CRs:
Thanks to Charles Andrew's previous remarks on stem height. I can add a few extra comments, based on my measurements and experience:
I've found that when either Cinelli or 3ttt quill stems (with a cone retention) are installed at the the maximum height (minimum insertion) mark, the bottom end of the stem is typically 1.5 cm below the steering tube threads, in the solid part of the steering tube.
The "grip surface" of a quill stem with a cone retention is typically 3mm wide. The band of stress is typically 1 cm above and below the grip surface. The grip surface + stress band together add up to 13mm, which is pretty close to the 15mm 'clearance' to the thread line.
I would set a Cinelli or 3ttt quill stem so that the bottom end is at NO higher that 1 cm below the thread line of the fork. This setting only gives a 5mm increase in height over the factory setting. This is still pushing your luck, and you do so at your own risk.
I've based the preceding on the measurements of my 1979 Pinarello. I rode the bike for 13 years with the stem at the same (maximum height) setting. The steering tube eventually stretched a slight bit, and consequently it was difficult to secure the stem at the original height placement. I had the steering tube replaced when I had the frame restored.
(I've never been heavy handed when installing stems.)
I've heard that a wedge stem is less stressful on the steering tube than a cone stem. This is most likely because the retention force is spread out over a wider area.
Consequently I've come to prefer wedge stems: they're easier to adjust on the road (you don't have to tap the top bolt to loosen the cone) and the reduced stress helps prolong the life of the steering tube.
I especially prefer the Nitto Pearl stems, which give a height increase of 5 to 10 mm over the Cinelli and 3ttt equivalents.
Andrew Gillis (Long Beach, CA)