Ask Sarah: Is gapless good?

Norby Urbano

 

Piston rings are important for maintaining good combustion pressures and preventing excessive oil consumption.  Different rings perform different functions.  All the rings stabilize the piston as it moves up and down the bore.  Compression rings are typically designed with a gap to allow them to expand outwards and provide an effective seal when they are compressed in the cylinder.  The top compression ring must also cool the piston by transferring heat to the engine block.  Second rings assist the top compression ring with sealing combustion as well as helping to keep oil from moving up the cylinder walls.  The second ring is a tapered face ring with the leading edge down and can be very effective at pulling oil off the cylinder walls.  The 3rd ring, or oil control ring, is located in the piston grooves below the compression rings.  Oil control rings prevent excess oil consumption by scraping excess oil off the cylinder wall and returning it to the crankcase.  They also make sure the compression rings are well lubricated.  

  

Pre-gapped stock rings are installed with end gaps that will allow the rings to safely expand and contract from the heat and friction generated during the combustion process.  This gap fluctuates but it should be wide enough to keep the rings from binding or not sealing properly.  Other options include file fit or gapless rings.  File fit rings start at virtually no end gap and are then filed down to the desired clearance before they are installed.  Gapless rings, such as those by Total Seal, are a two piece overlapping ring assembly with an interlocking rail that forms a gapless seal.  These rings are available in a gapless top or gapless 2nd ring version, and claim to improve the seal between the piston and the cylinder and close the path the gases can escape through.  

The Total Seal TSS ring set is for use with street or mild race buildups, no forced induction or nitrous, and compression ratios under 10.5:1.  The ring set includes a cast iron conventional top ring, gapless second ring, and three piece standard tension oil ring.

 Approximately 1800psi of combustion pressure pushes the ring against the cylinder wall and down on the bottom of the ring groove.  If leaking combustion pressure builds up between the top and 2nd rings, it could lift the top ring off the bottom of the ring groove, causing it to lose its seal.  This is a gas induced ring flutter.  The gas that gets past the rings is known as blow-by.  Blow-by leads to a loss of compression (less power), lower fuel economy, an increase in emissions, and contaminates the lubricating oil.  A tighter ring seal will lead to more complete cylinder filling in a naturally aspirated engine – more intake charge available for making power.  A tighter seal will also better contain the explosive forces during the power stroke, leading to more torque and horsepower.  But there really isn't a huge improvement when comparing a gapless ring to a stock ring with correct hot clearances.