This week I shall say a few words about piston and ring troubles because, hard-wearing as these seemingly simple components are, during operation they are subject to dynamic pressures, heat, friction, plus rapid changes of speed and inertia. Like most things, they eventually wear, but generally last for many years provided they are not abused by drag-type starts when cold, or by infrequent servicing or oil changes.
I have often seen piston damage as a result of broken rings, also rings seized in their grooves, resulting in the loss of ring pressure against the cylinder walls, leading to high oil consumption and the formation of carbon deposits .
One thing leads to another.
Carbon is abrasive and tends to score pistons and bores.
Because of heat expansion, there must be some side clearance in which rings operate. Without it they could stick or bind. Unfortunately, as the piston moves up and down on different engine strokes, due to ring pressure against the cylinder wall and inertia of the ring itself, the ring tends to lag behind as piston direction changes. Thus it moves up and down in its groove, resulting in wear on both components.
Wear in the groove increases side clearance and oil consumption and excessive ring wear makes things worse and breakage possible.
Cylinder condition also has an influence on this.
When a cylinder wears, it becomes tapered and in fact has a larger diameter at the top of ring travel than that of the bottom. This means that as the piston moves down, the rings are forced more deeply into the ring grooves, but the reverse happens as the piston moves up. Ring tension expands the ring to fit the worn part of the cylinder, resulting in continuous expanding and contraction – and more wear.
A gap in the ring ends is necessary to prevent the ends butting-up. To add to our troubles, we can have localised hot spots. Metal from a ring may become welded to the cylinder wall, thus causing further damage.
The causes of such hot spots are many and I have come across some due to manufacturing defects. But more likely you will find cooling system scale on the water jacket.
Pre-ignition and detonation may cause serious piston damage.
So if you hear knocking noises, seek expert opinion as soon as possible as both can lead to piston damage. The next time you take the cylinder head off for a valve grind, especially if the engine is beyond the flush of youth, examine pistons and bores carefully.