Engine knock

[alcodiesel]

hahaha, Ya think?

[HOTFLASH]

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
HotFlash
T27 Yawl

[GregH]

Did a piston blow up?

[CajunSpike]

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
HotFlash
T27 Yawl

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.

[blhickson]

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.

[blhickson]

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?

[CajunSpike]

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.

[JOHN COOKSON]

Post #22 got a thanks from me because of the effort it took to type it. Keep up the good work.


TRUE GRIT

[ndutton]

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.

[Al Schober]

We've had 'pin-ups of the month'. Perhaps it's time to start a 'chamber of horrors' with the photo in Post #15?

[jayw]

This is a great example of why not to ignore noises in the engine: doing so makes for very expensive repairs.
Engine started knocking on last 4 hour trip of season to bring boat to haul-out in Plymouth- didn't have much choice but to chug on. Engine failed about 1/2 mile from boat ramp, had to tow 35-foot sloop with my 3.3 hp outboard on the inflatable for the last leg in the harbor, which was quite exciting. Fortunately, the wind was light and not much traffic.
CajunSpike's detailed explanation of my photo is right-on, thanks for filling in where I deserted.
I'll probably not repair this engine since I've had a number of problems with it over the last few seasons mostly relating to being worn-out. Since it has already been re-sleeved before me and I did a valve job 2 years ago, time to move on.
I like Al's idea of "chamber of horrors", just don't like providing the first example.

[Scott312]

Pulled the spark plug wires one by one to see if the problem could be isolated.

Seems like it sounds better when #3 was disconnected and not firing. Any other advice, not sure what the next step should be.

Thanks in advance!

[CajunSpike]

If you pull the plug in the suspected cylinder and rotate the motor till its tdc, you can just see the edge of the piston in question. One tiny way to see if the piston is mostly intact at least.

[Peter]

Pulled the spark plug wires one by one to see if the problem could be isolated.

Seems like it sounds better when #3 was disconnected and not firing. Any other advice, not sure what the next step should be.

Thanks in advance!

From https://moyermarine.com/techtip001-2-2-2-2/ - Elevated Exhaust System Backpressure

"A valuable diagnostic technique was discovered during this episode. Whenever any one of the four spark plug wires was removed, this particular engine ran and even accelerated somewhat better. Clearly, by the time a restriction grows to this extent, the exhaust system is only capable of handling a three cylinder engine. As soon as the forth cylinder is added, the engine bogs down due to being unable to discharge its exhaust."

The sure fire diagnostic would be to measure the back pressure but unless you have the appropriate exhaust flange that will be hard to do.

Second would be to disconnect the exhaust from the manifold and see if it runs better. I have never done this and suspect caution would be required to ensure

1) water does not come back form the disconnected exhaust
2) exhaust fumes do not build up in the boat

I have no doubt more experienced forumites can advise further.

Peter

[Scott312]

Checked the exhaust with a borescope through the water injection tube, good condition.

Checked #3 piston with borescope, piston still there!

Pulled the manifold, and the valve cover - #2 had a stuck exhaust valve.

Not sure if we found THE problem, but we definitely found A problem...