Bike Forums - What is the difference between Reynolds 501 and 531 tubing?

531 was( is) a seamless butted tubing with a high Manganese
Molybdenum alloy content that gave it more ultimate tensile strength than most chromoly steel tubings available before 1983. 531 came in several butted profiles, .8/.5/.8 main tubes were the most common on lightweight road bikes

501 was a welded seam chromoly tube drawn over a mandrel, The top and down tubes were usually butted 9/6/9. The seat tube and stays were .9mm straight gauge.
501 came along in the early 80's for mass produced frames and was intended to compete in price with Ishiwata and Tange tubes. It was used in mass produced midrange sport road bikes and later MTB's. Many of us at that time thought it was crap that would ultimately fail in a dramatic way, but it has held up very well over the years.

Figure a 501 frame to weight anywhere from 100 to 200 grams more than a equivalent size 531 frame

Also it wasn't unusual for bicycle manufacturers to only use 501 on the main tubes and use cheaper hi-tensile steel for the fork, stays and sometimes the seat tube. read the sticker carefully when considering old frames.