I have been researching this for a while and cannot find anything definite. I build/run airboats and have decided to try to modify an Echo 23.6cc tree pruner engine to use the existing clutch with a propeller. I have already cut down the clutch housing to expose the clutch shaft and tapped the shaft (was a 1/4" square) to a 5/16x24. It worked perfect, leaving a perfect tap and threads. I have 2 very thick aluminum washers (about 3/16" thick each) to use with it, but I'm a little leary to fire this baby up for fear of the clutch shaft possibly failing!! Has anyone tried this yet? I figure the clutch shaft and clutchs themselves should be pretty strong, being as it was on a tree pruner...or pole saw some may call it. If anyone has tried this, or have any ideas before I temp fate, I would really appreciate it!!