--- title: Why Does My BluePrint Engine Have A One-Piece Or Two-Piece Rear Main Seal? description: Learn the difference between one-piece and two-piece rear main seals, how to identify which design your BluePrint Engine uses, and why it matters when selecting flywheels, flexplates, and replacement parts. --- [Skip to content](https://knowledgebase.blueprintengines.com/knowledge/why-does-my-blueprint-engine-have-a-one-piece-or-two-piece-rear-main-seal#main-content) English Show submenu for translations ![IN CRATE ENGINES..png\]](https://knowledgebase.blueprintengines.com/hs-fs/hubfs/IN%20CRATE%20ENGINES..png?height=40&name=IN%20CRATE%20ENGINES..png) Open main navigation Close main navigation - English Show submenu for translations - Go to www.blueprintengines.com Go to www.blueprintengines.com How can we help you? - There are no suggestions because the search field is empty. 1. [Knowledge Base](https://knowledgebase.blueprintengines.com/knowledge?hsLang=en) 2. [Technical Support](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en) # 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.   - [General Support](https://knowledgebase.blueprintengines.com/knowledge/general-support?hsLang=en) - [Technical Support](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#main-content) - [General Engine FAQ](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#general-engine-faq) - [BluePrint GM Style Engines](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#blueprint-gm-style-engines) - [BluePrint Ford Style Engines](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#blueprint-ford-style-engines) - [Transmissions](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#transmissions) - [Fuel Systems](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#fuel-systems) - [Parts and Accessories](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#parts-and-accessories) - [Carburetors](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#carburetors) - [Alternator](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#alternator) - [Term X](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#term-x) - [Valve Train](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#valve-train) - [Factory Five FAQ](https://knowledgebase.blueprintengines.com/knowledge/factory-five-faq?hsLang=en) - [Engine Specs](https://knowledgebase.blueprintengines.com/knowledge/engine-specs?hsLang=en) - [Discontinued Engine Specs](https://knowledgebase.blueprintengines.com/knowledge/discontinued-engine-specs?hsLang=en#main-content) - [Engine Support](https://knowledgebase.blueprintengines.com/knowledge/discontinued-engine-specs?hsLang=en#engine-support) [![Chill listening crop-3](https://knowledgebase.blueprintengines.com/hs-fs/hubfs/IN%20CRATE%20ENGINES..png?width=92&height=24&name=IN%20CRATE%20ENGINES..png "Chill listening crop-3")](http://navy-blueprint-logo) www.blueprintengines.com Help Center Copyright © 2026, BluePrint Engines