That mark is from the side of the piston pressing against the side of the cylinder because the bottom of the connecting rod ( big end ) connection to the crank journal bearing traveles through a region where it is not directly in line with the center of the cylinder, during every power stroke. ( un-like, it is in line, at TDC and bdc ). That area is a common area for wear because of that.
Don't use compression testers with the check valve near the gauge and at a distance from the sparkplug hole fitting. The hose of those type puts too much space into the setup resulting in inaccurate readings.
If you really want to know if it has worn to an extent that its a problem, do a compression test, ( with a tester with the check valve in the part that screws into the sparkplug hole, with the fuel system disabled, the spark disabled, and do at least 8 revolutions with 4 compression strokes in a row when testing each cylinder ).
If the leakage caused by that wear becomes excessive, ( compression too low ) you'll have a lot of blowby ( check to see how high the unscrewed oil fill cap dances when placed on the oil fil hole with engine idling ), and when it gets severe, the engine will not have enough compression to start on cold days when it has sat long enough to completely cool to ambient. When it does that, its time to replace the engine.
If the piston rings are stuck because of carbon buildup, then that kind of wear happens quicker, and the engine wears out quicker.
Because gumed up piston rings " might " be contributing to this problem, it would be wise to use VRP for the remaining life of that engine, however long or short that may be.
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Also, just in case you don't know it, those engines really require valve adjustment every 100 K miles. They are simple overhead cams with rockers and adjusters. There's no hydraulic lifter to take up slack with wear. The intakes in general become loose with high miles, resulting in a slight loss of power because they may not open enough, and might make more noise. But in general, the exhaust valves become tighter with use, resulting in a quiet valve that can destroy the engine if it gets too tight. If an exhaust valve gets too tight, it WILL melt as the super hot combustion gases rush past the small opening because it did not close all the way. The melted valve pieces usually manage to get a few pieces between the cylinder wall and piston and ruin the engine. Then the entire engine requires replacement.
Some people mistakenly think if it ain't making excessive valve noise, its alright. But you can have exhaust valves too tight without intakes making excess noise. It only takes one becoming tight enough to not close all the way, to ruin an engine. And that engine has 8 exhaust valves.
The hot gas byproducts after ignition ARE WAY TOOOOOOOO HOT for any exhaust valve to handle when the piston is still up. Only AFTER the piston moves down to provide room for those gasss to expand and drastically reduce temperature because of expansion, can an exhaust valve handle those gases going by it. That's why as soon as any exhaust valve becomes tight enough to not close all the way, its game over, and that engine can be ruined. Sometimes people get lucky and all the melted valve pieces get blown out the exhaust and it only needs a new valve seat or head, and valve. But usually the melted valve pieces manage to score the cylinder wall and ruin the engine.
If the valves have not been adjusted, adjust them ASAP.
