What Is a Rear Main Seal?

by Richard Rowe

Rear main seals come in many shapes and sizes, but all are responsible for sealing the back of an engine's crankshaft. These vital seals must keep oil inside the engine while coping with the crankshaft's rotation and heat expansion.


Rear main seals are located at the back of the crankshaft, at the rear of the engine block. Many of these seals can only be installed or replaced by removing the rear crankshaft main cap.

Rope Seals

Many older engines used "rope" seals that resembled a piece of cord. These rope seals did a decent job of keeping oil pressure in the engine but would eventually become saturated and start to leak.

Split Seals

These plastic seals are an improvement over rope seals but are still prone to leakage. These seals are often used because they do not require removal of the crankshaft to replace.

Once-piece Seals

One-piece seals are the best at sealing but require a specially machined crankshaft and cannot be replaced without pulling the crank.


Older seals may leak due to heat shrinkage, damage and pitting, defects in the crankshaft, improper installation or clogging of the Positive Crankcase Ventilation system (PCV).

About the Author

Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.

Photo Credits

  • photo_camera Image by Fotolia.com, courtesy of martin filzwieser