Piston Ring Compressor
A tool used to compress piston rings into their grooves so the piston assembly can slide into the cylinder bore.
A piston ring compressor is a specialized engine assembly tool used by engine builders to install piston and connecting rod assemblies into the engine block. Piston rings naturally expand outward past the diameter of the piston to seal against the cylinder walls. Because the rings project outward, they cannot slide into the cylinder bore unless they are compressed flush with the piston skirt.
The ring compressor consists of a flexible sheet-metal band or a tapered aluminum sleeve that wraps around the piston. The band is tightened using a ratcheting key, compressing the rings tightly into their matching ring grooves on the piston. The piston assembly is then aligned with the cylinder bore, and the mechanic gently taps the top of the piston with a wooden mallet handle, sliding the piston into the cylinder while the compressor holds the rings in place.
Using the correct size compressor and keeping it lubricated with engine oil is critical. If the compressor is loose, a ring can pop out and catch on the cylinder deck, snapping the ring or scoring the cylinder wall as it is tapped down. Fixed-diameter tapered sleeve compressors are highly favored by professional engine builders for their reliability and ease of use.