I have sripped and even sheared grade 5 and grade 8 capscrews, size 1/4-28NF which I believe is the thread size used on my OS 46 engines. It must be understood that not all prop threads are that size. It amazes me how easily that size capscrew can be stripped or sheared, though I have never damaged an engine thread. The prop nut and shaft thread are probably equal to a grade 8 bolt which has a recommended torque of about 13 foot pounds. A grade 5 is about 10 foot pounds. Let's assume grade 5. That is a 10 pound pull with a 12 inch long wrench. I just measured my cross wrench that I use on my glow plug and IN THE PAST USED TO TIGHTEN MY PROP NUT. The torque arm on that wrench, when it is in your hand is about one inch. To produce 10 foot pounds of torque with a 1 inch long wrench you need to push on the wrencj with 120 pounds. (10 foot pounds equals 120 inch pounds). This is why I use an 8" or 10" adjustable wrench. Here is a test I recommend: Put a grade 5 capscrew (3 bars on head) or grade 8 (6 bars on head) in a vice and put two nuts on it, preferably of a grade equal to the bolt. Use a 10 inch wrench and pull on it (NOT PUSH) till the bolt failes. You will have a better idea of what you are doing to your engine.