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Old 10-26-2015, 06:25 AM   #21
Ben's Avatar
City: Wilmington, NC
Country: US
Vessel Name: Sand Castle
Vessel Model: 1986 MT Sundeck 35.5
Join Date: Dec 2007
Posts: 685
Originally Posted by kchace View Post
Yeah this one is hard to diagnose at a distance. At 600 rpms the crank is turning 10 times a second and every cylinder fires 5 times a second. This sound is nowhere near that fast. To me that rules out anything to do with pistons, rods, injectors, etc. i agree that maybe something is loose inside and bouncing around?

I've never heard a broken crank. Wonder if that's possible?

I did the same math, and comforted myself. I've been off-boat a few days, so will go down this week and do the screwdriver check and pop the valve-cover for a look. After that, I'll call the mechanic. Want to have in mind the problem before the mechanic is on site. The good news is that the tone sounds tinnier than thumpy. So the valve cover pull is a good first look. I can go that deep, but after that, I start getting a little worried.



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Wilmington, NC
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Old 10-26-2015, 10:49 AM   #22
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City: Long Beach, CA
Country: USA
Vessel Name: Heads Up
Vessel Model: Grand Banks 42 Classic
Join Date: Mar 2013
Posts: 934
I had a harmonic balancer go bad on my diesel truck and it made a very similar noise when it started. It got worse in time and if that is the problem change it as soon as you can. If it turns and gets off balance they have been known to break the crank.

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