ski losing power

I had my flywheel off of my 1990 650 earlier this year. About a month ago , ski started to lose power gradually over the course of a couple of days. Eventually it would turn over but not start. After pulling my hair out for a couple of days, pulled cover off and noticed shattered woodruff key. As luck ( or unluck ) would have it, my 1994 701 engine is in pieces. Grabbed the 701 key and put it in the 650 ( 650 shaft seemed solid and woodruff key seemed tight). Ski ran great for about a week and is now starting to lose power again. Won't be able to look further into problem for a few days. Just wondering, are the woodruff keys the same for the 650 and 701 ? Does anyone know the size that I can get at hardware store ? If the woodruff key is shattered again , any idea why it would be ?
 
Location
Argo, Al
The keys are the same. What are you torquing the bolt to? The manual states, I believe, 50 ft lbs. I sheared a key one time and it scratched up the crank snout a little bit. I used some lapping compound to get the surface right and torqued to 70 ft lbs after reading on here that 70 is preferred and never had a problem after that. I have always torqued flywheels to 70 ft lbs after that with no issues. Also make sure the bolt isn't bottoming out for some reason before touching the flywheel.
 
I just tightened by feel. Need to buy a torque wrench. Will look at flywheel when I take it off. Seemed good but you know what they say about assuming.
 
Turned out to be the flywheel groove worn like Big Kahuna said. Question : is the flywheel softer metal than the shaft ? Only flywheel is buggered. And another thing - this happened on a 650 motor as opposed to my 718 motor. I mean this thing is so sluggish even when engine is up to par that I pull out the rocking chair and start knitting while on the 650 . Surprised that it would do this kind of damage.
 
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