The knobs on my 238 tuner all us an .050 Allen wrench. This may seem like a small size for the larger knobs on the tuner but that is the way it came from Ten-Tec. Also, replacements from Ten-Tec for the smaller knobs have been redesigned to help eliminate cracking. This is a problem with the older style knobs if the set screw is over tightened. 73, Bill K6WLM