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Old 09-18-2018, 08:46 PM
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Engine knock

Hello all,
A few weeks ago My oil pressure safety switch sprung a leak and I lost most of the oil in the crankcase. I was just docking when the engine got louder and there was quite a bit of banging going on. I looked at the oil pressure gauge and nothing..... I shut the motor down at once and did some troubleshooting. Put new oil in, found and fixed the leak. I did another oil change. It is much quieter but i have a few residual issues.
1. oil pressure at startup is 40lbs, when warm 20lbs at idle 25 lbs under load at
1000 rpm
2. there is a new noise in my atomic four i have never heard. I have had the
boat ten years and i know what it normally sounds like. something is not
quite right...a knocking noise that was not there before.
3. Engine is starting to burn oil slightly. I can hardly see it but I can smell it
Here is a link to the video and sound. https://www.youtube.com/watch?v=BmGLbmUHhp8
What do you folks think
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Old 09-18-2018, 09:27 PM
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Quote:
Originally Posted by Nick duBois View Post
Hello all,
A few weeks ago My oil pressure safety switch sprung a leak and I lost most of the oil in the crankcase. I was just docking when the engine got louder and there was quite a bit of banging going on. I looked at the oil pressure gauge and nothing..... I shut the motor down at once and did some troubleshooting. Put new oil in, found and fixed the leak. I did another oil change. It is much quieter but i have a few residual issues.
1. oil pressure at startup is 40lbs, when warm 20lbs at idle 25 lbs under load at
1000 rpm
2. there is a new noise in my atomic four i have never heard. I have had the
boat ten years and i know what it normally sounds like. something is not
quite right...a knocking noise that was not there before.
3. Engine is starting to burn oil slightly. I can hardly see it but I can smell it
Here is a link to the video and sound. https://www.youtube.com/watch?v=BmGLbmUHhp8
What do you folks think
I think the engine will need to be rebuilt to solve these issues. As a temporary fix, try using heavier oil.
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Old 09-18-2018, 09:59 PM
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Quote:
Originally Posted by joe_db View Post
I think the engine will need to be rebuilt to solve these issues. As a temporary fix, try using heavier oil.
Agreed but with this caveat: the longer you put it off the more risk of greater damage.

Case in point
When I acquired my spare engine it ran but with a deep sickening knock like John Henry was swinging his hammer inside the engine. A tear down and analysis found the #2 rod bearing was completely gone, ground into hamburger and glommed onto the oil pump pickup screen. I'm sure it happened over time, started small, maybe even an episode like yours but the deterioration continued. Additional damage was an oval rod end and a bent crankshaft. I believe neither would have been issues had the initial problem been dealt with early.
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Old 09-18-2018, 11:23 PM
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I can confirm that when I pulled the plug wires off one after another the knock pretty much stopped on the third cylinder. There was no difference in cylinders 1,2 and 4
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Old 09-19-2018, 08:11 AM
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Quote:
Originally Posted by Nick duBois View Post
I can confirm that when I pulled the plug wires off one after another the knock pretty much stopped on the third cylinder. There was no difference in cylinders 1,2 and 4
I would take the engine home and see what you have.
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Old 09-19-2018, 08:56 AM
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Quote:
Originally Posted by joe_db View Post
I would take the engine home and see what you have.
Yeah i think that is my best option. Im going to use it lightly at low rpm for the next month. then when the boat comes out of the water the motor comes out of the boat. Funny it does not always make that sound. It needed a paint job anyway. Pulling an atomic four out of a Catalina 27 is going to be no easy task....not much room on either side.
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Old 09-19-2018, 09:20 PM
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Quote:
Originally Posted by Nick duBois View Post
Yeah i think that is my best option. Im going to use it lightly at low rpm for the next month. then when the boat comes out of the water the motor comes out of the boat. Funny it does not always make that sound. It needed a paint job anyway. Pulling an atomic four out of a Catalina 27 is going to be no easy task....not much room on either side.
You might consider leaving that plug wire off for a while. Technically you should ground it.
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Old 09-19-2018, 10:54 PM
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Now if you go to my engine rebuild thread, you'll see I also had a knock in an A4, which ended up with a golf ball size hole blown through the block by an errant connecting rod.

