5.0 Mustang & Super FordsHow To Engine
Boss 302 Engine - It's Good To Be Boss
Ford Racing's Latest Killer 5.0 Block Revives The Famous Boss 302 Moniker
Horse Sense: Watch out for the episode of Overhaulin' in which we convert a '70 Mustang Sportsroof into a Boss 302 clone. Part of the build includes the car receiving a new engine based on the Boss 302 block from FRPP.
The more seasoned among us and even the majority of younger readers will have some memory of the rev-crazy '69 and '70 Boss 302 Mustang. A street car built, more or less, to homologate a Trans-Am race engine, the Boss' revered reputation among Fordophiles was firmly founded upon a tough nut of an engine block featuring sturdy bottom-end bulkheads, four-bolt main caps, and screw-in frost and oil gallery plugs. The lightweight 5.0 blocks issued in factory Fox or SN-95 Mustangs absolutely paled in comparison.
Naturally, that's all ancient history. In recent years, Ford Racing's catalog has carried two variants of 8.2-inch-deck cast-iron 302ci blocks for those in need of stouter stuff than their factory 5.0 bottom ends could offer. Those offerings are the $1,050 M-6010-B50 block and the $1,995 M-6010-R302, a sturdy piece cast for Ford Racing Performance Parts by an outside contractor. But when the dwindling current stocks of the strong B50 Sportsman and stronger R302 blocks run out, they will both be replaced by the new, and stronger yet, M-6010-BOSS302 block. These are being cast-as we speak- in the same Indianapolis facility that pours the PowerStroke diesel blocks. In fact, the high-tin alloy used for the new Boss 302 block is exactly the same as found in the diesel blocks, for reasons of vastly improved tensile strength and hardness. The Boss blocks are also specifically heat-treated in order to gain the best metallurgical properties.
The Boss 302 nomenclature is more than just nostalgia; the new block bears more than passing resemblance to the original's girder-like bottom end. Yet the four-bolt main caps (on journals 2, 3, and 4) of the new Boss use splayed outer fasteners-a stronger configuration than the parallel outer bolts on the original Boss 302. The old and new Boss also share screw-in freeze and oil-gallery plugs.
One major departure from the old Boss design-as well as that of the B50 Sportsman-is the new Boss 302 is of Siamese-bore design. The production-based blocks used traditional water jackets. Aside from obvious rigidity improvements, the Siamese construction provides more meat between the holes to safely allow a larger finished-bore dimension (up to 4.125 inches).
The outgoing R302 block is also of Siamese design, but lacks the new Boss' drilled coolant crossover holes between bores. These holes provide improved cooling by reducing or eliminating steam pockets-maybe not critical on a drag application, but certainly welcome on a street, road-course, or roundy-round car. Of course, such coolant holes can be-and often are-drilled into the R302 by savvy builders, but at increased machining cost. With the Boss 302, you get 'em from the factory.
The new Boss 302 casting also incorporates revised oiling passages so that its lifter bores are pressure-lubed from both front and rear, as opposed to the rear-only arrangement on all preceding 5.0 blocks, including the R302. This is particularly useful with high-lift, mechanical flat-tappet cams in keeping the lifters and pushrods thoroughly oiled.
In all, the Boss 302 is a noteworthy improvement over even the previous cast-iron king-of-the-hill R302, and costs about 12 percent less, at a suggested list price of $1,759. This is amazing-especially considering the massive increase in iron prices in recent times-and is mainly due to the efficiencies and consistent quality of casting in an automated OEM production style, rather than in small batches from a manual-pour boutique casting facility. Ironically, the diesel-tough, high-tin iron alloy of the Boss flows better into the casting molds, aiding greatly in that consistency.
According to Ford Racing sources, the rejection rate on the Boss block is a mere fraction of the R302's. From Indianapolis, the heat-treated Boss blocks then trundle up the road to the Detroit suburb of Sterling Heights, where machining takes place at the same highly automated facility that machines most of Ford Racing's other blocks (see sidebar).
Despite the modular V-8's absolute dominance in current production vehicles, the new Boss 302 block-and the crate motors based upon it that will in all likelihood follow-proves that Ford Racing has no intention of abandoning its pushrod, small-block roots. In fact, about 55 percent of FRPP's crate engine sales are 302-based. The Boss is indeed back.
