Cleaning a Piston

Before cleaning the piston, check the top ring groove for excessive wear.

The top of the piston should be cleaned with a scraper.

Use a scraper to clean the top of a piston.

The top (and only the top) of the piston can also be cleaned on a wire wheel, but be especially careful not to round off the edges of the piston head.

Experts often advise against cleaning any part of the piston with a wire wheel because it requires skill and care. A beginner can accidentally ruin an otherwise good piston by trying to clean the skirt and ring grooves using the wire wheel.

It is a good idea to clean the piston top before removing the piston rings to avoid accidental ring groove damage.

Cleaning Piston Ring Grooves

It is easier to clean the piston ring grooves if the piston and rod assembly is held in a vise. Be careful not to damage a slipper piston skirt when clamping the rod in the vise. The figure below shows the right way to clamp the rod on a slipper piston in a vise.

The right way to mount a slipper skirt piston and rod assembly in a vise.
The jaws of the vise must be soft metal. Steel-toothed vise jaws can mark the connecting rod and weaken it. Damaging a metal surface raises stress, weakening the part. This is called a "stress raiser."

During engine operation, carbon forms in the back of the compression ring grooves. The carbon must be removed; otherwise, the carbon deposits in the ring grooves might prevent the new rings from compressing enough to enter the cylinder during piston installation.

Clean the carbon from the ring grooves with a ring groove cleaner.

Cleaning carbon from the ring grooves with a ring groove cleaner.
  • When using a ring groove cleaner, be careful not to remove aluminum from the rear of the ring groove after all of the carbon has been removed.
  • The ring groove cleaning tool works very well on compression ring grooves but can easily nick oil ring grooves, which do not usually get as much hard carbon buildup anyway.
  • Oil return holes sometimes become plugged and must be cleaned, or the engine will continue to use oil. Drilled oil holes can be cleaned with a drill bit that correctly fits the oil hole.

If a ring groove cleaner is not available, an old ring can be broken off and ground sharp to clean out carbon.

Cleaning a piston ring groove using a piston ring.

Tape the ring so it does not cut you. Use a ring to double-check for correct groove depth. Roll the ring around the entire groove to ensure that the ring does not bind up on the edges of the ring land.

Knurling Pistons

To compensate for worn piston skirts and cylinder walls, piston skirt diameter can be enlarged by knurling.

Knurling a piston skirt: (a) a knurled piston skirt. (b) Cutaway of a knurled piston skirt.

A knurled piston requires only half the clearance of a stock piston.

A piston skirt that has collapsed (become smaller because of heat) should be replaced, rather than knurled.