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I have Cane Creek Thudbuster Seastpost (aluminium) 27.2mm x 400mm which i want to use for a bike with a carbon frame (31.6mm seat-tube).

I'm thinking about getting a seatpost adapter/shim from 27.2mm to 31.6mm, but I'm not sure if that's not going to damage the carbon frame. Shim itself is ~80mm in length.

Is it safe to use?

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    I'd be checking with the maker of the frame to ensure warranty will not be voided. Get their answer in writing and keep it safe. – Criggie Jul 21 '16 at 11:07
  • Another option is to get the seatpost remade as a 31.6mm by a competent engineering firm. I had one make me a 750mm 28.6mm seatpost for a folding bike and other than being brutally expensive, it worked great. – Criggie Jul 21 '16 at 11:08
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    I'm not 100% but I can't imagine there'd be a problem with the two materials, as long as the fit is good. Of course a shim is not as good a solution as a seatpost that fits, but I suspect you know this already. But @Criggie is right - check how you stand as regards warranty. – PeteH Jul 21 '16 at 12:42
  • Random thought - can you get a carbon fibre shim rather than an aluminium shim? That way it can keep any corrosion away from your frame. The shim should be at least as long as your seatpost's minimum insertion line, and longer is totally fine. – Criggie Feb 10 at 5:42
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80 mm length should be long enough to provide stable and good grip. Do make sure to use carbon mounting paste, of course, wiping any traces of grease (if they exist) off the adapter and the seatpost. If the seatpost stick long above the frame and if it's an off road bike, I might consider it a tad bit risky move. Otherwise, you should be fine.

And, like the previous poster said, to occasionally check for traces of galvanic corrosion. You don't want the seatpost stuck to the frame.

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Been using similar for perhaps 10 years with no issues just check and use a bit of lube every few months and more if youre in all weathers

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Carbon fiber can cause galvanic corrosion on aluminum.

But the reaction is slow, and the carbon fiber would cause the aluminum to corrode, and not vice versa. So it should be safe to ride and not damage your frame.

This question covers it a little more in depth -- Metal seat post with carbon fiber frame?

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