the rodent is right about the crank ... your already half way there. and i'm not sure exactly what happened to your top end, which could have an impact on the crank depending on exactly what the hell happened to the top.
i've heard arguments both ways on 951 cranks, the one I tend to agree with is that once you get over 100 hours, your entering the danger zone, but that's really just a #, nothing more nothing less,
the age, hours, condition of the top end,and how hard it has been ridden are certainly huge factors, and honestly, pure dumb luck, i've seen neglected ski's with over 200 hours and i've seen ski's that were meticulously maintained only last 120 or even less. but if a ski isn't pushed that hard, doesn't spend all its time in a salty enviroment, good oil, yada yada it can last a really really long time, there are lots of 951's out there with 2-300 hours+ of zero problems, we just don't hear from them here because they bought the dam thing in 99, and have ridden it 350 hours with just gas&oil
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re: pulling from the ski, its not a requirement but it is a recommendation. do you have to ? nope.. and if your doing pay work with a flat rate for a top end you probably just leave it right there, bang it in, bang it out, if its a personal ski and your doing the work and this is your first, take the extra time to pull the engine and do it on the bench, it will be easier.
don't dig into the crank unless you feel comfortable, and you have money budgeted to replace said crank if it turns out to be questionable, sometimes you are better off not knowing
IMO its MUCH easier for your average guy to do the top with the engine on a workbench than inside a ski, you dam near need to be an octopus to get the cyl's back on as it is, but on the bench at eye level is easier than when bent over a ski on a stand (or a trailer, i hate working on a ski on a trailer, i bang my dam shins all the time)...
but for an experienced wrencher who can do a top end in his sleep, he probably just bangs it out while in the hull.