Volume III, Issue 5, Page 11

Beware though, GM also installed the virtual look-alike 348 in trucks from 1958 to 1966. Sharing the 409’s 7.75:1 compression and single four-barrel, the Work Master 348 delivered 220-horsepower and 325-lb/ft of torque. A sure way to identify the more desirable 409 is the “X “ cast into the leading end of the driver-side of the block. Another tip for field spotters is that 1962 and 1963 Work Master 409’s are generally painted orange while the 1958-up truck 348’s are generally painted gray. Unfortunately, around 1964 GM started applying gray paint to all truck 348’s and 409’s. If the heads are off, a 4.313-inch bore diameter (standard) will differentiate the 409 from a 348’s smaller 4.125 bores.

The big snag with using a truck-sourced 409 block is the machined pocket shown here. Without a conventional combustion chamber in the W’s cylinder heads to make larger or smaller, GM’s combustion ratio strategy relied on piston dome shape, head gasket thickness, and machined pockets to tailor compression to suit the job. The pockets shown here add volume and once worked with specific truck pistons to drop compression from the passenger car’s 11:1 (10:1 in 1963 to 1965 340-horse applications) to 7.75:1 for extra durability and low-grade fuel compatibility – big bonuses to truckers. By contrast, performance-oriented passenger car-sourced 409 blocks do not have these pockets. Today, you must compensate with custom piston domes to achieve higher compression. We’ll cover that in a moment.

All 348 and 409 W blocks were delivered with conventional 2-bolt main caps which have proven very reliable. Even the ultra exotic 1963 Z-11 427 was built with this bottom end configuration. Shaver Specialties handled all machine work which included magnaflux and pressure check, a 0.060-inch overbore and align honing. Since this engine had been stored in a damp environment, the 0.060 overbore restore the bores. Remarkably, the bores displayed healthy 0.300 average wall thickness after the overbore.