Hey guys. Back in 2018 I built this engine for my boat. Some of you may remember the thread.

350 EFI TBI Mods, cam, heads, intake, tune, 383?

Last summer on Ky Lake, which is a massive lake, my engine locked up while underway in the main channel of the lake, running around 45 mph. The engine was turning 4000 RPM when it happened. I had already had the boat at WOT several times that day without any problems, but had backed off due to choppy water. When it happened we were stranded right in the middle of the lake, which has large barge traffic. We were able to call Ken Lake marina, gave them our location, and they came out with a pontoon and towed us in. For the most part the boat had been extremely reliable since I built it and got it tuned. Oil pressure was great, no oil usage, no noises, no vibrations....etc. Everyone told me the drive would give me problems, but it never has other than an oil leak. I did install a drive shower. On a good day the boat would run around 65 MPH per GPS.

This rotating assembly was balanced internal front and rear with a neutral balance flywheel and harmonic balancer. No counter weights.

Just curious . . . . which crank is it ? They have 2. One for 5.7 rods and one for 6"

The rear main looks like it was getting ready to spin. Wonder if that had anything to do with it ?

That sucks.
I’d be willing to bet I was not part of the ‘everyone.’ Alpha’s can take a beating. Just not high idle speeds going into gear.

I do not like aftermarket non forged cranks. Just my opinion. Sorry it bit you. The cracks / fatigue probably started a while ago and got bigger till it broke apart.

I agree. The rear main looks like it was doing some spin welding.
Friend had the same thing happen to his 300 Tempest. Running 4500 after running hard all day. Let off and BOOM! Carnage everywhere.

IDK, since the bearing damage is toward the edges on main, I think as it failed or right before it failed that rear crank piece wallowed around and did that. OP has had it together since 2017 or 2018, them bearings would be down to copper and woulda kabooomed years ago if there was a bearing issue, IMO, Smitty

Exactly

PS,
What part number crank and pistons ? Looks like the pistons are H860CP ? Cant see the crank number in your old pictures. I have a theory but it depends on the part numbers.

And THAT is why we don't use cast crankshafts in boat engines.

I don't necessarily agree with that. How many lower hp Mercruiser engines came with cast cranks ? As long as you understand the limitations they are fine. I did a 400 Small Block for a friends boat probably 20 years ago. It gets abused pretty regularly and it is still running after all these years

I agree with this.

The crank likely cracked and was starting to beat the bearing out of it right before it fully let go.

It's the risk you take when buying low quality China cast cranks. They may go forever or they may fail after a few years for seemingly no reason. Part of the game.