But I'm curious, in the OPs case, the knocking goes away when he disconnects #3 spark plug. If the problem were a crank or piston bearing going out, how would removing spark make it go away? The crank is still turning, the piston is still slapping madly up and down. Is it just because the power is removed from the power stroke?
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Old 09-20-2018, 07:15 PM
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Quote:
Originally Posted by wristwister View Post
Now if you go to my engine rebuild thread, you'll see I also had a knock in an A4, which ended up with a golf ball size hole blown through the block by an errant connecting rod.

But I'm curious, in the OPs case, the knocking goes away when he disconnects #3 spark plug. If the problem were a crank or piston bearing going out, how would removing spark make it go away? The crank is still turning, the piston is still slapping madly up and down. Is it just because the power is removed from the power stroke?
Without the combustion there is much less force on the piston-rod-crank. It is just going up and down and round and round. Combustion is a huge force on the piston, any play in the crank journal might make the knock.
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Old 09-20-2018, 09:19 PM
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Quote:
Originally Posted by romantic comedy View Post
Without the combustion there is much less force on the piston-rod-crank. It is just going up and down and round and round. Combustion is a huge force on the piston, any play in the crank journal might make the knock.
OK, that makes sense. But that has me thinking that when my daughter and I were motoring her boat to it's new home with the engine knocking away, we might have actually made it if we pulled the plug from the bad cylinder and motored on 3 cylinders. Good lesson learned, and probably applies to any piston engine; gas, diesel, boat, car, whatever.
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Old 09-21-2018, 09:50 AM
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funny thing... under load in gear there is no knock at all. At 1500rpm i can not detect a thing. I went down and listened for it while someone else had the helm and nothing. Only in neutral or when reducing the rpm can it be heard. Can anyone venture a guess as to why???
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Old 09-22-2018, 02:50 PM
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Nick, if you like you can give me a call and I might be able to drive over and have a listen to it for you. 483 8524
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Old 09-23-2018, 09:45 PM
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Thank you Mo that would be awesome. I will give you a shout tomorrow and we can pick a time that works for you.
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Old 10-24-2018, 06:34 PM
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Suspense is killing me. What did you find?

Mine is knocking too, sounds almost identical.
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Old 10-29-2018, 09:57 PM
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Unhappy Knock, knock

I wondered why it was making so much noise right at the end....
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Old 10-30-2018, 07:57 AM
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hahaha, Ya think?
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Old 10-30-2018, 11:05 PM
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Red face Knock knock. Who's there?

Hate to expose my ignorance so, but please explain to we uninitiated just what we are looking at. Or should not be seeing if all were well.

Mary
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Last edited by HOTFLASH; 10-31-2018 at 08:21 PM.
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Old 10-31-2018, 08:58 AM
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Did a piston blow up?
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Old 10-31-2018, 09:45 AM
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Quote:
Originally Posted by HOTFLASH View Post
Hate to expose myself ignorance so, but please explain to we uninitiated just what we are looking at. Or should not be seeing if all were well.

Mary
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The photo above is from a disassembled engine. A part called the cylinder head has been removed, so you can see inside the engine. The cylinder head covers up round holes that are supposed to contain pistons. The pistons move up and down from burning gasoline, eventually spinning the propeller.

Each of the holes in the engine are supposed to have round pistons in them. The piston in the center hole has self destructed leaving free metal in the hole doing all sorts of damage.

Normal is 4 holes and 4 pistons.
In this case, there are 4 holes and 3 pistons + the remains of a piston.
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Old 10-31-2018, 11:57 AM
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Great explanation!

