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I have a Day 6 ebike; it's a large frame--the Samson model. The seat post (the frame member) is 31.4325mm or 1.2375 inches (I.D.) at the clamp collar. The seatpost (the seat support post) is likewise on the O.D. I would like to switch to a more comfortable seat, and the seats I'm looking at require the old-school seat clamp that bolts to a 7/8 inch end post atop the seat post. With me so far? These seat posts come in 25.4mm (1 inch) and 27.2mm (1-1/16 inch) seat posts (with the 7/8 inch seat mount at top). If I switch seat posts, I will need a seatpost shim; either 25.4mm to 30.9mm or 27.2mm to 30.9mm. There is no seatpost shim for my existing Day6 size: 31.4325mm--it is an odd ball. Here is the question: using either of these shims will leave a gap of 0.5325mm (31.4325 - 30.9mm = 0.5325mm)between the shim and seat post tube inner wall. The next size up, 31.6mm, is too large to fit. So, if I use a 30.9mm shim in my 31.4325mm seatpost tube, will I be able to tighten the seatpost collar/clamp without cracking the tube frame (aluminum alloy)? Anybody have experience with this? The ebike weights 54 lbs and I weigh 195 lbs. Thank you!!

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    Is there any reason you can't simply switch the saddle and leave the correct seatpost mount in the bike ? 99% of saddles use the same rail connection. – Criggie Nov 4 '17 at 4:59
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    The engineering tolerances are completely screwed up. Seat posts are manufactured in 0.1mm increments, probably 0.05mm tolerances, certainly no more accurate than 0.01mm. Why on earth (let alone how) would you measure seat tube to 0.1 micrometer accuracy (100 time more accurate than the manufacturers would use). At these levels of precision, the temperature the reading was taken at is important. – mattnz Nov 4 '17 at 5:16
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    Cane Creek do a 27.2 ID-31.4mm OD shim universalcycles.com/shopping/product_details.php?id=27081, but don't use a shim, just buy a 31.4 seat post. Dozens of 31.4OD Shims and posts listed on ebay – mattnz Nov 4 '17 at 5:22
  • @mattnz So what if he reported more accuracy than need in the measurements. – paparazzo Nov 4 '17 at 6:07
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    It took me 20 seconds to find multi listings for a 31.4 seat posts and another 10 for shims. By looking (presuming he looked) for a 31.4325mm post or shim, the OP has not been able to find the products he needs. That is important – mattnz Nov 4 '17 at 7:16
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To answer the question of whether you can get away with the .5mm gap, absolutely not. It probably won't be able to clamp securely, and even trying it once could destroy the seattube opening. Bikes are essentially never tolerant of this type of mismatch.

I presume you are in this spot because you want to use a seat like a double-rail Brooks or similar that requires a pipe-style seatpost, as opposed to a more common cushy seat, most of which come with a usually-superfluous clamp that can be removed in the case of a post that has its own clamp. If that's the case, I think all you need is a 27.2-31.4 shim, which is oddbal but does exist.

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You're stuck looking at a small selection of seatposts because you are focused on getting one with a 7/8" top. Presumably you think you want to attach a seat which has a clamp like this:

http://www.jensonusa.com/Velo-78-Single-Rail-Saddle-Clamp

But every standard bicycle seat made in the past several decades uses a twin-rail system with standard spacing and almost-standard rail diameter. So don't look for 7/8" topped posts, look for any post that fits your frame, then attach any standard seat to it using the standard twin rails.

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