Made in USA
BB Chevy Big M Engine Block 4.250" Bore
Product Discontinued
DARTPART NUMBER: 301-31263354

BB Chevy Big M Engine Block 4.250" Bore

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This product is no longer available. Please call customer service for assistance at 1-800-345-4545.

Iron Blocks W/Billet Steel Main Caps
    Big Block Chevy Big M Engine Block
  • 4.250'' Bore
  • 10.200'' Deck Height
  • Billet Steel Main Caps

    Specifications

    Material: Superior Iron Alloy
    Bore: 4.250'
    Bore & Stroke: 4.625' x 4.750' max recommended
    Cam Bearing Bore ID: 2.1195' - 2.1205'
    Cam Bearings: Special coated, grooved, with 3 oil holes
    Cam Bearing OS: +.010', +.020', +.030'
    Cam Bearing Press: .002'
    Camshaft Postion: Standard
    Cam Drive: Standard Timing chain, gear drive or belt drive
    Cam Plug: Standard BBC 2-7/32' (2.218' dia)
    Cubic Inch: 632' max recommended
    Cylinder Wall Thickness: .300' min @ 4.625' bore
    Deck Height: 10.200'
    Deck Thickness: Adequate for all applications
    Fuel Pump: Mechanical pump provision
    Fuel Pump Pushrod: Standard BBC
    Freeze Plugs: Press in cup plugs
    Head Gasket: Fel-Pro 375-1037, 1047, or 1067
    Inner Head Stud: 2 slotted bosses per side
    Lifter Bores: BBC .8427' - .8437'
    Lifter Galley: Raised .350' for longer lifter bore
    Lifter Type: Roller - Gen VI (+.300' longer), Solid - Mark IV
    Main Bearing Size: Standard BBC
    Main Bearing Bore: 2.937' - 2.937'
    Main Caps: 4-bolt steel billet
    Main Cap Register: Deep stepped register on each side (No need for dowels)
    Main Cap Press: .005'
    Main Cap Bolts: All 1/2', #2, #3, #4 have splayed outer bolts
    Oil System: Wet or Dry Sump - Main Priority Oiling
    Oil Galley, Main: Stepped, 9/16' - 1/2' - 7/16'
    Oil Galley, Filter-Main: 5/8'
    Oil Filter: Stock Oil Filter Location
    Oil Pan: Standard pan bolt pattern, extra bolt holes provided for strokers
    Rear Main Seal: Standard 2-piece seal, Fel-Pro 375-2918
    Rear Main Thrust Width: 1.622' - 1.624'
    Serial Number: On main caps
    Starter: Mounts in standard location
    Stud Hole, Head: Blind holes
    Timing Chain/Gears: Standard
    Timing Cover: Stock Cover
    Torque Specs: All 1/2' bolts - 105 lbs
    Weight: 4.250' bore - 280 lbs

    The Strongest and Most Reliable Big Block Available!

    Dart has reinvented the "Rat Motor" with their cast-iron Big M big blocks. Working with top builders and Dart's own championship-winning Pro Stock engine department, they have designed a no-compromise, user-friendly block that solves the problems that have plagued big-block racers for years. Each unit is in-house machined on precision CNC equipment to ensure the highest quality and to eliminate the need for expensive blueprinting.

    Show More

    The Dart Big Block Chevy Big M Engine Block is a purpose-built foundation for serious horsepower, engineered for racers and high-performance builders who demand strength beyond OEM limits. Designed specifically for extreme big block applications, the Dart Big Block Chevy Big M Engine Block delivers reinforced construction, expanded displacement capability, and race-ready durability. Whether you’re assembling a high-horsepower street engine or a competition-only drag racing build, this aftermarket engine block provides the structural integrity needed to handle substantial torque capacity and cylinder pressure.

    Unlike factory castings, the Big M block is engineered with thicker cylinder walls, priority main oiling, and reinforced main webs to support elevated horsepower levels. For enthusiasts pushing forced induction, nitrous, or large cubic inch combinations, starting with a high-strength performance engine block like the Big M is critical to long-term reliability and consistent results.

    Performance Benefits & Key Features

    The Dart Big Block Chevy Big M Engine Block is manufactured from premium high-strength cast iron to withstand extreme loads and high RPM use. Key features often include siamese cylinder bores for increased displacement potential, four-bolt or cross-bolted main caps for added bottom-end stability, and priority main oiling to ensure consistent lubrication under racing conditions.

    Performance builders value specifications such as bore size flexibility, deck thickness, improved oil control, and compatibility with stroker rotating assemblies. These attributes allow for large cubic inch combinations capable of producing significant horsepower and torque. Enhanced rigidity reduces block flex, supporting improved ring seal and long-term durability in high-output builds.

    This block is ideal for drag racing applications, high-performance street builds, marine engines, and competition-oriented projects where power levels exceed stock limitations. It’s particularly well-suited for naturally aspirated big cubic inch engines as well as supercharged or turbocharged combinations requiring a robust bottom-end foundation.

    Fitment, Compatibility & Applications

    The Big M engine block is designed for Chevrolet big block compatibility, making it a natural fit for custom builds based on GM big block architecture. Popular applications include classic muscle cars such as Chevelle, Camaro, Nova, and full-size Chevrolet models, along with C10 and other GM truck platforms configured for big block installation.

    Engine builders appreciate the block’s flexibility for stroker assemblies and high-performance cylinder head combinations, making it adaptable for both street performance and dedicated race vehicles. Its compatibility with standard big block Chevy components allows experienced builders to tailor the engine to specific displacement and performance goals.

    When you’re building serious horsepower from the ground up, start with a proven foundation. Explore the Dart Big Block Chevy Big M Engine Block options at JEGS and choose the strength and precision your high-performance build demands.

Part TypeEngine Block
Product LineDart Big Block Chevy Big M Engine Block
BrandDART
Package Height15.0
Package Width21.0
Package Length25.0
Weight250.0 PG
Part CategoryPerformance Parts
Maximum Bore Diameter4.6 in (116.84 mm)
Engine Block Weight (Lbs.)280
Deck Height10.2 in (259.08 mm)
ToolStroker Combinations
Vendor Part Number31263354
Package Qty1
Package TypeEA
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