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Johnson / Evinrude

PISTON DESIGNS

There are three basic piston designs used in Johnson / Evinrude outboard engines;
Crossflow, Loopcharged and Four-Stroke.

Crossflow Loopcharged Four-Stroke

The oldest design, and the In 1968 Johnson / Evinrude Four-stroke / four-cycle engines
one used on all Johnson / introduced an outboard engine are fairly new to the marine
Evinrude engines until 1968, with loopcharged pistons. This industry. They have become the
is the crossflow piston. As the engine was the 55 HP 3-Cylinder. engine design preferred by most
air / fuel mixture travels through A loopcharged engine has a marine engine manufacturers
the transfer port and into the transfer port cast into the cylinder to meet government mandated
cylinder, a deflector, cast into wall in such a way that it will direct exhaust emission standards. Oil
the top of the piston, directs the air / fuel mixture towards the stored in the crankcase of these
the air / fuel mixture towards the combustion chamber and spark engines is splashed onto the
combustion chamber and spark plug tip without the need for a cylinder walls for piston and
plug tip for maximum efficiency. piston deflector. All engines with ring lubrication.The pistons are
All engines with deflectors on top a flat top or a slight dome on equipped with oil control rings
of the pistons will be crossflow the top of the pistons will be to scrap excess oil off of the
engines. External inspection of a loopcharged. Loopcharged cylinder walls. Four-stroke / four-
crossflow engine will reveal engines will not have removable cycle pistons are easily identified
removeable transfer port covers transfer port covers on the side by a three piece oil control ring
on the side of the cylinders. of the cylinders. in the bottom ring position. The
pistons almost always have a
flat top or a dished top and never
have needle bearings between
the wrist pin and connecting rod
(like two-stroke engines).

Removeable Transfer Port Cover Non-Removeable Transfer Port Cover

Piston with Flat Top and Oil Ring
Piston with Deflector Top Piston with Flat Top in the Bottom Position

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