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Forged S/B Chevy Rotating Assembly, 421, .283 Dome, 400 Mains, 6 Rod
Forged S/B Chevy Rotating Assembly, 421, .283 Dome, 400 Mains, 6 Rod

Forged S/B Chevy Rotating Assembly, 421, .283 Dome, 400 Mains, 6 Rod

400 Chevy Small Block V8, 421 Chevy Small Block V8, 400 mains Main Journal Style

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Forged S/B Chevy Rotating Assembly, 421, .283 Dome, 400 Mains, 6 Rod
Forged S/B Chevy Rotating Assembly, 421, .283 Dome, 400 Mains, 6 Rod
Forged S/B Chevy Rotating Assembly, 421, .283 Dome, 400 Mains, 6 Rod
Forged S/B Chevy Rotating Assembly, 421, .283 Dome, 400 Mains, 6 Rod
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We estimate this item will ship on or before March 12th.
Part # 9151274
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  • For 400421 Chevy Small Block V8
  • Kit includes 2-pc rear main seal cast steel crank, rods and pistons
  • Forged aluminum pistons
  • Connecting rod and crankshaft
  • 14:1 compression ratio


Main Journal Style:
400 mains
Cylinder Bore Size:
Select Oversize to see
Engine Stroke:
Connecting Rod Type:
Connecting Rod Length:
Connecting Rod Material:
Forged 4340 Steel
Connecting Rod Weight:
634 grams
Connecting Rod Bolt Diameter:
Piston Style:
Piston Diameter:
Select Oversize to see
Piston Material:
Forged Aluminum
Dome Height:
Piston Weight:
427 grams
Piston Ring Style:
Crankshaft Material:
Forged 4340 Steel
Rear Main Seal Style:
Wrist Pin Diameter:
Main Journal Diameter:
Rod Journal Diameter:
Bearing Construction:
Bearing Undersize:
Narrowed Bearing:
Coated Bearing:
MFG. Part #:
Select Oversize to see
Sold in Quantity:


Speedway introduces forged rotating assemblies, with an optional balance job. Includes new 2-pc rear main seal crank, rods and pistons. Cranks are new so they use standard size bearings.

All Scat 4340 cranks are 2-piece rear main seal, internally balanced and weigh approximately 51-55 lbs.  All balancing is done with our state-of-the-art Hines balancer by race mechanics with over 30 years of experience!  A kit purchased with the optional balance job will ship in 24-48 hours. These rotating assembly engine rebuild kits may require additional charges for heavy metal.

If you are going to run this crank in a sprint car motor, you must have the crank flange turned down prior to balancing the crank to allow it to go through the motor plate. 

NOTE: Piston rings are not included and must be purchased separately. Use Style E rings.

64 cc 14.0:1 compression ratio


  • 415/421-ci SBC (3.875-inch stroke)
    • Displacement: 421 ci
    • Bore x Stroke: 4.155 x 3.875 inches
    • Compression: 10.7:1
    • Rod: 6.00-inch



Kits feature: 

  • Scat 4340 Forged Steel Crankshaft   
    • Standard size journals
    • Straight shot and chamfered oil holes
    • Nitride hardened and shot peened
    • Lightening holes in all rod throws
    • Large radius on all journals
  • KB Signature Series Forged Pistons 
    • 2618 aluminum alloy construction
    • T-6 heat treated 
    • 4.125" bore pistons available in standard, .020, .030, .040 or .060 oversizes
  • Speedway 4340 H-beam Connecting Rods
    • Full floating pin in silicon bronze bushing
    • Precision bolt bushings to positively locate cap
    • 6.0" lengths
    • 634 grams
    • Weight matched set of 8 rods
  • King HP Series Main & Rod Bearings
    • 0.015" thick top layer for superior macro particle embedability and conformability
    • 40 Vickers hardness to resist overlay migration and extrusion
    • 450°F heat threshold for superior protection against friction and overheating
    • Mains are 3/4 grooved for improved oiling to the rod journals
    • Bi-metal aluminum silicon alloy featuring fine micro-bore finish

Important Messages

  • Get FREE shipping when your order includes this item and exceeds $99! Ground shipping only in the contiguous US.
Some parts are not legal for use in California or other states with similar laws/regulations

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What's In The Kit

Each kit from Speedway Motors is carefully packaged and delivered right to your door. Below is a list of all the items you should expect to receive when you order this kit. Some of these items may also be available for sale individually if you're here looking for replacement parts.
Select a Oversize above to see what items are in this kit.

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Q & A

Videos & Articles
  • Learn about the importance of getting the chain tension set correctly, and why it is critical that we get it as near perfect as we can. Walkthrough a basic procedure that effectively and consistently gets the chain tension set properly every time.
  • Step back in time with our modern reproduction of the iconic Chevy 461 double hump cylinder heads, now cast in lightweight aluminum! In this video, Joe and Wyatt discuss the features that bring the vaunted camel hump heads into the 21st Century.
  • In this Tech Tip, John discusses LT1 rotating assembly options for a customer looking to build a solid autoX, drag, and occasional strip powerplant for his Firebird.
  • Check out this helpful piston ring style chart.

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