Ford Racing Boss 351 - The Boss Is Back!

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.

The Metal
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.

The Design
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.

Casting
The casting of the Boss 351 block starts with a brand-new tooling. Over a century of experience pouring blocks was put into the design of these molds and the way they are poured. Critical elements to a high-quality, repeatable casting are the sand used, number of cores, keeping the molten iron moving, proper venting, quick cooling, and keeping areas thin enough to cool properly but thick enough to give the required strength. Ford Racing uses flow dynamics and analysis to help determine the best way to cast the block, with what end up and where to put the vents. Unique to their Boss blocks, Ford Racing uses a two-block mold that doubles productivity without hurting quality.

Features

Cross Drilling
The Boss 351 block is a Siamese bore, meaning the cylinder walls are cast solid with no cooling passage between them for increased strength. Ford Racing has found that without the cross drilling on Siamese bore blocks, it was common to get hot spots near the deck from poor coolant flow. While this rarely results in a head gasket failure, they did see piston softness from the heat in their sealed circle track applications. They've also seen heat-related issues in street cars that have inadequate cooling systems. This is especially common in street rods where radiator space is at a minimum. A non-cross drilled version will be available shortly for those individuals who have an adequate cooling system and are seeking a larger bore. The drilled sections extend 5/8 inch below the deck surface.

Main Caps
Though two-bolt main caps are good and four-bolt mains are better, four-bolt splayed mains are the best. Unique to the Boss version, Ford Racing has angled the outer main drillings for two reasons. One is to change the clamping load from one direction to two. This gives the cap additional strength without adding material. The second reason is that the angle of the drilling puts the bolt in a meatier part of the block, which also increases its strength. Ford Racing left caps 1 and 5 as two-bolt mains for oil pan fit. While this may seem to compromise strength, drag racers running this same design in the Boss 302 are making north of 1,400 hp.

Screw-In Freeze Plugs
There are a couple of different reasons to use a screw-in plug instead of the typical factory press-in style. The first is that the screw-in style is much easier to service. For teardown and cleaning, this is crucial. Also, the aluminum Allen-head anodized plug looks the part on a race block. Ford Racing hasn't done any testing on this theory, but some claim the screw-in style increases the block strength. Ford Racing doesn't promise these results, but has found cases where tapered thread pipe plugs can stress the block and ultimately lead to failure. That's why they use straight-thread fittings sealed with an O-ring.

Oil Passages
Oiling has been improved for this block dramatically. Most production blocks, and some aftermarket ones, feed the lifters solely from the rear. The new Boss 351 block feeds the lifters from the front and rear. Instead of the passages being tapped for pipe threads, they use -AN thread for O-rings to keep the load off of the block.

By The Numbers