Engine Mechanism & Function -- Mechanical (H4SO): Piston

• The pistons are of a slipper skirt design for reduced weight and friction. The piston is also a thermal flow type whose oil control ring groove is provided with round oil drain holes.
• The piston pin is offset toward the thrust side (No. 1 and No. 3 pistons downwards, No. 2 and No. 4 pistons upwards) to reduce piston slap noises.
• The piston head has recesses to prevent interference with the intake and exhaust valves. It also has engraved marks to identify the piston size and the direction of installation. On type I models, the piston heads are concaved at the entire exhaust side to gather the tumble flow around the spark plug and improve combustion. Only on the type I models the pistons are distinguished left or right.
• Three piston rings are used for each piston; two compression rings and one oil control ring. The top piston ring has inner bevels and the second piston ring has an interrupt (cut) on the bottom outside to reduce oil consumption.

Fig 1: Identifying Inner Bevel Of Piston

Courtesy of SUBARU OF AMERICA, INC.