Mercruiser 470 crankshaft sprocket play on shaft

[jabfd]

Hi All,

I had a knocking sound only at idle over 1200 rpm and you would not hear anything and the engine will run great at 3800 rpm. As soon as I drop it back down to idle. It sounds like someone is in there with a hammer. Sound like it was coming from the front of the engine.

I pull the engine and checked crank shaft and rod bearings. Everything seems tight no metal was seen in the oil.
What I did notice is when I rotate the crank shaft there is play in the crank timing gear. Like the key way is sloppy. Is this normal or at least is there a tolerance on this. When I move the crank a little back and forth in slop it sound like the noise I hear when the motor is at idle just a lot softer.

Any help of ideas will be greatly appreciated thank you

1984 Mercruiser 470 2 barrel carb

 

[ScottinAZ]

is the slop in the fit of the gear/sprocket on the crank snout, or is it the chain slapping? there should be no appreciable slop between the crank snout and the gear/sprocket. If there is, it WILL wear into the crank rendering it pretty much useless. Not to mention the cam timing will be all over the place

 

[jabfd]

Thanks for the reply. I can rotate the crank forward and back a little and the crank and the crank damper pully moves Insync. However while doing this the timing gear on the crank will not move until I spin the crank a little further then it engages almost

 

[jabfd]

Hi all so it turns out the key way in the sprocket is bad. See pic

 

[ScottinAZ]

yeah, obviously you will want to change that. You will also want to replace the crankshaft key, as that is probably eaten up as well. With any luck, there is no appreciable wear on the crank snout from the sprocket going back and forth while eating out the keyway. If there is, well, there goes the crank as well (you will only really know when you get the timing set off.... unfortunately).

 

[jabfd]

Got the timing gear off. The key was bad as well replaced both. Just waiting on some gaskets to come in so I can put everything back together.

 

[jabfd]

P.s the crank was fine

 

[ScottinAZ]

P.s the crank was fine

awesome news. makes the repair a LOT easier and cheaper. Hopefully the root cause was just a poorly heat treated crank gear allowing the keyway to waller out