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It's getting hard to find any selection for my old Avanti 12 speed. it currently uses 27x1.25 inch tires. Can I put 700cc 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
    
Yes sorry not had my coffee yet, 27x1.25. LBS just has almost no selection as its a out-dated size for the most part –  CylingWolf Jul 9 '11 at 16:07
    
Got it, have edited the question. Please feel free to add more information about the bike, the kind of riding you do, et cetera, if you think it's relevant to the question. –  Neil Fein Jul 9 '11 at 16:09
    
I ride mostly roads, the odd time its hard packed gravel so I tend to keep my tires under inflated a little bit. I realize puncture resistant tires are not a cure and never get a flat. But it gives me more options tire/tread design. Bike is pure steel 12 speed bike is almost 30 yrs old and uses shrader valve tubes. Some bike paths here are that real fine stone shard stuff so would prefer to use different tire maybe also a tad wider, it has old school brakes and would allow the clearance width wise. I ask to use the same rims as I really dont want to have to replace the brake system aswell. –  CylingWolf Jul 9 '11 at 16:20
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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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2 Answers

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
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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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