How to put bigger tires on it I have a 1985 Murray Phoenix high tech racer. It has 26 X 1 3/8 tires. I want to switch it over to mountain bike tires. How would I do that?
26x1-3/8 is ISO diameter 590.  Common Mountain bike tires are ISO 559.  I'm not aware of any 1-3/8 tires that are knobbies, and the only way you're putting Mountain tires on that would be to change the rim or wheel.  The problem you'll run into with that is the brakes won't be lined up and there may not be a brake that will reach the rim.  Long answer short: No.