Mercruiser Knocking

[Midnight Thunder]

I just replaced the rod bearings after blowing one out that caused a loud knocking noise.

After getting it all back together, she runs fantastic. However, I still have a knock at 3500+ rpm that get's louder with the rpm.

I had no play on the cam or the crank but the noise is deffinitley in the lower end. I will be pulling the motor again - but what did I miss to check?

 

[lime4x4]

Re: Mercruiser Knocking

Did u check/replace the crank? Did u check the main bearings? Where any of the rod bearings spun if so did u replace the connecting rod as well? Is it the same knock as before or a different knock?

 

[Midnight Thunder]

Re: Mercruiser Knocking

Thank you for the reply.

One rod bearing was spun. Replaced them all and have no play in them. The cam and crank bearings seemed fine so I left them alone as they had no play.

I'm going to pull it again. I'm going to re-check all the tolerances with my manual on the crank and rods.

The noise is not coming from the flywheel.

If I've got play between the rod and crank (left to right - not up and down) - what are my options for repair? That's the only thing I can think of as they was 'barely' a play left and right. (I mean barely)

 

[Coors]

Re: Mercruiser Knocking

Read the specs for the side-play; if you spun a bearing, the rod needs replacing, and the crank journal needs miking, and a close inspection, as the bearing seized on it.

 

[searay3]

Re: Mercruiser Knocking

When a rod bearing goes, the rod gets hammered into an egg shape. Probably can't see it by eye. Also the wrist pin takes a beating from the hammering. As was stated, crank will need attention, possibly ground to an undersized bearing, and rod/wrist pin need replaced. If the piston is questionable replace it too. Hate to see you have to go back in a third time....

 

[Midnight Thunder]

Re: Mercruiser Knocking

Excellent advice - thank you.

I was hoping not to have to pull the pistons - new head gasket and all that.

I will replace the wrist pins and crank bearings while it is off as well as re-inspect the rod bearings for wear.

Will a machine shop be able to inspect it all and match rod bearings with just the crank and the piston rods? (really hard to get the block to them)

Is there any way to be able to pull the rod wrist pin without the piston coming out? (save me a gasket guys lol)

 

[Coors]

Re: Mercruiser Knocking

Nope, pull block, and take it to machine shop, or you will do it twice.

 

[Midnight Thunder]

Re: Mercruiser Knocking

Good enough. I will find a way to pull the block.

To lighten my load - can I remove the head and bring just the block with pistons and crank intact? lol - please say yes.

 

[Coors]

Re: Mercruiser Knocking

Sure, they would have to take the head off anyway.

 

[Midnight Thunder]

Re: Mercruiser Knocking

thanks for all your help. I appreciate the assistance and will post how it goes.

 

[Coors]

Re: Mercruiser Knocking

The more accessories you can take off it, the less $ it will cost

 

[Midnight Thunder]

Re: Mercruiser Knocking

for sure. I've done most of my work on a 2-stroke.

With the 4 stroke, I will of course go to my manual, but removing the crank and the cam - is it difficult to get it back together - I'm thinking about the timing aspect - I do not want to get it wrong.

If all they need to do is the rods - 'everything' else is done - then I will save some $$ there.

I would think I need to ensure the cam, the crank and the distributor need to align perfectly.

I'll read up on it tonight - but am I going to be into any serious challenges?

 

[Coors]

Re: Mercruiser Knocking

No, leave the cam, crank, and pistons in it; they will want to eyeball all of that. Basically, give them the short block, with heads.

 

[Midnight Thunder]

Re: Mercruiser Knocking

ok - so block with pistons and cams together, head on the side. lol will do and thank you again.

 

[Bondo]

Re: Mercruiser Knocking

Ayuh,..........

Strip the motor as much as Possible in the boat,......
All the acillerary componets,.. Starter, Alt., Induction, Exhaust, Heads......
That'll Lighten the load for Pulling it out of the boat.....

You want to deliver the Complete Short Block, with the Heads on the side, to your Machinest,.....
Leave the Oil Pan on it,.... It makes it Easier,+ Cleaner moving it around town.....

He'll tell you How Much is Salvageable,+ How Much Work has to be Done to get a Quality Repair........
Your Machinest will want to do the Disassembley to see Why it happened,.......

All Said,+ Done,........ You'll end up with a Rebuild motor,.... Going that Deep means all the Usual replacement parts should be used.....
Rings, Bearings, Gaskets, Etc........

 

[Coors]

Re: Mercruiser Knocking

Yeap, what Bondo said is what I meant. They want to see it whole, as much as possible, in order to see if a periphial took it out, or it was just worn out. Rebuilt a lot of engines, back in the day, and we needed the info (was it a valve hitting, was it not changing the oil, did the timing chain stretch, etc)

 

[Midnight Thunder]

Re: Mercruiser Knocking

Good enough - will do and I thank you for your time.

 

[Midnight Thunder]

Re: Mercruiser Knocking

update:

I spoke to the 2 machine shops - one is a performance machine shop about the prob. They are familiar with the engine (a bit) and said all they need is the crank and pistons together - save $$ without the block - they said to replace the bearings there and check tolerances but they can inspect the crank and pistons np with them out - they already did the head. So I'll pull that this weekend.

Putting the crank back in - thinking about timing here - am I in for any serious challenges? The manual seems pretty straight forward. Should I replace the timing chain even if there is no wear?

thx!

 

[Coors]

Re: Mercruiser Knocking

What did the other machine shop say? as you can't pull the crank and pistons out as a unit.

 

[Midnight Thunder]

Re: Mercruiser Knocking

lol - for sure - they both said bring the crank and the pistons marked (cylinder 1,2,3 etc) and they can check it like that - does not need to be in the block - they would pull it anyways. Think I need to get a third opinion?

Please keep in mind that the knock all started when I burnt a rod bearing - not sure if that helps you. As far as we have been able to tell - it's age.