Cylinder Wall Scuffs (Pictures)

[Sureshot]

I'm hoping someone may be able to help me diagnose the cause of these scratches / scuffs on my cylinder walls. I have a 50 HP Johnson, 2000 model. On the top cylinder, the scratches are on both the left and right side of the cylinder walls. On the bottom cylinder, the scratches are only on the left side. The rest of the walls still have the cross hatching. I noticed the scratches after a trip on the lake where I had a few problems:

1) When I was slowing down to take a break if felt like I lost a cylinder and the engine died. I tried cranking it for almost 30 minutes but couldn't get it to fire. I didn't have my proper tools with me so I couldn't tell if I was getting spark or if I had flooded the engine possibly. I got a tow back to a dock and then tried cranking and the engine fired right up.

2) After I had gotten the engine going again after my 1st problem, we were cruising around 4200 RPM down the lake and then all the sudden the engine just cut out and shut off. It was very quick, not enough time to try pumping the bulb to see if it was starving for fuel. Never got an overheat alarm and my horn had been tested. The engine immediately started back up and I idled it back to the dock. Checked compression when I got home and I was at 120 on both cylinders.

Any idea what likely caused those scratches based on the problems I had? Also, am I ok continuing to run the motor? I took it out one time after I noticed the scuffs and it ran great. Thanks in advance.

 

[cr2k]

Re: Cylinder Wall Scuffs (Pictures)

My guess, since the scuffs are perpendicular to the crank, is maybe the wrist pins are loose and sliding back and forth rubbing on the cyl walls.

 

[wrench 3]

Re: Cylinder Wall Scuffs (Pictures)

If I'm reading the pictures right, the scuff marks are at 90 degrees to the wrist pins. Or maybe 80. Definitely not lined up. If the compression is good, the marks won't cause a problem. However, if the marks are bad enough to catch a finger nail in them, you should find out what caused them. It could be overheating or poor lubrication. The effects are almost identical.

 

[emdsapmgr]

Re: Cylinder Wall Scuffs (Pictures)

There is a significant indicator: the cylinder liner scuffing is happening on both cyls. So, probaby not related to a single carb jet restriction that would affect one cyl only. Looks like the cyls were recently honed and the pistons look to be .050 over, so it could be a recent overhaul. Those pistons are not factory, so need to be accounted for when overboring each cyl. Non-factory pistons usually expand thermally at a different rate than the factory pistons, so different boring clearances ares usually planned for by the machinist that does the boring. The wear pattern appears to be generally on the high thrust sides of the piston (90 and 270) degrees and not the top/bottom, so it could be a fit/clearance issue. Also, when overboring beyond .030 over I believe it's recommended that you increase the main jet size somewhat to account for the higher displacement. Your overbore tech can probably make a suggestion for any jet size increase.

 

[Sureshot]

Re: Cylinder Wall Scuffs (Pictures)

Thanks fellas. This engine was recently overhauled. The top and bottom cylinders were bored and honed to .020 over. WSM .020 pistons were installed. I took it to an outboard repair shop of good reputation. They did the machine work and I installed the parts. Where I'm confused is to why these scuffs appeared after I had run the engine the entire season. I've been babying this engine, checking compression after each outing and using a flashlight to look at the cylinder walls. The wear only showed up after the outing I described in post #1.

 

[emdsapmgr]

Re: Cylinder Wall Scuffs (Pictures)

Even though the horn was replaced, the head overheat sensor could be bad. If so, possible the engine overheated and it never set off the horn. If that block overheats, the pistons can swell just enough to instantly seize inside the cyls, locking up the engine. Usually, it will restart after cooling off after a few minutes. The compression still sounds ok, but if an overheat lockup situation repeats, it is possible to break a ring and then you'd see lower compression. I assume it has a strong telltale....? Was the thermostat replaced at overhaul?

 

[Sureshot]

Re: Cylinder Wall Scuffs (Pictures)

Thanks emdsapmgr. I replaced the thermostat and tested the heat sensor by grounding it to the block. The horn went off so I'm assuming the overheat warning system works as it should. Telltale was good too.

So maybe when it shut down on me it was a seize and not an ignition / fuel problem? One other thing was I had a VRO fuel pump when this issue occurred which I've since replaced with premix.

 

[gm280]

Re: Cylinder Wall Scuffs (Pictures)

If this IS due to a rebore and oversized pistons that were not clearanced enough for thermal expansion, then you could easily get a snap gauge and micrometer and see if the cylinders are now egged shaped as well. If it still is in spec, then carry on and be watchful...