5.0 Mustang & Super Fords
Boss 351 Cylinder Iron Blocks - Big Brother - Boss 351 Block
Meet The Ford Racing Boss 302's Sizable Sibling
Hard to believe, but it's been almost three years since Ford Racing Performance Parts took the wraps off its modern and muscular Boss 302 block (PN M-6010-BOSS302)-a foundation that was engineered and cast to be even tougher than the infamous R302, A4, and B50 blocks it replaced. To say the least, the Boss 302 has been a huge success. Now the family has quite literally grown to include three new cast-iron Boss 351 blocks for those rare but serious situations where a 5.0-liter foundation just won't do. We predict that they, too, will be a huge success.
You might be wondering, why does Ford Racing need yet more 351 castings when the FRPP catalog already contains multiple examples in both iron and aluminum blocks? One reason is the current iron M-6010-N351 and -W351 castings are going away (the two-bolt -C58 Sportsman casting will soon be offered only as part of a 392ci crate motor assembly.) The other answer is more to the point: The Boss 351-in 9.2- and 9.5-inch deck heights-is a more modern block, with superior alloys, oiling, and casting procedures, combined with modern and precise machining techniques. These are, in other words, likely the best wet-sump 351 iron-blocks Ford Racing has ever offered, and they come at a suggested retail price of only $1,999.
Like the modern Boss 302, the new 351 blocks are poured by Indianapolis Casting Corporation using the same high-nickel, high-tin, 41,000-psi iron alloy used in the PowerStroke diesel blocks, which are also cast there. Then they go to a state-of-the-art Detroit-area facility, where five-axis horizontal CNC-machining centers and three-axis vertical machines whittle them with robotic consistency before shipment to the FRPP warehouse. The general manager of this machining facility claims the alloy uniformity of the Boss 351 (and Boss 302) is among the best of any block he's encountered.
Design-wise, the Boss 351 is essentially just a tall-deck version of its already proven 302-inch brother, aside from necessary 351-style revisions, including the rear China wall and distributor pocket.
"All the lessons learned in casting and machining the [Boss] 302, we applied to this product," says Ford Racing's Jesse Kershaw. "Bottom-end construction is virtually identical to-and just as strong and massive as-the smaller Boss, which routinely shrugs off 1,400 or 1,500 hp in such rides as that of EFI Renegade's Bob Kurgan.
"Like the 302, the 351's center three main caps fasten with four bolts, while the front and rear main caps are of two-bolt design. Why? The majority of loads are taken by the No. 2, 3, and 4 mains, and this arrangement also accommodates conventional oil pan designs rather than requiring a race custom piece. Nonetheless, for those who may be anticipating outputs well north of 1,500 hp, there is ample material in the block to convert to four fasteners on all five mains if desired."
To maximize bore potential and rigidity, the Boss 351 is of Siamese-bore design, meaning there is more metal between the cylinders (though there are small coolant crossover passages drilled between the cylinders to improve cooling by eliminating steam pockets). "Regular" Boss 351 variants, the 9.2-inch-deck (PN M-6010-BOSS35192) and the 9.5-inch (PN M-6010-BOSS35195), have a maximum bore capacity of 4.125 inches, while a third variant (PN M-6010-BOSS351BB) lacks these drilled coolant crossovers and can be bored to as much as 4.185 inches. The short-deck version can take up to a 4.00-inch stroke; the 9.5-inchers can handle up to 4.250.