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It's getting hard to find any selection for my old Avanti 12 speed. it currently uses 27 x 1.25 inch tires. Can I put 700c tires without needing to buy new rims?

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Welcome to bicycles.se, CyclingWolf. Do you mean 27x1.25 tires? – Neil Fein Jul 9 '11 at 16:03
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According to Sheldon Brown, a 27" wheel has a 630mm bead seat diameter (essentially the inner diameter of the matching tire), while a 700c wheel has a 622mm bead seat diameter. So the 700c tire is 8mm smaller in diameter -- about 1/4 inch. This is in the "iffy" range, I'd say. (But you can certainly interchange 700c and 27" tubes, if they're approximately the right width. Many tubes are sized both ways.) – Daniel R Hicks Jul 9 '11 at 16:36
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FYI, if you've got 27 x 1 1/4 (32-630) tires, you might have the widest available. I don't know that any current manufacturers make a wider tire. Michelin used to make one (World Tour) that was 27 x 1 3/8 (35-630), but they no longer list it. You may need to look into switching to 700c rims if you want wider tires. Depending on your brakes this may be possible - the 8mm circumference change means your brake pads only need to be able to move 4mm further down the slot. – lantius Jul 10 '11 at 0:55
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Yeah... I too learnt the hard way.... 700C won't fit a 27 inch rim and you'll end up nipping and damaging the inner tube if you struggle trying to persuade it with your tools.... I didn't even get close to fitting it on, so believe me don't even try. – user13678 Sep 7 '14 at 12:12
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I have managed to put one on, but it was a lot of work and the beads sat in the centre of the rim, they wouldn't seat at all. Bike was rideable, but only just. It got me 10km to the bike shop though. I suspect it would only be possible with a fairly deep rim trough, as you do really want that 8mm extra diameter from somewhere. – Móż May 1 at 23:58

No!!!

I tried this when I was knee-high to a grasshopper and it so-did-not-work! I actually managed to get a Michelin Select 700 x 28c tyre on the 27" wheel but the inner-tube was so pinched and pierced that no amount of that gunk you can spray into tubes could save it. I did get the size wrong but after many hours of struggle 'hoped' to get a low-profile tyre on my front wheel.

The bead diameter of the 27 inch wheel is 630 mm, the bead diameter of a 700c is 622 mm (I properly learned those measurements that day). 8mm is a big difference, believe me!

If your LBS does stock 27" tyres, do not part with your cash until you have read 630 on the sidewall. If they do not stock them, ask them to get them in. This will save you the postage of mail order. Their suppliers will carry them and they should be able to get them in for next week if you ask nicely and put down a deposit.

There are specifications for wheel sizes and tyre sizes - ETRTO. Schwalbe have a good intro and list of sizes:

http://www.schwalbetires.com/tech_info/size_markings

The 27" size for your bike will be 630, 609 is the size for older bikes with 'rod' brakes and no sidewalls to the rim.

Normal 700c inner tubes will go in a 27" wheel with no problems.

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That's what I was afraid of, thanks for the reply. I will just tell them to order me in some 27" schwalbe marathons then or similar tires in a slightly wider width. – CylingWolf Jul 9 '11 at 16:33

No, a 28" tire is 700c generally, but a 27" tire will not fit. Tubes, on the other hand, will work just fine.

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i actually have a 700c tire on a 27'' rim but the tire bead wont sit properly. it sits slightly off center so the tire is high in one spot and low in the other. it makes for little bit bumpy ride.

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Yeah - don't do that. Your extreme case would be a fast corner or other lateral movement, and the unseated side separates from the rim, unpeeling the tyre from the tube and your bike moves further and faster sideways than you intended. Generally leads to a washout or a wipeout, with resulting blood and tears. There's no quick fix - get and use the right size tyres. Perhaps you can sell the old ones on ebay/craigslist/etc for a small sum? – Criggie Oct 12 '15 at 23:56

700 c tire will is too small to put on 27" rim...

I spend hours try to pull it in, and end up puncture inner tube... not worth the trouble, definitely get 27 tire... not worth the time

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Welcome to Bicycles SE. We ask that people on this site [write to the best of their ability](bicycles.stackexchange.com/help/how-to-answer(. Please edit your answer to use proper sentence structure and punctuation. Additionally, this answer has not covered anything that has not already been stated. It would be better to simply upvote the existing answers that already cover what you have said. – jimirings Jun 20 '14 at 13:04

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