I believe the original was quarter inch. Measure the gap in the pulley. I don’t like the bronze core because when the covering wares off from lack of waxing, the bronze can eat into the aluminum. (Yes, the life of the rope is greatly extended by wax on the cotton covering.) This was aggravated when they went to a center pull starter to try to counteract the torque that made the motor turn when the rope was pulled.The owners manual says to replace the rope when it shows signs of ware. Good advice not always followed. When the hand grip gets pulled off’ there is a tendency to shorten the rope to get rid of the frayed end. This adds to the strain because you run out of rope quicker. Use as much rope as you can on the old rewinds. The ’56 and newer use a predetermined length to synchronize the compression stroke to the wide part of the pulley. The woven nylon is stronger and self lubricating. Just be sure there are no rough edges inside the housing or pulley. . . 😀