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Flywheel Question

637 Views 5 Replies 3 Participants Last post by  jrapp
Ok here is the break down. 1987 350's rebuilt in the past few years practically no use since re-build. Port motor had a bad vibration, shop pulled it found the flywheel was 180 out from correct. Re-installed and runs much smoother. After inspecting the flywheel it is weighted. Engine builder said to not run that flywheel as it will tear up the crank as 350's are internally balanced. I agree with that entirely. My question is would you notice if the flywheel was weighted other than looking at it? Would the motor vibrate? Any other tell tale signs? Reason is starboard motor runs great. I wonder if I have to have it pulled also to check the flywheel.
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I know what you're going through. Been there and done that. I had to replace the engine in my last boat on very short notice. (milky oil at the beginning of a season) I was fortunate to find a performance engine builder who happened to have a low hour 305 small block which he had just pulled from a customer's boat he was building up. Seems to me, it was a late 80's or early 90's vintage like the one you are dealing with. The price was right and I thought I'd be back on the water in 24 hours. The engine was totally complete and assembled except for the flywheel, bell housing, and starter motor. As it turned out it was a much newer block that the one I was replacing and the flywheel, bell housing, and starter motor from my old engine would not fit. The engine builder found the parts I needed to complete the installation but it vibrated like crazy. He said it was probably the wrong flywheel and he gave me another which also vibrated. He then said the same thing your engine builder said about some of those engines were internally balanced and the fly wheel should not be counter balanced. He machined one of the fly wheels to remove the counter balance and then balanced it. It too vibrated. After trying 6 or 7 variations of flywheels he had laying around (I got pretty proficient at pulling the engine that year), I finally gave up and went to a Merc Cruiser dealer and tried to order a new flywheel. He told me that Merc Cruiser listed numerous flywheels for that vintage of engine and that the only way to get the proper flywheel was to get all the numbers from the engine and call Merc Cruiser directly to identify the proper combination for that particular engine. Sure enough, the flywheel they sent ran smooth as silk.
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jrapp said:
Engine builder said to not run that flywheel as it will tear up the crank as 350's are internally balanced. I agree with that entirely.
This is not true of all 350's and thus why this mistake is made all the time ! It shouldn't be as a quizzilion sbc's are around and being rebuilt/modified daily.

The one piece seal motors have a externally balanced flywheel but still a neutral balanced harmonic balancer.

There are three flywheels for the one piece seal crank 305/350's.

GM#14088646 : 12.75" diam, 153 tooth, for 10"clutch, lightweight nodular iron (16lbs),

GM#14088650: 12.75 diam, 153 tooth, for 10.4"clutch, standard weight flywheel

GM#10105832: 14" diam, 168 tooth, for 11"+11.85" clutches.

Supposedly GM#14088646 is what Merc uses on these 305/350 one piece seal motors, but this is not from my hands on notes.


If an engine is rebuilt and the shop balances the assembly for neutral balance, that changes everything of course. Will then need a neutral balance flywheel. Issue with this, I don't know of any company that makes a neutral balance flywheel for these motors, therefore the shop will also have to neutral balance the flywheel.
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the motors are 2 piece rear mains. I was told by my shop that the current flywheel is a 117 tooth.
Okay, I assumed GM 1987's not Merc 1987's.

GM went to one piece crank in 1986. Merc can lag behind because of inventory and although the longblock is a pre'86 Merc did not put rest together until afterwords and then ship it as a '87. That's when parts sourcing can be tough at times - the GM year of production vs the Merc year of final marine assembly/retrofitting/etc.

The pre-86 350 engines (two piece rear main seal) always had neutral flywheels.
GM#14085720 12.75"diam, 153 tooth
GM#3991469 14"diam, 168 tooth

GM#14085720 is supposedly the flywheel that came on these Merc two piece seal motors. Again, in my notes but not my hands on notes.
Thanks for all the info cfm!
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