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Old 09-13-05, 06:55 PM   #1
dstrong 
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Seatpost size: 27.0 vs. 27.2 - No big deal, right?

As the result of an eBay buying frenzy I've ended up with two Thomson Elite seatposts. I'm about finished building up my Interloc Impala (steel) which states a seat tube diameter of 27.2.

I have the two seatposts and boy, putting them end to end, you can't tell the difference in the diameters. Slide 'em into the seatpost though and you can feel a slight difference.

Here's the dilemma. The 27.0 is newer and has been cut shorter by the previous owner. It appears to be long enough for my needs. The 27.2 is the right diameter, older and hasn't been cut.

The collar seems to grip the 27.0 just fine but I'm concerned about stress on the frame around the post.

Is this something to stress about (actually I know the answer to that...would just like an opinion or two).

Thanks.
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Old 09-13-05, 07:07 PM   #2
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Don't try to run the smaller seat post or you may have problems. The seat post collar will have to pinch down the seat tube to get a good grip on the post. On some bikes the collar will run out of room and bottom out. Not good.
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Old 09-13-05, 07:14 PM   #3
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find a .2 millimeter shim and you are good to go. Seriously though I would stay with the 27.2 post.
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Old 09-13-05, 07:39 PM   #4
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Thanks guys...that's kinda what I figured. Look for a 27.0 Thomson Elite on eBay soon!
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Old 09-13-05, 09:08 PM   #5
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Beer can is .1 mm thick (or .2 mm on diameter). Bend tabs at the top of the shim so it doesnt drop down the seat tube.
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Old 09-13-05, 09:26 PM   #6
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I'm pretty confident a 27.0 will slip quite consistently over time. We have had bikes with seat tubes manufactured less than 2mm out of spec and had issues with slipping seatposts.
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Old 09-13-05, 09:34 PM   #7
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AndrewP...is that thickness true for Shiner Bock cans also??? ;-)

Actually...I had read the beer can shim "trick" in a previous thread (yes...Newbie can search) but just can't bring myself to do this on my first bike build ever and first roadbike since I bought my green Peugeot in 10th grade with money earned landscaping!

Although...after reading about how my aluminum post is going to fuse to my steel bike, maybe a sacrificial shim is a good idea?
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Old 09-13-05, 09:39 PM   #8
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It won't fuse if you grease it.
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Old 09-13-05, 11:46 PM   #9
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Yup seatposts need lube too
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