5/16" x 24 hex bolt. 1" or 1 1/4" long. It's a common size you might be able to find at a hardware store or by mail order. If you can't find one that short, you can always cut a longer one down. The bolt is the same on both Ramtec or Dynamax although I think Ramtec may have been the shorter one. Use white lithium grease on the starter probe.