427/454 Bare Block

427/454 Bare Block

The classic Chevy Big Block production engine was introduced in 1965. In the late 1980s, a new version arrived, designed for marine and fuel-injected applications. The early-style engines are known as Mark IV Big Blocks, while the later style is referred to as the Gen V (and Gen VI) Big Block. You can tell them at a glance by checking for a mechanical fuel pump mounting pad. If it has one, it’s a Mark IV. If there’s no fuel pump pad, it’s a Gen V block.

Despite the fuel pump mounting pad difference in their castings, the cylinder blocks of the Mark IV and Gen V are based on the same design architecture. However, there are several other differences—particularly in the water jackets near the deck surfaces—that make some Mark IV and Gen V parts incompatible, including crucial components such as the cylinder head gaskets.

Within the last few years, Chevrolet Performance revised the basic Big Block architecture to commonize the Mark IV and Gen V, creating an all-new cylinder block casting that combines the features of both generations. It also incorporates significant updates and strength-enhancing features that make the Big Block a stronger engine foundation with provisions to support 21st-century performance.

Although the basic Big Block architecture is revised, Chevrolet Performance continues to offer two versions, each differentiated by performance and displacement capability. The Bowtie block continues to be the block of maximum performance. All of our crate engines use the revised Big Block design.

19170538 Chevrolet Engine Block

• New design casting incorporating the best designs of Mark IV and Gen VI
• Production type cast iron 4-bolt block
• 4.250” finished bore
• 4.310″ max bore (non-siamese bore)
• Machined fuel pump pad
• New water jackets for use with Mark IV or Gen VI heads
• Revised oiling to allow for bigger cam bearings/cam lift
• Bolt boss (not machined) added near distributor hole like 8.1L
• Can be drilled for use with 10-bolt front timing cover
• Additional clearance added for roller timing chains
• Auxiliary oil pressure line added to front of block
• Racing style oil filter cast feature with added oil pressure port
• Additional boss for manual transmission clutch pivot (machined)
• Additional material added around lifter bosses

Here’s what distinguishes the latest Chevrolet Performance Big Blocks from earlier castings:

• Water jackets are revised near the deck surfaces so that Mark IV or Gen V head gaskets can be used interchangeably
• Oil pressure feed holes are added to the oil filter boss and front bulkhead to support oil feeds for superchargers, turbochargers, etc.
• The oil hole next to the camshaft bore at the front of the block is repositioned to enable safe machining of the cam bore to accept a 50mm roller camshaft bearing
• A mechanical fuel pump mounting pad is included, similar to the Mark IV
• A boss next to the distributor hole in the valley supports hardware for digital ignition equipment
• The front bulkhead is thicker and stronger, with marked provisions for a 10-bolt timing cover (non-Bowtie blocks are delivered with drilled and tapped holes for 6-bolt covers; remaining holes must be drilled and tapped at the prescribed positions)
• Non-Bowtie blocks are machined for 4-bolt parallel main caps; Bowtie blocks are machined for 4-bolt splayed caps
• Revised rear-of-block allowed for the machining of 1- or 2-piece main seals (similar to Gen V design)
• A front clutch boss is included for older muscle car applications
• 454 blocks have a slightly beefier main web than previous blocks
• All blocks are made with the standard production roller camshaft and lifter machining
• 502 and Bowtie blocks share the same main web, which is strengthened considerably from the Mark IV and the first-generation Gen V Bowtie block
• Bowtie blocks feature a distinctive water jacket design to allow up to 4.600″ bores. These blocks can be identified by a “B” suffix behind the casting number

Note: Two additional core plugs are featured in the rear bulkhead. They enhance the manufacturing process at the foundry and help improve overall quality. Also, a “Bowtie” logo and other identifying marks are incorporated on the Bowtie block, distinguishing it from previous generations.