My 27" x 1 1/4" Araya rims have an OD of 19.6mm, and an ID of about 14.2mm.
They should handle 1 3/8", or 35mm, tires...right?

According to the ETRTO, whose chart has been published by Sheldon Brown and others, a 32 mm tire would ideally be paired with a rim whose interior width is 15 mm or greater and a 35 mm tire should be mated with a 17 mm rim. But that chart's known to be awfully conservative. You'd almost certainly be fine, although I might personally stick to 32's.

Schwalbe has the chart on their web site:
Tire Dimensions | Schwalbe North America

Is it a true 35mm width? Have run various 1.25 width on 27 arayas, some are narrower closer to 31 but some have been wider like 34. No issues with either but currently use a narrower tire to fit with fenders. Wanted to put a pair of 1 3/8 sand canyons but never did as too wide for fender but would think you would be fine.

Make sure you have room between the chainstays too, near the seat tube. I ran into that problem with 27 X 1 1/4 on a 1985ish Prelude and a Moto Jubilee Sport, both needing 27 X 1 1/8 or narrower.

can You Even Find 27x1_3/8" tires (630-37)?

Not that Im Looking for any.. whats 1/8" matter so Much?






./.

Loose Screws' 1&3/8" Sand Canyon tire, as mentioned by dailycommute above

Kenda makes a few, the k161 and k40. Sunlite also has at least one that I know of.

SwiftTire 27? x 1 3/8? Tire & Tube Combo ? Made by Panaracer ? SWIFTTIRE
https://www.amazon.com/Kenda-Hp-85ps.../dp/B00IFROH9U

Thanks for all the great replies!

The Kendas are in the Lexco catalog, but only offer a Kross Cyclo tread (K161) or a trekking tread (K40) - neither of which I want. I may go to an 1 1/4" size when the time comes.

I actually don't even have a frame yet, but am looking forward to building up something for my Seattle daughter-in-law. With their prices, I figure I can find and build her a bike, ship it, and still come out ahead.

She recently rode my Koga-Miyata, and fell in love with the mustache/bar-end setup. It's an 8-speed Shimano 105, and truly a dream to shift. She's leggy, and does not want a mixte, so that should help me locate a 56-58cm men's donor frame.

She also loved my B17, so of course I'll have to snag her a woman's version, being as her tastes apparently run towards upper end! The leather saddle combined with Seattle's weather worries me some, but I'll ponder that and decide later.

I dont have the bike either , not planning to get one ..

Hook Bead rims will take even wider , if it Matters ..