This is the M-6010-B50 Sportsman block, still available at this writing, but in rapidly dwindling supply. This upgraded two-bolt piece was the final 5.0 block design produced at Ford's Cleveland casting plant. Notably stronger than factory 5.0 blocks, the B50 is a solid step up due to extra bottom-end beef and tougher metallurgy but offers nowhere near the vigor of the R302, or especially the new Boss 302. Also note the production-style, pressed-in freeze plugs.
A rear comparison view of the new Boss 302 (left) and the B50 Sportsman shows the revised cast oiling passages in the Boss that deliver oil more efficiently to the valve lifter area. The Boss is unique in having this improved lubrication-even the mighty R302 lacks this provision. Note the screw-in plugs in the Boss casting; these are straight, not tapered, and use O-rings forpositive sealing.
These oiling revisions are also supported by casting revisions on the front of the Boss block that are not overtly apparent in comparison with the B50 block.
At last, we show you the business end of the new Boss 302 block. Note that this was the first one received by Ford Racing and is not 100 percent machined. Our arrows point to casting locators that would normally be machined off. Note the massive web, or bulkhead, dimensions as well as the splayed outer fasteners on the four-bolt, nodular-iron main caps. This splayed configuration is superior from an engineering standpoint; it allows these fasteners to work in both shear and tension. You can also see the part number proudly displayed.
The dimensions, material, and fasteners of the three Boss 302 four-bolt main caps are identical to those used on the R302. The vertical fasteners are 11/42 inch, while the splayed bolts are 31/48 inch; all are Grade 8. To illustrate the mass of these main caps, we've placed a two-bolt cap from the B50 Sportsman block at the bottom for comparison. We're guessing bottom-end rigidity will not be an issue.
Here's a Boss 302 (left) parked beside an R302. Aside from its oiling and cooling improvements, the Boss also has a lot more around the lifter bores. This is good news for those wanting to sleeve or use larger-diameter lifters-roller lifters these days can be as large as 1.0625 inches in diameter. Not everyone will take advantage of this, but it's nice to have the option.
The Boss 302 comes predrilled with coolant crossover holes between the bores, which is something the R302 lacks. It costs extra to have the holes added by a machine shop. Like the R302, the Boss bores come semifinished to a 3.990-inch diameter so final boring/honing to your desired bore diameter can take place at assembly. Ford Racing recommends a maximum bore of 4.125 inches, but we're sure some of you will push that a bit.
A major machining facility in Sterling Heights, Michigan, is responsible for taking the raw Boss 302 casting and forming it so it can be boxed and shipped to customers. A longtime Ford and Ford Racing associate, this firm's 150,000-square-foot headquarters is chock-full of five-axis CNC horizontal machining centers and three-axis vertical machines.This is not your mom-and-pop machine shop.Amajor machining facility in Sterling Heights, Michigan, is responsible for taking the raw Boss 302 casting and forming it so it can be boxed and shipped to customers. A longtime Ford and Ford Racing associate, this firm's 150,000-square-foot headquarters is chock-full of five-axis CNC horizontal machining centers and three-axis vertical machines.This is not your mom-and-pop machine shop.
Each Boss block undergoes about three hours worth of highly automated computer-numerical-controlled machining before it's ready to slide under the hood of your project. The following captions give an abbreviated account of what is involved.Each Boss block undergoes about three hours worth of highly automated computer-numerical-controlled machining before it's ready to slide under the hood of your project. The following captions give an abbreviated account of what is involved.
This is what the heat-treated Boss 302 castings look like when they arrive at the machining facility. As you can see, there is a lot to be done.
This is just one of the massive and expensive Makino A99 machining centers in the facility. The raw blocks take two separate passes through these dual-pallet behemoths-the first to have the fore and aft surfaces machined, and the second to do most of the other necessary machining, including roughing in the cylinder, cam, and crank bores, as well as drilling and tapping. These things can remove as much as 244ci of material per minute.
The blocks then visit a second, slightly smaller, machine, which is made by Ex-Cell-O. Its major task is to finish boring for the cam and crank, but a myriad of smaller items are also looked after. When the block emerges from here, it's almost ready for shipping.
It can't be shipped until the nodular-iron main bearing caps-which are also machined in this facility-are bolted on. You can see in this comparison of before and after that the roughly three hours of machining is time well spent. We took a little editorial license and kind of dummied up the "finished" block. Actually, there were no completely machined blocks lying around during our photo shoot, so the one in this shot is still not quite ready for shipping.