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Why Does My BluePrint Engine Have A One-Piece Or Two-Piece Rear Main Seal?

One of the most common questions customers ask when ordering flywheels, flexplates, oil pans, rear main seals, or replacement parts is whether their engine uses a one-piece rear main seal or a two-piece rear main seal. This is an important distinction because many components are not interchangeable between the two designs.

What Is a Rear Main Seal?

The rear main seal is located at the back of the engine where the crankshaft exits the block.

Its purpose is to:

  • Prevent oil leaks
  • Seal the crankshaft
  • Keep engine oil inside the crankcase

Without a properly functioning rear main seal, engine oil can leak between the engine and transmission.

What Is A One-Piece Rear Main Seal?

A one-piece rear main seal is manufactured as a single seal that installs around the rear of the crankshaft.

Advantages

  • Improved sealing
  • Reduced leak potential
  • Modern design
  • Easier serviceability

Most current BluePrint SBC, BBC, SBF, and LS engine platforms utilize a one-piece rear main seal design.

Always verify your specific BluePrint engine specifications before ordering parts.

What Is A Two-Piece Rear Main Seal?

A two-piece rear main seal consists of two separate seal halves installed around the crankshaft.

This design was commonly used in earlier engine platforms.

Advantages

  • Traditional design
  • Common in many classic vehicle applications
  • Direct replacement for older engine combinations
Why Does It Matter?

Rear main seal design affects the selection of:

  • Flywheels
  • Flexplates
  • Oil pans
  • Rear seal housings
  • Crankshafts
  • Gaskets

Using the wrong components can result in fitment issues and improper operation.

Small Block Chevy (SBC) Applications

Two-Piece Rear Main Seal

Common on:

  • Traditional pre-1986 Small Block Chevy platforms
  • Many early 350 applications
  • Many early 400 applications

One-Piece Rear Main Seal

Common on:

  • 1986-1999 Late Model Small Block Chevy platforms
  • Many BluePrint 350 engines
  • Many BluePrint 383 engines
  • Current BluePrint SBC engine families

Always verify your specific BluePrint engine model before ordering flywheels, flexplates, or oil pan components.

Big Block Chevy (BBC) Applications

Two-Piece Rear Main Seal

Common on:

  • Earlier Mark IV Big Block Chevy platforms

One-Piece Rear Main Seal

Common on:

  • Gen V Big Block Chevy engines
  • Gen VI Big Block Chevy engines
  • Current BluePrint BBC engine families

Typical BluePrint BBC applications are based on the 1991-1999 Gen V / Gen VI platform.

Small Block Ford (SBF) Applications

Most BluePrint Small Block Ford engines utilize:

One-Piece Rear Main Seal

Examples include:

  • 302
  • 306
  • 347
  • 351W
  • 408W
  • 427W

These engines are generally based on the 1984-1995 Ford 5.0 H.O. architecture.

LS Engine Applications

All BluePrint LS engines utilize:

One-Piece Rear Main Seal

Examples include:

  • LS3 376
  • LS427
  • LS-based stroker combinations
How Do I Identify My Rear Main Seal?

The easiest methods are:

  • Review your BluePrint engine specifications
  • Verify your engine model number
  • Review flywheel or flexplate requirements
  • Contact BluePrint Technical Support with your engine serial number
Flywheel And Flexplate Selection

When selecting a flywheel or flexplate, always verify:

  • Rear main seal style
  • Engine balance type
  • Crankshaft flange pattern
  • Transmission application

Flywheel and flexplate balance requirements often differ between one-piece and two-piece rear main seal applications.

Always verify both the engine balance requirements and rear main seal design before ordering components.

 

 

Frequently Asked Questions

Why does my flywheel ask if I have a one-piece or two-piece rear main seal?

Flywheel and flexplate balance requirements may differ between one-piece and two-piece rear main seal engine combinations.

Can I use a two-piece rear main seal flywheel on a one-piece rear main seal engine?

No. Always use the flywheel or flexplate designed for your specific engine configuration.

Do LS engines use a two-piece rear main seal?

No. BluePrint LS engines utilize a one-piece rear main seal design.

Why is rear main seal design important when ordering parts?

Many drivetrain, balancing, sealing, and oil pan components are selected based on rear main seal style.