What causes Kohler Piston be scored above first ring.

[southbend9]

I have not done that many small engine overhuals. So when I saw this piston come out of the block looking like this I wondered why ? I know some of you have seen this so I thought I would ask you what happen.

[steamin10]

My opinion is high combustion temp causing a condition that allows scoring. after the top ring is glued in from burnt oil, there is a flame leak that amplifies the problem, literally softening the aluminum to weld on to the top of the cylinder.

Blocked cooling , high head temps, lean fuel mix all contribute. Dirt in the cumbustion air and cause the beginnings too. Really a lot of small problems can grow along this line, but high running temp is my answer.

In small yard amchines, grass clippings are notroious for causing cooling failures of this type by filling the ducts with clippings, limiting the air flow. Death to larger engines, as they are more sensative to heat.

[southbend9]

Now that you mention the heat. This I was told usally happens on the exhaust valve side and it did.

[lynmn]

The Kohler service manual calls that condition, carbon cut.
Hard carbon builds up on top edge of cylinder near ex. valve.
A very common problem on the K series engines.

lynmn

[Mid Day Machining]

We have the same problem with Zenoah engines used to power Gas RC race boats. It happens on the exhaust side of the piston, and and will ususlly ruin both the piston and the cylinder.

As a cure, we will then the top of the piston down .004 about 1/8 inch below the ring.

Granted, our engines run quite a bit faster ( 16,500 to 18,000 RPM )

Don't sand it. Turn it on a lathe with a tool that has at least a .045 tool nose radius. The machine grooves created bu the turning tool will carry oil for lubtication. If you polish .004 off, you won't have the oil grooves.

That cured the problem for us.

[jim rozen]

There seems to be a lot of carbon between the top two rings. The
oil ring(s) seem to be working fine, but you might check the side
gap on the top ring, and be sure that the ring land is dead flat.

If the land is not flat you get a 'wavy' ring groove - and even though
it WILL check fine with a feeler gage at ever point along the ring,
the piston will blow by something terrible because the ring simply
cannot seat on a land that is not flat by any appreciable amount.

And yes, I've seen that effect before, on a motor of mine. Drove
be crazy because it would blow by so bad it would use a great deal
of oil. Finally had to set the piston up in the lathe to check the
ring grooves - dang, out by about 15 thou!!!

Have no idea how it got that way but new pistons put that motor
right.

Jim

[flutedchamber]

Synthetic oils will do a lot to prevent the carbon buildup in that area.

My buddy raced Honda 4 stroke dirt bikes and they had much the same trouble that you list, along with eating camshafts and rocker arms twice per season. Switching from Honda regular oil to their full synthetic cured both problems. Camshafts and rockers lasted more than one season..often two seasons.

[BClem]

We used 'Blue Max' synthetic oil in a Hirth flat four two cycle fuel mix. These ultralight aircraft engines were torn down every 100 hours and it was amazing how clean and carbon free they were. The synthetic oil definitely helps keep the carbon under control if not eliminates it entirely. Although a four cycle may not benefit as much as a two cycle by the use of synthetic oil since the lube oil is controlled away from the combustion chamber as much as possible. Most engine manufacturers recommend a 'de-carbonizing' head removal periodically so it is nothing new or nothing to be alarmed about in any case. The rings can become frozen due to a build up of carbon and actually begin to score the cylinder walls if the engine is left either running rich or under a lengthy running load condition. Carbon build up is usually due to too much fuel or a too rich mixture. The correct carbon should be grey, not black - as can be checked on the spark plug porcelain.

An engine can benefit by doing a carbon elimination run at the end of each season by fogging the running engine down with a product called 'store-n-start'. Doing the procedure will eliminate a considerable carbon build up. An old method was to rev the engine up and pour water into the intake - steam the carbon off the combustion chamber surfaces - but that method will clog an exhaust system badly.... Kerosene also works with this method - especially if you have a mosquito problem too.

[steamin10]

Ha! We used to decarb race motors that started to get problems, and small briggs engines too. Running water into a screaming engine sounds like death on the move, but really solved lots of problems of carbon build up from fuel quench on high compression engines, where carbon caused knock or even filled the compression space to the point it closed the edges of the space completely. Now you have carbon with the piston hammering the head. A few seconds of water eliminates all that without any wrench time. Circle track motors were a problem for us until we got the boosters dialed in where we did not lean out on WOT. In theory we were using excess fuel to cool the cylinder, to prevent a high rpm preignition, much of which can be caused by hot carbon acting as a spark source. Real world, a mess with poor engine design. It is reaaly not a fuel problem, but go-fast parts that dont work well together.

The Navy buys fuel all over the world, at foreign ports. It has a lab and tests that diesel for its quality, and finds hotter and colder fuels all the time. Airlines have the same problems and of course, monitor the quality they recieve.

Yep I confirm the right mix will leave a gray dusty film on plug tip porcelans. Watching what a working engine is doing by this method alone will detect a bad cylinder seal, coolant leaks, bad fuel , or fuel mix, and more, if you wanna take the time to learn.