How to Size Oil Seals

by Francis Walsh

At the bottom of an engine is a reservoir for oil which is capped by the oil pan. At each end of the pan, there is a seal to prevent oil leaks. These seals come pre-cut or require a custom cut to fit correctly. Knowing how to size oil seals will assist you when you assemble the oil reservoir after repairing, inspecting or rebuilding an engine with a dual seal oil pan.

Measure the curvature of the bottom of the timing chain cover with a shoestring. Lay the shoestring into the oil seal seat so that it fills the seat and runs out both sides of the seat where the oil pan mates to the motor. Add an extra 1/8 of an inch to the distance from one side to the other and mark it on the shoestring.

Lay the shoestring out flat on a level surface and mark the length on a cutting surface. Make note of which measurement this is by writing "Front" or "Rear" before measuring or marking the next distance for the remaining oil seal.

Measure the rear oil seal seat at the back of the engine. Place the shoestring into the curvature of the rear oil seal seat and lay it out so that both ends extend past the beginning and the ends of the seat. Add an extra 1/8 of an inch to both ends and mark the length on the shoestring.

Lay the shoestring out and mark the length of the oil seal. Retrieve the new oil seal(s) and inspect them for damage. Lay an oil seal next to one of the measurements so that the oil seal extends past the marks at each end. Mark the distance directly onto the rubber oil seal. Repeat for the remaining oil seal and cut to length.


  • check Cutting an oil seal perfectly is an art. What most people find happens is that a perfectly good looking cut ends up leaking because it is too short. Leaving extra is always recommended, along with proper tension on the retaining bolts.


  • close Oil seals that are too short or too long may leak if not properly mated with the oil pan. Leaking oil can drain the engine of important lubricants which will result in engine damage if not fixed.

Items you will need

About the Author

Francis Walsh has been working as a freelance writer since 2003. He has contributed to websites such as Shave, Autogeek and Torque & Chromeas, as well as provided content for private clients. Walsh has worked as a performance part-packer and classic car show promoter, now serving as crew chief for Nitrousfitz Racing.

Photo Credits

  • photo_camera Motor - Hot Rod image by Jeffrey Zalesny from