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Old 01-12-22, 07:17 AM
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Join Date: May 2008
Location: Fredericksburg, Va
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Bikes: '73 Bottecchia Giro d'Italia, '83 Colnago Superissimo, '84 Trek 610, '84 Trek 760, '88 Pinarello Veneto, '88 De Rosa Pro, '89 Pinarello Montello, '94 Burley Duet, 97 Specialized RockHopper, 2010 Langster, Tern Link D8

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I am not familiar with that stem. If the "backer nut" is press fit into the stem, then the issue is not a design flaw as much as a manufacturing quality issue of the boring the hole for the nut (too small). I have not seen the shape of the press fit so can't speculate too much more. Just one more!
The crack is on the peak of the shape as compared to the breaks shown in the follow-up posts which are across or nearly parallel to the bar. This is important because if the break did propagate all the way in the direction of the existing crack, what would fail? The only failure that makes sense to me is that the press fit "nut" would be more likely to rotate. Even then, I don't think it would because of the remaining force around it.
I don't like apparent failed parts or those that might fail any more than the next guy. It just doesn't look good, and it attracts all kinds of opinions, some expressed well and others in an unnecessary sensational way.

I went out and checked 9 of the 11 stems I have with the same clamping design. One was suspect but the loop showed it was a scratch. I will need to file and sand it down to minimize the stress point.

If any Cinelli stem would fail as designed, it would be the 1R. The weak point is right across the middle of the insert. There is a hole for the insert and the thickness of the material is the thinnest around the bar. The wedge pushes the bar against that line but also on either side if the size of the bar is matched to the stem mounting hole.
Bikes don't stand alone. They are two tired.
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