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Stinking 37 rod bearing

gibler3

Member
"Ok. I have a 1987 3.7 Merc &

"Ok. I have a 1987 3.7 Merc (470 block). Last year I spun a rod bearing. So I pulled it out, tore it down, had the crank turned and put it back together with new bearings. The shop said my crank was not bent. I had the rods checked (replaced 1 w/a cast piston weighed and matched to the other 3) and honed the cylinders lightly. I also blew through all of my oil galleys and washed the block with soap and water. My exhaust riser is in great shape and everything went back together with new gaskets. I had my Merc manual on the workbench next to me the whole time. The first trip out, it ran great. Had power, planed out fast, ran cool and sounded good. Was a good day.

The following weekend, I put it in the water and no more than 1 minute from the ramp, I had a rod knocking again. My temp was very low (just put it in), my oil pressure was 60 (until my rod started knocking - then dropped to 20) and hadn't even hit 2000 rpm yet. I have a new temp sender and gauge and have verified with a laser temp gun.

When I had it apart before, there was no water or fuel in the oil. My coolant level was fine. I've had the 4" heat exchanger apart and it looks great - never in salt. I know by the sound what it is. How the heck could I wipe out a rod bearing like that? This one doesn't make sense to me - looking for ideas. My boat is 19', alpha 1, and 19 pitch prop.

Wish I could find a reasonably priced 3.0 to drop in and get rid of the thing - the boat isn't worth a new engine. I don't ever want to own a 3.7 after this one
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"Mike, sorry to hear it's

"Mike, sorry to hear it's not going well for you. I can imagine your frustration at having spent all that time and money to rebuild your engine and still have that rod knock! I have a good running '83 470 that has a slight rod knock from the rear. It seems to disappear when the engine warms up but returns when its cools down after idling for a while. I'm still running it that way, It has NOT gotten worse in about 25+ hours of operation. I know I'm pushing it, but I'm not going to pull the engine in the middle of the season.
Here is what I think happens to these engines. Water enters the exhaust manifold from plugged riser or cracked exhaust manifold, missing/defective exhaust flapper and gets into the rear cylinder. Since the engine can't compress the water, you get engine damage. Usually a spun #4 bearing.
Do you think you spun the bearing or maybe like me, you have excessive clearance? What I'm thinking of doing, which might apply to you is to pull the engine and drop the pan. Pop the caps off the rods, and if all looks good, install new rod bearings. The rest of the engine can stay together. I may do this in the off season. Since you had the engine out before, this might go quick for you. I did this once in a car and pulled it off. It was a quick fix! I'm sure you have the correct size bearings and that the crank was not cut, right? Well, that's my 2 cents. I don't think you are going to get a quick easy answer here. Good Luck!
Chris
Some helpful links
http://www.flickr.com/photos/m170/
http://www.breezeworks.net/cgi-bin/MBoard/forum/display_forum_topics.asp?ForumID =1&PagePosition=1&ThreadPage=1"
 
It's more than too much cl

It's more than too much clearance. It's making a pretty nasty noise. And it wiped out #3 last time. It's possible that water is making it's way into a cylinder but I didn't pick up on any signs of it last time. Not to say that isn't what is happening though. I have no choice but to pull it out and will give it a check. Thanks!!
 
"So - if I find that I have wa

"So - if I find that I have water entering my #3 or #4 cylinder from a manifold crack.....
Does anyone know why mercruiser switched from a cast iron exhaust manifold to an aluminum one for any reason beside weight? I have a good cast manifold off an older 470 that I might try and use with this engine if I find that the alum is cracked. Otherwise, new alum is $750+ or take a chance on ebay. Thanks for any input - mike"
 
"I pulled my alum exhaust mani

"I pulled my alum exhaust manifold today and capped off the ports, pressurized it and checked it w/soapy water - nothing. Got the torch out and heated it up to 170 - nothing. So I filled the exhaust side full of water and let is sit for an hour thinking I might get a bubble or two while under pressure - nothing. All I could get on it was about 30 psi with the test cap I have. I'm thinking at this point that my manifold and riser are both good. My exhaust flapper is intact and ok. My next step is to do a leakdown test on each cylinder to see what happens. I'm not against the idea that I might be getting water in a cylinder. Maybe a cracked head? I have a new head gasket and followed the torque sequence per Merc specs. My spark plugs aren't fouled or nasty. Any suggestions? Scratching my head on this one."
 
"The leakdown test will confir

"The leakdown test will confirm if you have a leak anywhere, including cracked head or block. Go for it.
Rod"
 
"Mike, I'm sure you would

"Mike, I'm sure you would have known if the manifold or the riser looked bad before you put it on. I have heard of cheap or in-correct riser gaskets causeing problems. Risers plug up and blow water out the gasket and into the exhaust chambers.
I'm starting to think it waas a fluke that you through a bearing a second time. Did you check the clearence with Plastic gauge when you rebuilt. Since your crank was cut, you would need the right size bearings. I noticed you mentioned your oil pressure droped. I was in a car with my friend and his oil light came on, first thing that happemed, he did throw a rod bearing. Check the manifold/riser/flapper. Then check the heads for cracks, then mic crank, install new bearings, and new oil pump. My 2 cents
Chris"
 
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