Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 2 Post(s)
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It could be simply a stress crack in overly brittle paint, but it looks more like it's in the metal. You might watch carefully to see if it widens when the clamp bolt is cinched, but the only way to know for sure is to remove the paint and check the metal beneath.
If you're concerned and want decent post holding power with reduced stress, consider using coarse lapping compound (available in auto stores) in lieu of grease in the clamping zone of your post. The grit engages both the post and frame providing lots of holding power without having to tighten the clamp as much. (it's the same principle as throwing sand under tires for traction on ice)
BTW, if it is cracked, you can carefully file it down to a standard diameter, and fit a replaceable alloy clamp. It takes a bit of hand skill and patience, but isn't really difficult - a perfect DIY project to save a frame for little or no $$$.
An ounce of diagnosis is worth a pound of cure.
“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin
“One accurate measurement is worth a thousand expert opinions”
- Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance