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Forged Small Block Chevy Rotating Assembly, 421 Flat Top, 350 Mains, 6 Rod
Forged Small Block Chevy Rotating Assembly, 421 Flat Top, 350 Mains, 6 Rod

Forged Small Block Chevy Rotating Assembly, 421 Flat Top, 350 Mains, 6 Rod

400 Chevy Small Block V8, 421 Chevy Small Block V8, 350 mains Main Journal Style

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Forged Small Block Chevy Rotating Assembly, 421 Flat Top, 350 Mains, 6 Rod
Forged Small Block Chevy Rotating Assembly, 421 Flat Top, 350 Mains, 6 Rod
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Part # 9151271
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  • Get FREE shipping when your order includes this item and exceeds $99! Ground shipping only in the contiguous US.


Main Journal Style:
350 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
Piston Style:
Flat Top
Piston Diameter:
Select Oversize to see
Piston Material:
Forged Aluminum
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: A balance rotating assembly option is currently not offered for this package but the proper balancing service can be determined after your order is placed.  If you would like our balancing option service for 395 and 421 engine builds, contact customer service.

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

50 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 
    • 4.000" bore pistons available in standard, .030, .040, or .060 sizes
    • 4.125" bore pistons available in .030 or .040 sizes
  • 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
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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