Piston rings

by Had Robinson
updated August 10, 2023

Note: operations on paramotor pistons do not require any special tools.

Please review the Wiki on piston rings, if needed.

Removal

Use your fingernails to gently spread the tips of the ring so that it will just clear the piston.  Slide the ring off the piston.  Clean the lands (the groove) of the piston with a used ring of the same size that has been broken in half.  If an old broken ring is not available, use the ring you just removed from the piston.  Use a file to nick the outer and inner ring surface and break the ring in half.  There must not be any goo or combustion residue in lands of the piston.

 Installation

Coat the ring with 2 stroke oil.  Spread the tips with your fingernails just enough to slide the ring onto the piston.  Rotate the ring on the piston so that the tips of the ring are on either side of the holding pin that is pressed into the lands of the piston.  If the ring is not lined up properly with the holding pin, the piston will not slide into the cylinder.  If there are markings on the flats of the ring, make sure they are on the top side when installing it.

Worn rings

Like everything else in an engine, piston rings wear out.  If the cylinder lacks oil while running, even for a moment, damage to both the ring and the cylinder will occur.

The photo below shows a new ring on the left and a damaged ring on the right.  If your ring looks much different than the one on the left, it should be replaced.  The ring on the right is from a new engine that had no lubrication for a brief period.  The pilot had fuel that was contaminated with water (he sent me a sample).  However, from the looks of the ring, he probably forgot to mix oil with the gasoline.  Typically, when lubrication is lost, the piston will seize from overheating, as this one probably did.

Notice that the chromium/nitride plating is missing from the outer edges of the damaged ring (red arrows) and the ring core is exposed and badly worn.  Instead of relatively sharp outer edges, the ring is slightly rounded and is unable to do its job of sealing the combustion chamber.

The black arrow shows what is left of the badly scuffed plating.  If the pilot had run the engine a bit more, there would have been nothing left of the plating.  When lubrication is lost while the engine is running, the cylinder, piston, and ring wear is severe and all of the components must be replaced.

The piston ring is necessary to seal the combustion chamber and to transfer heat from the piston to the cylinder walls.  If the piston ring is replaced, it is a good idea to also rehone or replace the cylinder.  If the cylinder wall has any scratches or gouges, it must be replaced.  See Rebuilding a paramotor for more information on restoring a worn but otherwise serviceable cylinder.

photo courtesy of Precision Surfacing Solutions