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I have a 1970s Schwinn World Sport bicycle that has all of its original parts and in working condition except the wheels. The wheels it's designed for are 27s, circus tires. They are real hard to find and expensive.
I tried to substitute my 700c from my Norco but the hubs are too big to even fit in the fork dropouts. Is there any tire that would fit this bike besides the old 27s?
Rather than replace the whole rim and wheel to use modern tyres, consider that 27" tyres are still made and can be purchased new.
Downsides are there are still at least two sizes and the selection is smaller. A modern road bike wheel is 622mm across the Bead Seat Diameter (BSD) and 27" and 28" are much more likely to be 630mm or even 635mm BSD.
Example: https://www.sjscycles.co.uk/tyres-27-630/ shows fifteen different 630mm tyres, with two Continental Gatorskins and a Schwalbe Marathon variant.
https://www.sjscycles.co.uk/tyres-28-635/ has three options in 635mm.
On the other hand, swapping to 622mm BSD rims gives you hundreds of choices, and probably an aluminium brake track as well. The old Schwinn rims from the 70's are likely chromed steel which has poor braking normally, and almost nothing in the wet. For safer braking, that alone could be worth the cost of replacement brake calipers too.
Assuming the frame in question is steel, you have four options.
Theoretically it is possible to cold set (bend) the drop outs (the parts of the frame and fork where the wheel is set) so they fit wider modern hubs. I strongly suggest you do not try to do that yourself, and you might not be able to find a shop willing to do it; however, you could easily find modern wheels to substitute if you do.
If the difference between the width of the frame/fork dropouts and the hub is less than maybe 5mm you can squeeze a modern hub in there and call it a day. Probably it'll be not great for bearing durability, but nothing terrible. Sheldon Brown thinks it's fine.
Do make sure you're getting a freewheel, not free hub, wheel as a replacement if and once you do, unless you are sure your friction shifters can sweep a 7 or more speed modern cassette.
You can get a shop to re-lace your current hubs to 700c rims. You'll need new spokes, nipples, rims, obviously, and rim tape, plus wheel building labor is costly. On the upside, you're not altering the vintage frame and if you ask to keep the old stuff you can theoretically put it back the way it used to be before.
If you are switching to modern rims, you need to make sure your brakes can reach them beforehand. See if you have the ability to lower your brake pads by a further 8mm. Of course, you could swap calipers too, and cheap Alhonga dual pivots will in all likelihood brake better than whatever you currently have.
There are absolutely decent 27" tires available. Schwalbe Marathons, for example. Inch sizes, especially obsolete inch sizes, are ambiguous. Your Schwinn surely has 630mm rims, so you should look for 630mm tires. 635mm tires were found on "roadsters", ancient cruising bikes, not old road bikes.
You’ll need to measure the distance between the dropouts (OLD) to figure out what sort of hub you need. A regular tape measure or ruler is fine.
https://www.halowheels.com/frame-spacing-hub-o-l-d-information/
From there you can figure out if an off the shelf 700c/650b wheel could be used. If it’s an unusual size you might need to hit eBay/FB market place to get a 2nd hand replacement.
I ride more or less this bicycle (1980s Raleigh, nothing special, but same problems from the dimensional point of view). I cold-bent the forks (eventually) with an improvised jig---before then I just sprung them open when changing the wheel with a wooden wedge---and swapped the front caliper for a spare rear caliper I was given at the local (2nd-hand) bike shop. (Rear calipers often have longer reach.) The bike rides fine for what it is, maintenance is easier since I can use modern cassettes, tires/tubes are cheaper.
Check your brake reach before you go this way.
Obviously if you care about preserving the bike you should think hard before substituting anything. If you're just out to ride it I've seen plenty of older road bikes adapted for modern wheels in English university cities. Get someone to do the bending if you're not comfortable with it, or go slowly. (Cold bending like this is hard, because you have to get the tubing beyond its elastic limit, but then in yield it... well, yields, i.e. doesn't require any increase in force to keep moving. Levers help. Expect to overshoot on one side slightly and adjust. Steel is quite forgiving if you're gentle.)