Thanks for the description, was wondering the same as Mary. And the remaining piston are the bits of metal on top of the case? the walls of the cylinders don't look very thick, is this how a rod can bash into it and cause a hole, thus "throwing a rod?" Appreciate any info on the inside of these wonderful engines. Fairly knowledgeable about the outside parts but rarely see the insides. Thanks!
You know, some descriptions of these "interiors" would be awesome somewhere on this site. I've always wondered how do cylinders get "re-bored" and why would they?
I've heard pistons break down and the cylinders have to be re-sized somehow, or maybe its the reverse. How does that get accomplished?
And old engines, like ours, need work on the rings and bearings, but I don't even know where those are.
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Old 10-31-2018, 11:59 AM
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and Bill L., is that the "head gasket" that gets blown and has to be replaced? How does that get blown? (I think I may need a new one). What are the 2 round metal pieces to the right of the "almost" empty cylinder?
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Old 10-31-2018, 02:03 PM
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Yes, the bit of metal is a broken off part of the failed piston.

When a piston fails, the metal shards get bounced around inside the engine. These shards can cause further damage as they interfere with the correct engine operation. What gets damaged is pretty much a random result of the broken piston.

The rod is normally held in a center position by the piston. Because its in the center, it does not make contact with the cylinder walls.

When the piston breaks, the rod may not be held in position. Between the rod moving where its not supposed to be and the metal fragments floating freely, there is a lot of very bad things that can happen.

A connecting rod has a opening at the rod top. This is where a connecting rod pin attaches the piston to the rod. You can see the pin in the picture. The rod also has a opening at the bottom of the rod, where it connects to the crank shaft. 'Throwing a rod' means the rod itself has failed in such a catastrophic manner as to break free from the crankshaft or break apart. Again broken metal from such a failure does very bad things, including punching holes in the engine block.

A cylinder hole in an engine is supposed to be a certain diameter when its new. As wear occurs, the piston hole may grow in size or get scratched/damaged slightly.

'Boring' a cylinder basically means to 'drill out' or enlarge the piston hole slighly so you get back to smooth usable cylinder walls. When you enlarge the piston hole, you also need a matching size piston to go into that hole.

Yes, that grey material you see IS the head gasket.
'blown head gasket' means the gasket leaks where its not supposed to.

You have to unbolt the head, remove the gasket, replace the gasket, then reinstall the head, tightening the bolts in a specific order with a specific amount of tightness, to replace the head gasket.

The two round objects are the cylinder valves.
The exhaust valve lets burnt fumes out the cylinder.
The intake valve lets air/gas into the cylinder.

This video explains the basic operation of an engine.
https://www.youtube.com/watch?v=Pu7g3uIG6Zo

Rings are a round thin part that goes around the piston head. Their job is to keep the burning fumes at the top of the piston and not let them escape into the rest of the motor. As things age, rings wear out and have to be replaced. Worn rings can be one reason a piston has low compression. Leaky valves is another reason. Rusted/broken/blown head gasket is another.

Bearings are usually installed on a crankshaft or camshaft. Their job is to allow metal things to move against one another without generating friction and heat.
Rub two of your fingers together dry. if you do it enough they will warm up.
Now put a little oil between your fingers. They slide easier. A bearing behaves like the oil on your finger.
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Old 10-31-2018, 05:41 PM
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Post #22 got a thanks from me because of the effort it took to type it. Keep up the good work.


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Old 10-31-2018, 08:08 PM
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With that kind of damage I recommend getting the crankshaft checked for straightness. Our crank with no center main bearing is more susceptible to bending than most any other crankshaft that typically has mains every 2 rod journals.

If you take it to a crankshaft specialist be aware he will number the cylinders - - and journals - - starting at the timing gear end, exactly the opposite of our convention. You'll be talking about the same journal, you'll call it #3 and he'll call it #2.
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Old 10-31-2018, 08:14 PM
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We've had 'pin-ups of the month'. Perhaps it's time to start a 'chamber of horrors' with the photo in Post #15?
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