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.
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.
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.
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.