Re: Broken Stainless steel bolt
First off, it should be a 5/16 bolt with a 1/2 head.
Second: Is this bolt absolutely necessary? Well, there are seven of them and only one would be missing. Mercury uses five or six to hold on the lower unt. My thought is that you could get away with it under duress but the factory engineer put that bolt there for a reason so I would try my damndest to replace it.
Use a branded Vice Grip or Sears or other. The non-branded ones just are not accuarate enough and will slip. This is a case where the extra cost of the branded tool is worth it.
Before trying to remove it, spray the bolt stub for a couple of days if you have the luxury of time and heat in between to try to get the liquid to suck into the corrosion. Then, when you finally do get the Vice Grip on it, it needs to be so tight that it hurts your hands to clamp it. Then, wiggle the bolt back and forth. This helps to break the corrosion weld. Each time you wiggle, work it out about another 1/8 to 1/4 turn. It is a long process but you can be successful.
NOW: If all fails and the stub gets so munged up that you can no longer get it out, buy a COBALT drill set. --Cobalt alloy, NOT the cheap Cobalt brand. DeWALT makes a nice set. These drills work quite nicely on stainless. Grind down the stub to a flat surface and drill it out. If yoiu are accurate it is possible to re-tap the same sized threads. However, it is also possible to re-tap to 3/8-18.
