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Jan 23rd, 2013
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Yamaha broken crankshaft? #14583016 01/02/23 12:38 AM
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SteezMacQueen Online Happy OP
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today, while fishing, my 2004 F150 experienced a major malfunction. The crankshaft snapped right under the flywheel. I stated the motor, slowly approached planning speed…normal as could be. I started to let off the throttle and I heard a severe rattle/clanking. I shut off the motor and decided I needed to figure out how I was going to get back to the ramp. I started the motor right back up. I started idling towards the cove with the ramp and the noise was horrible. but….the noise vanished above idle speeds …so off I went. When I started the boat again, to trailer it up, it again sounded really bad and normal above idle. I noticed that the water pressure was above 30 psi. No oil lights…nothing abnormal other than the noises at idle.

once on the trailer, I removed the cowling and the plastic engine trim and saw the flywheel was wobbly loose. Scary loose. But the end of the crankshaft was still very tight to the flywheel. Upon further inspection, I realized the end of the crankshaft had snapped(I’m assuming). What was holding the flywheel to the top of the motor I’m still not sure of. Even more impressive is the flywheel still has enough rotational “grip” on the rest of the crank to allow the starter to start the motor.

is there something else that could be wrong? I looked at the exploded view on SIM and it shows that the threaded end of the crank is one piece with the rest of the crank. If the threaded end is still very firmly attached with the nut on the flywheel, and the threaded end wiggles with the flywheel, doesn’t the crank HAVE TO BE BROKEN?

anyways. Guess I’m motor shopping. I don’t want to invest several thousand dollars to repair the motor and have a 19 year old lower unit, fuel system, etc. SAD! it still runs so smoothly. And very strong for a 150.


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Re: Yamaha broken crankshaft? [Re: SteezMacQueen] #14583134 01/02/23 02:42 AM
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Man hopefully you just have the flywheel "key" sheared and that will be easy enough to repair. But my thinking it's just sheared the key, so it's still in contact with the flywheel, but it would be messed up. This repair will be easy and cheap to repair. Cleaning all the old and Sheared key from the crankshaft and flywheel will be Tuff enough, but certainly cheaper than a broke crankshaft.
Usually the reason for this to happen is the flywheel nut loose. They do use a locknut on it and they have some patch lock on the threads to keep this from happening. Let's just hope this has happened and you can repair this easy enough...

Re: Yamaha broken crankshaft? [Re: Rowdy] #14583155 01/02/23 02:58 AM
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Originally Posted by Rowdy
Man hopefully you just have the flywheel "key" sheared and that will be easy enough to repair. But my thinking it's just sheared the key, so it's still in contact with the flywheel, but it would be messed up. This repair will be easy and cheap to repair. Cleaning all the old and Sheared key from the crankshaft and flywheel will be Tuff enough, but certainly cheaper than a broke crankshaft.
Usually the reason for this to happen is the flywheel nut loose. They do use a locknut on it and they have some patch lock on the threads to keep this from happening. Let's just hope this has happened and you can repair this easy enough...

The end of the crankshaft is still very tight to the flywheel and the nut is still torqued and has a blue paint dab on it and the threads to show movement. I wish, and hope still, that there is something less expensive going on. I’m pretty sure it’s worst case scenario.

I’m still kinda trying to wrap my head around how it could happen. The flywheel only sees minimal load, and only for very short 1 or 2 second intervals when the starter engages. Other than that, it is a free ride! Makes me wonder how it hasn’t thrown a rod or worse. If the forging had a flaw, I guess it could happen. I started researching the issue after I got home…seems as though there have been a few instances of the motor cracking it’s crankshaft and slinging the flywheel THROUGH the engine cowling. Mine is still attached. Somehow.


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Re: Yamaha broken crankshaft? [Re: SteezMacQueen] #14583158 01/02/23 03:02 AM
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I’ll try to remove the flywheel this week and see what happens. All my big sockets and pullers are locked in my tool cabinet at work. Won’t be able to access them till Tuesday


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Re: Yamaha broken crankshaft? [Re: SteezMacQueen] #14583224 01/02/23 11:55 AM
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The nut would must likely still be tight and the gaulded sheared key would be enough resistance for the starter to fire up the engine.
Now for something else to check, pull the dip stick and see what the engine Oil looks like. Your looking for really nasty oil that could have metal in it.

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