possible causes for a crankshaft to snap in half ????

[hypersniper2010]

i have a 96 mercury sportjet 120xr motor. when i bought it i was told it had no spark. bought it and threw a battery in it and tried to crank it and it wouldnt crank. so i pulled the plugs and it still wouldnt crank so i pulled the head and there was a small piece of metal wedged between the piston and and cylinder wall so i thought it was a bad ring so i split the block only to find the crankshaft had snapped right at the top cylinder and the small piece of metal was from the crank. could this have happened if someone tried to start it while it was already running? or a backfire cause the ride plate is blown off on 1 corner. no obvious other damage. impellar and jet drive are all 100%...
only other thing i noticed was the flywheel was bent slightly and 1 tooth on the starter is chipped pretty good.
any other ideas ???? im open to any and all. i already bought a new crank and it will be shipped tomorrow and i dont want to break it again.
thanks

 

[hypersniper2010]

hello... tap tap tap ... is this thing working ????
lol

 

[seadooguynj]

Sorry man, I got nothing on this one...

 

[hypersniper2010]

thanks for at least replying. was wondering if anybody was even coming to this section of the board anymore. i will wait patiently for an answer:cheers:

 

[biffdotorg]

It's a bit quiet for us up North yet, as I am still putting my snowmobiles away. Boat season is close, but we had snow last night and 25 degrees this morning.

I would think most guys brains have shifted to boat mode around the rest of the country though.

Good luck,

 

[Igetwet]

I've never heard of a starter being strong enough to over power a running motor...but it is strange the with the evidence you've found. Damage like that usually comes from the motor ingesting water, water really doesnt like to compress. With the bent flywheel tho... Where exactly was the crank broken?

 

[hypersniper2010]

no evidence of water in the motor. ie washed piston,head etc. the crank was snapped right at the top cylinder just above the piston rod journal. thats the first weakes point closest to the flywheel. and seeing the bend in the flywheel and chunk out of the starter the starter im just taking a wild guess.

 

[Dr Honda]

If it was severely over-heated... you could loose the temper on the crank, and have it crack/split. OR... it could just be a defect in manufacturing.

Fortunately... the 3 cyl cranks are easy to get on ebay. A new crank for my 240 EFI is a $1k.

 

[hypersniper2010]

it was a 4 cyl merc sport jet 120xr. and was a little bit hard to find. bought one from ebay and guy swore up and down it was the right one and when i got it it fit in the block ok but the rod journals were only 3/4" and they needed to be 13/16" but i did end up hopefully finding the right one and am waiting delivery this week. in the mean time im getting my other jetboat running. its a merc sport jet 90 and needed a piston. cant complain about that one as i got the entire boat for free. only problem with having his and her jet boats is now i had to redo my entire dock to fit 2 boats..... life is so tough... lol...:thumbsup::cheers: