Ford Racing has been producing 351ci Windsor-based blocks for years, but when it came time to start fresh with all-new tooling, they decided to make some major changes. In 2006, Ford Racing started manufacturing the new Boss 302 block. The original Boss 302 and Boss 351 found in Mustangs in the late '60s and early '70s both had Cleveland-style canted-valve heads and were both small-blocks, however, the original Boss 302 used a 302 block with a Windsor architecture. The original Boss 351 used a Cleveland-style block that had many differences from its Windsor cousin. Ford Racing's new Boss 351 is based off of traditional Windsor architecture. Aside from small differences like the china wall, distributor, and main journal size, the most obvious difference between any 302 and 351 is the deck height, 8.2 and 9.5 inches respectively in stock applications. The 9.2-inch deck height became common in racing to get the center of gravity lower and still have a fair rod/stroke ratio. When designing what would become the new Boss 351 block, Ford Racing incorporated many of the successful qualities from the Boss 302 and their previous 351 blocks.
After casting, each batch of blocks undergoes a 171-point inspection performed by a coordinate measuring machine (CMM). Each coordinate on the casting is measured to ensure it is to the correct tolerance. Should they ever have an issue, they can trace back to that exact batch and have data on measurements, as well as the exact iron mixture and temperature.
Ford Racing plans to have a line of crate engines featuring this block in late 2009. One that we're particularly excited to see is the 427. Though the block is new, customers have already began building 700-plus horsepower 429-inch motors using the Boss 351 block. They can boast Boss 429 emblems with a stronger, lighter, and more reliable package.
Ford starts with extremely high-grade iron ore, then adds other metals to get the mixture they are looking for. The tensile strength well exceeds that of stock and many aftermarket blocks. The typical strength of a stock block is about 30,000 psi while the Boss 351's diesel-grade iron is rated at 41,000 psi. It's the same material that was originally developed for the Ford diesel 6.0L and 6.4L engines. Hard-core racing blocks used in NASCAR and professional Pro Stock drag racing engines use a compacted graphite iron with a strength that can be as high as 60,000 psi. Similar to the use of chrome-moly, racers use the high-tensile strength iron as a tool for weight savings. The Boss 351 lands right in the middle, offering extreme durability and a cost-effective price tag. The science of metallurgy has dramatically improved in the last decades; now Ford uses tin in addition to nickel to maximize the strength for these blocks. Aside from starting with an impressive material, the plant performs metallurgy sampling for every batch of blocks they cast.
When you hear Boss 351, you may think Cleveland, but this block isn't one. The blocks in the Cleveland family aren't nearly as in demand as in the Windsor group, and that's why the Boss 302 and Boss 351 are both Windsor-based. The Boss 351, however, has adopted the Cleveland's 2.75-inch main bearing diameter. The Boss 351 comes in two basic models, one with a 9.2-inch deck height, and one with a 9.5-inch height; both of which are very popular. The maximum stroke is at 4.00 inches, depending on the crankshaft manufacture. Though most of the blocks come punched with a 4.125-inch bore and cross drilling, a non-drilled version will be available rough honed to 4.120 inches to accommodate larger bores, assuming the vehicle's cooling system is sufficient.