Merc 470 Alpha 1 knocking please help

[T.LOWE]

I have a 1982 Glastron with a 470 in it. Last year I updated the outdrive with a used Alpha one. The outdrive was professionaly serviced and installed on the boat. The boat has been in storage since that time and now I am ready to ride.
I pulled the boat out and ran it on the muffs with no issue. I took it to the lake and when it is in gear, i have a pretty loud knock. The knock only occurs when the boat is in gear and the severity increases with rpms.
I do not seem to notice the knock when running on muffs and in gear.

I had my wife drive the boat so I could try and pinpoint the knock but i have not been able to find it's origin.

Any ideas would greatly be appreciated.

 

[Daddy O]

Re: Merc 470 Alpha 1 knocking please help

Without more info, sounds like a rod knock. Goes up and down with rpm, and while in gear. Has a load on the engine.

 

[T.LOWE]

Re: Merc 470 Alpha 1 knocking please help

if it were a rod, would I not hear it when running on the muffs. in addition, I have used a stethascope and do not hear the knock in the engine. I was leaning more toward the drive line but really need some ideas on pinpointing the issue.

 

[Don S]

Re: Merc 470 Alpha 1 knocking please help

Sometimes it takes a load on the engine, or the drive to be able to hear a rod knocking. If you still have the boat in the water, put it in gear and listen to the engine with your stethascope. If you don't hear it on the engine, then it's time to pull the boat out of the water, REMOVE THE PROP, and run the engine in gear and see if you can hear anything on the drives lower unit with the stethascope.

Last year I updated the outdrive with a used Alpha one. The outdrive was professionaly serviced and installed on the boat

Since it was a used drive you put on, I doubt the professionals pulled the drive apart to do a service on it. Usually that consists of changing the gear lube, possibly the impeller, and putting it back together. Even if they ran it, they didn't hear the noise either.

 

[T.LOWE]

Re: Merc 470 Alpha 1 knocking please help

thanks, I will give that a shot Don. I know they pulled apart the upper unit becasue they checked those bearings and installed a new upper seal kit but I know they did not break open the lower unit.
In the event it ends up being the engine, I have an extra 120 setup in another boat I have been using for parts. This is actually the boat this outdrive came from. Is it possible and how much trouble would it be to switch out these engines

 

[Coors]

Re: Merc 470 Alpha 1 knocking please help

If the knock wasn't there before replacing the outdrive, maybe the outdrive?

 

[Don S]

Re: Merc 470 Alpha 1 knocking please help

The 470's and 120's are completely different engine. The outdrives have different gear ratio's, so don't expect things to be the same. You can probably really overrev that 470, which is NOT a good thing.

 

[T.LOWE]

Re: Merc 470 Alpha 1 knocking please help

hummmm. I was not aware of that. I was led to believe they were the same ratio when I switched them out. Will I be ok if I watch my RPM's and not go over the recommended WOT RPM?

 

[T.LOWE]

Re: Merc 470 Alpha 1 knocking please help

Went back to the lake this weekend and was able to get a mechanic to come by and take a ride. He fees strongly that the knock is from a bad wrist pin.
I guess I will be pulling to engine apart but going to try and wait until winter.
Any thoughts?

 

[wire2]

Re: Merc 470 Alpha 1 knocking please help

I learned a trick to find a sloppy rod or wrist pin.
Pull all the plugs and turn the engine over slowly by the crank bolt. With a light on the spark plug hole, watch for #1 piston to come up to TDC, then rotate crank a bit more to make it start to come down. Now put a screwdriver or punch in the hole and push down on the piston. If the piston drops even a bit, there's play there. If not, repeat for the remaining cylinders.
If it's difficult to get something in there to push the piston, bring #1 past TDC and down a bit, then reverse the crank a few degrees. If the piston remains stationary while reversing, there's slop there.

 

[Don S]

Re: Merc 470 Alpha 1 knocking please help

No problem waiting till winter, just don't run the motor again till it's taken apart and fixed. If the rod goes thru the side of that aluminum block, you will find out what a new engine will cost, not just a single rod or piston.