I just saw this posting and would like to offer some additional input. If you still have the pieces of the stud, examination of the fractured ends will tell the story (magnification will help).
If the surfaces are rough, then the stud sheared off. More than likely, what you will see is a smooth area and a rough area. The smooth area defines a crack, and the rough area denotes the ultimate failure of the fastener. There are two sources of the cracking. The first would be a flaw in the base metal, such as a carbon inclusion (very common to this type of failure). The second would be the formation of a stress riser in the threaded area.
When threads are formed, they are rolled, a forging process. This process reduces the likelihood of stress riser formation, but it could occur. Another source of stress riser formation in threads would result from the use of a sharp die. If the threads had been damaged during the installation of the engine (very possible) and then a die run over them, the subsequent failure would be possible. If you have the pieces, you should be able to see where a die would have cut sharply into the root of the thread.
I really don't like the design of this mounting. All of the engine weight and vibration are concentrated on the threaded portion of the stud. A better way would be to place spacers (they would have to be machined after taking careful measurements to maintain engine alignment) under the engine block tabs and on top of the rubber mounts (here I am assuming that there is a metal surface on the top of the rubber mounts). Above the engine block tabs, use a heavy flat washer and either a locknut (upset thread, not Nylon) or double nuts in a jam configuration (simply, one on top of the other with their flats misaligned). Use a thread locking compund, such as Loctite red.
You probably will never have any problem with what is in the boat now. The size of the studs are way over-spec'd to withstand the abuse of any compromise. However, if you continue to have problems, you also have the remedy.
ps: Please take a look at my post in this forum regarding jib rigging. I need your input. Many thanks.
Last edited by jones2r; 12-02-2006 at 02:32 PM.