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Piston wear marks bad news or business as usual

T

tomf

Regular Contributor
"1990 Mariner 15hp, unknown hi

"1990 Mariner 15hp, unknown history, acquired as partly disassembled non-running engine.

Q1: Found what appears to be minor scoring or wear marks on one of the pistons. The marking is not deep, and there are no tangible grooves as such - one can just feel a slight roughness of the surface.

293200.jpg
Mariner 15 hp, piston wear marks"">

The cylinder had no visible scoring, though there were some wear marks at the corresponding location, along with a pile of accumulated carbon gunk in the same sector at the end of the piston stroke. Also noticed that one of the piston rings is sticking in its groove from one end. Does this look routine, worrisome or like a lost cause?

Q2: The other piston looks ok on the sides, but I'm wondering about the piston crown. It looks noticeably "curvier" than the other one, which has a much smaller "dip" in its curvature. Is this normal or somehow deformed?

293201.jpg
Mariner 15hp, piston crown"">

Any advice is appreciated, as I'm fairly new to working on outboards at this depth. Thanks in advance!"
 
"If the piston rings are OK th

"If the piston rings are OK then you could leave the top piston as is but make sure you remove the aluminum that has stuck to the inside of the cylinder sleeve. Basically, the piston has collided with the cylinder wall and the softer aluminum has "rolled" on to the cylinder. Take a sharp sturdy knife and scrape the aluminum off. Remove any high spots from the damage on the piston.

As for the second piston I am unsure, but it may be detonation damage which is known to pit and gouge metal away over time. The process is similar to cavitation erosion seen on props.

Basically it looks as though the motor was running at improper timing for some time (wrong fuel even). The cause of the first piston impact may have been from overheating at some time. I would replace both pistons if it were me and check the timing."
 
"OK, thanks Derek. Overheating

"OK, thanks Derek. Overheating does seem more than possible, since the thermostat on the machine was corroded shut and there was also some "bubbling" evident on the quality control sticker on the cylinder block. My current working hypothesis for at least a partial history of the engine is as follows:

1. Engine left sitting in brackish water for extended period; thermostat corrodes shut.

2. Owner starts engine; no water circulation; overheating results.

3. Owner starts pulling things apart to find problem; gives up, abandons engine in shed.

Could be totally off the mark, of course..."
 
"Here's a better look at t

"Here's a better look at the "mystery piston crown", minus some of the fossilized carbon...

293468.jpg
Mariner 15 piston crown"">

What do you guys make of this? To my eye, the surface doesn't really look too damaged, but the "dip" in the curvature of the crown is weird.

If it helps in any theorizing, this piston also had a large amount of carbon buildup on the inside of the piston head, whereas the other piston (with the score marks) had almost none."
 
"Revisiting this thread with a

"Revisiting this thread with a further thought...

During disassembly, I noticed that the ignition coil wires on the engine were crossed - not sure if somebody tried to run it this way, or if they just put them back on the wrong way after partly disassembling it.

The scuffing in the top piston looks pretty standard, but would running with the coil leads crossed be enough to lead to this kind of piston crown deformation (see bottom pic)? Upon comparison with a replacement piston, I noticed that both pistons in fact have this defect.

I haven't yet been able to come up with a full theory for the history of this engine, but whatever it is, it's definitely AFU.
happy.gif


Even the carbon on the piston isn't like anything resembling a burnt-in petroleum product - more like fossilized rock, totally impervious to cleaning solvents and even a Dremel wire wheel.

Luckily, both cylinders are within spec for diameter and out-of-round, so hopefully with replacement pistons and new rings I might yet get this baby to run..."
 
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