Engine Rebuilding Part Three - Piston Rings

Low compression and oil burning are usually a sign of worn rings and/or cylinders. Replacing the piston rings can restore compression if the cylinders do not exceed service specifications. But if the cylinders are worn or damaged, reboring the cylinders to oversize will be necessary to restore proper clearances and compression.

Replacement rings come in various materials and sizes. Most compression rings are cast iron, though many import engines have steel rings. Rings may be plain faced, chrome-plated, inlaid with molybdenum ("moly") or nitrided for added durability. Replacement rings should generally be the same types as the original.

Ring sizes can be confusing because ring thickness and width may change from one model year to the next. You may have to refer to the VIN number to determine the correct rings for the engine. Oversize rings and pistons of the corresponding size will obviously be needed if the cylinders need to be bored or honed to oversize.

Some shops "plateau" the cylinders after honing. This can be done various ways, but one way to do this yourself is to give each cylinder a few strokes with a flexible brush-type "Flex-Hone" in a drill. This helps remove surface debris and knocks the sharp peaks off the ridges left in the bores by honing.

Cylinders must always be cleaned before new rings and pistons are installed. This means scrubbing the bores with warm soapy water and a brush to remove all traces of honing residue and metal.

Always use a ring expander to install new rings on pistons, and a ring compressor to install the piston assemblies in the block. Cylinder walls must also be lubed to protect the rings and pistons against scuffing when the engine is first started.

Engine Rebuild Tutorial Links:

• Engine Rebuilding - The Basics Part One
• Engine Rebuilding Part Two - Crankshaft Bearings
• Engine Rebuilding Part Three - Piston Rings
• Engine Rebuilding Part Four - Camshafts
• Engine Rebuilding Part Five - Valve Train
• Engine Rebuilding Part Six
• Engine Rebuilding part Seven - The Break In