Piston Fit. & Oil Control
A pistons role in blow-by and oil control is esential. The elements of a piston which affect oil control are the piston skirt fit in the cylinder bore, the piston skirt design, oil drainage, the piston's stability, the piston's land and groove width and squareness and the connecting rod alignment
The piston fit in the cylinder bore is crutial due to the fact that on the downward stoke it is the piston's skirt which first shears the oil film prior to the piston ring passing. If the oil film is too thick the piston ring will hydroplane over the oil and it will be scraped to the combustion chamber on the upward stroke where it will be burned, thus reducing the power output of the cylinder.
The piston's skirt design determines how effective a pistons oil shearing ability is. Full skirt pistons which are bore size around their entire circumference are more efficient oil scrappers than the current open slipper type pistons of today. The open slipper pistons are necessary to clear crankshaft counter balances although full skirt pistons are still used in some engines.
The draining back of oil into the oil ring area of the piston is important for good oil control. When the oil scraped by the oil ring cannot be removed or drained rapidly behind or under the oil ring, it will cause the oil ring to ride over the oil film leaving too thick a film on the cylinder wall, the oil ring is responsible for scraping the largest part of the oil film from the cylinder wall, the draining of this oil down the inside of the piston must be efficient.
In racing engines friction and weight reduction are a prime consideration, pistons have become much shorter. Both ring grooves and piston lands have been reduced in width. The Piston's stability is affected by a shorter piston when the piston is protruding from the bottom of the bore at B.D.C. At the start of the upstroke the piston tends to shift in the bore, this movement will interfere with the piston's ring's ability to stay in contact with the bore , thus increasesing the piston's groove and ring side wear and accelerasting the piston's skirt wear and collapse. The more stable a piston is the less wear will occur.
In a hi-performance piston that has a reduced piston height and reduced piston ring groove and land width it is important that the second land under the top piston ring be strong enough to resist deflection during the engine's power stroke.
The ring grooves of the piston must have no downward tilt. If it is titled the top edge of the piston ring will be wear excessively and during the upward stroke of the pistrion it will cause oil to be scraped up to be burt during the power stroke. A piston's ring grooves must be perpendicular to the center line of the piston. When the piston's ring grooves are tilted, it will cause the piston ring to spin rapidly and the piston moves up and down in the cylindewr bore, this will accelerate a piston's ring side and could result in breakage.
When a connecting rod is misaligned certain situations occur. The piston rings are not perpendicular to the center line of the cylinder bore and will rotate rapidly and the connecting rod bearings will be misaligned on the crank journal causing abnormal bearing wear and excessive oil will be spun onto the cylinder wall.