust marks when I adjusted it higher to fit me. Being a conscienc ious bike owner, I put grease on the seat pin before putting it back in. I suppose it was fortunate that the seat clamp bolt was an aluminum one be cause on my first longish ride with the seat at the proper height, the pin started slipping down. I could not tighten the seat clamp bolt tight eno ugh to keep the pin from slipping, even after wiping off as much of the gre ase as I could. I ended up using a trail-made shim cut out of a coke can in order to keep the seat from slipping.
It forced me to go ahead and replace the Selle Italia seat and SR pin with a straight aluminum pin and Ideale clamp and aluminum frame saddle th at I'd wanted to put on it all along. Of course, I had to spend an hour or two sanding down the pin to get it to fit, but at least I know it fits.
Lynn Kuehl, Concord, CA USA 925 685-9999