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Bought a used frame and am not quite sure what size seatpost to use. I have 26.8, 27, and 27.2 seat posts, and they all fit in the opening! Obviously we're talking 4mm difference between the smallest and the largest, but I wouldn't think that the 27.2 would fit at all if the frame wasn't built for that size post? It is a tight fit, but I can still "press" the post into the frame with a little twisty motion.
 

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chamois creme addict
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More info please

Please post the manufacturer and model of the frame and I'm sure someone amongst us will know the answer.
 

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crestlinefarm said:
Bought a used frame and am not quite sure what size seatpost to use. I have 26.8, 27, and 27.2 seat posts, and they all fit in the opening! Obviously we're talking 4mm difference between the smallest and the largest, but I wouldn't think that the 27.2 would fit at all if the frame wasn't built for that size post? It is a tight fit, but I can still "press" the post into the frame with a little twisty motion.
Try www.sheldonbrown.com. Do a search under seatpost database. He has most frames listed and the approriate seatpost size
 

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Banned forever.....or not
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Most shops have a tool that can make sure of the size. It just slides into the seat tube until it stops. You then read the dia. that is the last one to go in.
 

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crestlinefarm said:
I checked Sheldon Brown before posting...can I do any harm by using the 27.2?

Yes, you can do harm to your frame. If you put in a size that is too small, you will have to overtighten the pinch bolts to get the post to not slip. It can damage the frame and crack it at the pinch bolt if you have to over do it. To be fair the chances of that are slim, but why risk it over something so easily fixable like putting in the right size Take the frame to a shop and have them measure for the right size.

BTW the three posts you have all vary by 0.4 mm, not 4. It's a small difference, but it's important to get the right post size. By the way, if all 3 posts you have fit, then the obvious is that your seat tube is at least 27.2mm If the 26.8 and 27.0 also fit, it's because they are too small. If a 27.4 fit as well, then 27.2 would be too small as well. Go to a shop and ask them to measure it with some Vernier Calipers. Every shop should have at least one set.

Russ
 
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