It is vital that pistons be properl

It is vital that pistons be properly cooled. Both the AT25 and 27 engines use oil under pressure to cool the piston crown in order to reduce the potential for detonation and damaging piston expansion. On the AT25 and previous AT27 the oil travels from the crankshaft through the connecting rod and through the piston pin. It then enters the piston via a cast passage and into the cooling cooling coils where it circles the crown before exiting the piston through a drilled passage on the other side of the pin bore. The oil flow through the current AT27 piston is the same other than it uses an open chamber in the piston crown instead of the coilThis cut-a-way is of an AT25 piston. Note the location of the forth and fifth oil control rings. Beneath each oil control ring is a shallow half round groove. These accumulator grooves allow for only a very specific amount of oil to be retained for lubrication. Excess oil is returned to the crankcase via holes in the grooves. Previous AT27s used only one oil control ring and accumulator groove. Current AT27s do not have the accumulator groove.