I'm restoring a 70's vintage Schwinn Collegiate and Raleigh Colt, each with 26x1 3/8 tires. I purchased Kenda tires to replace the rotting originals, but have found they are a bit too large--can't seat the beads. Can anyone suggest a better brand and source?

26x1 3/8 is not the same as a standard 26" tire (which I am assuming you bought). Check Sheldon Browns site for more info.

There are 2 different 26x1-3/8" sizes.
590 & 597MM bead seat diameters.
Sounds like you might have picked up the 597 (Schwinn size) when you need the 590.
Look on the sidewalls for the MM.

Thanks for the info. I have the S6 Schwinn tires. The bead set must be 597mm. I'll try the 590mm