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Boss 351 Cylinder Iron Blocks - Big Brother - Boss 351 Block
Meet The Ford Racing Boss 302's Sizable Sibling
January, 2010
By Dale Amy
Photography by Dale Amy
Ford Racing's new Boss 351... Ford Racing's new Boss 351 block-three of them, in fact-take over in displacement where the Boss 302 leaves off. In 9.2-inch deck height, this means up to 427 ci, or 454 cubes in 9.5-inch form. Like the Boss 302, these blocks are strong enough for serious race duty, yet affordable and civilized enough for street service. 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. The Boss 351 incorporates... The Boss 351 incorporates the lifter-bore oiling improvements introduced on FRPP's Boss 302 block, allowing pressure lubrication of the bores from both front and rear. Another similarity is the O-ring-sealed, aluminum, AN-style, non-tapered oil galley and freeze plugs used throughout the block. By the way, our photographic subject is a blue-painted display piece; the normal Boss block finish will be as-cast or, more accurately, as-heat-treated. 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. Our arrows point to the 0.090-inch... Our arrows point to the 0.090-inch coolant crossover tubes on this cutaway (cut about an inch below the deck surface). The tubes would intersect about a quarter-inch above this point (no other aftermarket Siamese-bore 351 has these coolant crossovers). Also note the water-jacketing (lots of cooling) and the generous cylinder wall thickness (shown here in as-machined 3.990-inch bore diameter). Ford Racing is also proud of their casting techniques; core shift is practically non-existent. 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. A block is only as good as... A block is only as good as its main bulkhead and cap structure, and it's easy to see that the Boss 351's bottom end is the stuff of railroad bridges. The Number 2, 3, and 4 caps use four bolts, with the outer fasteners being splayed. This arrangement is superior to parallel fasteners, as the splayed bolts provide extra strength under both shear (side) and tension (end) loads. Like many recent high-performance Windsor castings, the Boss 351 uses a 351 Cleveland-style 2 3/4-inch main-journal diameter. "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." This is an as-cast Boss 351... This is an as-cast Boss 351 block awaiting machining. The blue/gray color is a result of a heat-treating process done to temper the block to final hardness. If you're familiar with the Brinell hardness scale, the Boss blocks generally run around 235 to 245, whereas a production 351 block would have been notably softer at around 200. 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.  The heavy lifters in the block-finishing...  The heavy lifters in the block-finishing process are Makino A99 horizontal machining centers, which can whittle away up to 60 ci of material per minute. The blocks take two separate passes through the Makinos, the first to machine the fore and aft surfaces, and the second for most of the other necessary machining, including roughing in the cylinder, cam, and crank bores, and drilling and tapping. Each block will spend nearly 3 hours on the computer-controlled Makino, and each will consume about 12 to 13 man hours at the machining facility in total. And, yes, that's actually a Boss 302 block in our photo. |  At another station, the web...  At another station, the web faces are machined for crankshaft clearance, the separately machined main caps are then torqued in place, and the assembly then goes through a separate line-boring station. The blocks should require no further clearance machining for common Scat or Eagle cranks up to 4.200-inch stroke. |  Though they start off from...  Though they start off from the same casting, the Boss 351's center three main caps differ from the Boss 302's in their crankshaft bore diameter. This larger crank bore also necessitates moving the center bolt holes further outboard. |  The lifter bores are finish-machined...  The lifter bores are finish-machined to size (0.8753 to 0.8768 inches) and the block is machined to accept factory roller lifter guides and the guide retainer. The decks are cut at 0.010 to 0.015 inches above nominal 9.2- or 9.5-inch height. |  Throughout the machining process,...  Throughout the machining process, Boss blocks go through a battery of manual and automated quality inspections. About one block per shift gets to spend about an hour's quality time with this coordinate measuring machine, which checks it against a computerized blueprint. |  Even the packaging is high-tech....  Even the packaging is high-tech. Before shipment to FRPP's warehouse, each Boss block is sealed in a volatile corrosion inhibitor bag. The inhibitor vaporizes at room temperature and coats the metal in a thin anti-corrosion layer. The block is then shipped in a wood-braced plastic and cardboard container. |  This is a Boss 351-based,...  This is a Boss 351-based, 427ci FRPP dyno mule that has been undergoing durability testing. Short- and long-block versions of the 427 should be available by the time you read this. A hydraulic-roller cam is onboard, and when topped with heads of around 60cc chamber size, compression will be right around 10:1, so this is intended for naturally aspirated applications where big torque and kickass throttle response are the goals. | | |
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