Steve Baur
Former Editor, Modified Mustangs & Fords
June 1, 2007
Contributers: Tom Naegele Photos By: Courtesy of D.S.S. Racing, Ford Racing Performance Parts
Screw-in freeze plugs and oil-galley plugs increase the strength of the block walls. These were utilized on the original Boss 302 block. Also present is the oil-dipstick hole, which allows use of the factory oil pan and dipstick.

Naegele's Take on the Legend Reborn
"For all practical purposes, FRPP's new Boss block is the R302 replacement," Naegele says. "Features include a four-bolt block similar to the R302 block. Main bolts are 351W 1/2-inch fasteners (R302 are 7/16), and in order to do this they had to move them outboard. This required FRPP to provide a new oil pump pickup due to space limitations. Keep in mind that there are no 302 blocks out there with 1/2-inch mains. We're going to install different main studs that are a little longer so that we can use the D.S.S. main support system.

"The head bolt holes are 1/2-inch pieces, like 351W and R302 blocks, but they're slightly deeper for better gasket retention and a lesser chance of ripping out threads. To utilize the extra length, you'll need different fasteners.

"The Boss' cylinders measure roughly 3.990 inches. The undersized bore needs to be finished bored and honed, just like the R block. The Boss' thick cylinders offer a comfortable 4.125-inch overbore capability-the R302 was stretched at 4.125 and often required sonic checking. It was more of a 4.100 piece.

"The new Boss block uses common outside diameter (OD) cam bearings or conversion bearings for use with most off-the-shelf cams. The R302 block was the first to come out with common OD bearings (Dart blocks use them also), and from a manufacturing standpoint, it's easier to bore the cam bearing holes all the same size. Stock cam journals, however, are all different sizes. Most cam companies sell common OD cams through special order, but FRPP came up with conversion bearings to facilitate the use of common off-the-shelf 302/351W grinds. The conversion bearings are a bit expensive, though the cost has come down some from when they first came out.

"Another feature that FRPP added to the Boss block is its improved front and rear lifter-galley oiling feeds. The R block just oils the lifter galley from the rear and can starve the front lifters of lubrication. The Boss went the way of the Dart block, as it feeds the front and rear lifter galleys. In blocks with only rear oil feeds, FRPP noticed scuffing of the front lifter bores in some endurance applications. For the average Mustang enthusiast, this is not much of a concern, but it does add performance and value to the package.

"In the lifter valley, the Boss has finish align-honed mains and lifter bores-R blocks must be align-honed, as the bores are small out of the box. The Boss' big lifter bosses allow machining for offset or bigger lifters.

"One feature the Boss block shares with early Boss blocks are screw-in freeze plugs and oil galley plugs. The R block has screw-in galley plugs for oil but press-in plugs for the water. Dart blocks use press-in freeze plugs as well, unless machined by the builder for the screw-in type pieces.

The R302 block has screw-in galley plugs for oil but press-in plugs for the water. Dart blocks use press-in freeze plugs as well, unless machined by the builder for the screw-in-type pieces. The Boss 302 sets the standard with screw-in freeze and oil-galley plugs. The screw-in type of plugs adds more structural rigidity to the block and won't pop out in high-horsepower/high-stress applications.

"When you drop a stroker crankshaft in a stock 5.0 or R302 block, you need to machine the block for counter weight clearance, but FRPP has provided ample room with the Boss, and it has also done a nice job of cleaning up the crank case casting. Normally you could expect an hour to an hour-and-a-half of grinding on an R302 block to fit the crank.

"The Boss has a shorter cylinder length, which makes rod bolt clearancing of the cylinders obsolete, but it may limit the amount of stroke and compression height you can run. The bigger you go on stroke, the more the piston protrudes out of the bottom of the cylinder. The Boss cylinders are approximately 0.400-inch shorter in length than a stock or R block.

"The deck heights on the Boss are 0.010-inch tall and will need to be equalized and decked just like the R block. This is fairly common among aftermarket blocks. The Boss features siamesed cylinders, and when FRPP was designing the block, the people in the NASCAR and endurance racing arenas that they talked with said the siamese design could overheat the head gaskets, so FRPP provided the Boss block with siamese cross-over coolant bleed holes to help remove trapped steam from trouble areas.

"In order to make the new Boss readily accepted by early Mustang and Ford enthusiasts, FRPP included a boss and tapped hole for the clutch cross-shaft from early '60s/'70s cars. This wasn't present on either the R block or the A4, which put a lot of people out when it came to building good motors for old cars with manual transmissions.

"Unlike most of the competition, the Boss block has a provision for the use of the factory dipstick, rather than requiring one to purchase a special oil pan-mounted one.

"The three center main caps are four-bolt and, more importantly, splayed. Splayed caps help keep main-cap walk under control, as they tie the cap laterally on an angle, which secures the cap, minimizing the sliding on the parting surfaces. Front and rear are still two-bolt pieces. The D.S.S. main support system that we'll use in this build will keep them from walking, and it also allows the use of a full-length billet-aluminum multilevel scraper/windage tray. Aluminum deadens the harmonics that make main caps walk. It does need longer main fasteners, and the main support comes with all of the necessary ARP custom hardware.

"The Boss block's big-bore nature is a win/win situation. While you can take it out to 363 ci (basically a 347 with a 4.125 bore size), this offers challenges when it comes to street-car longevity. The best engine combination is a result of the best compromise of all aspects of the engine, including rod ratio (rod length divided by stroke) and piston-ring placement. Engines that have poor rod ratios and use short piston designs tend to have the pistons rock in the bores, which causes excessive wear and added friction at a higher rpm. For racers, seasonal maintenance (rings and bearings) is not an issue, but it's not something someone with a street car is going to want to do. The short pistons also usually end up having the wristpins in the oil ring, and that can lead to oil control trouble if the proper ring package is not used.

FRPP added a boss and tapped hole for the clutch cross-shaft from early '60s/'70s cars. This wasn't offered on the Sportsman blocks. Now the old-school crowd can build a hot small-block, too.

"This is why many engine builders recommend the 331ci stroker assembly for street cars. The common 331 comes from a 3.250-inch stroke, a 4.030-inch bore size, and a 5.315-inch connecting rod. This combination results in a 1.250-inch compression height, which provides proper spacing for the piston rings. It is far superior than the 1.090-inch height found in common 347ci motors. The better rod ratio and taller piston results in better ring placement and spacing. Subsequently the piston rocks less in the bore and promotes greater ring-and-piston life. The taller piston prevents the wristpin from intersecting the oil ring and a better 1.63 rod ratio is achieved.

"The common 347ci engine, with its 3.400 stroke, 5.4-inch rod, 4.030 bore, and 1.090 compression height (the distance from the wristpin center to the top of the piston) makes for the same inefficient piston/ring/rod ratio design and results in a 1.58 rod ratio just like the 363.

"With the Boss block's big-bore capability, we can obtain the 347 ci along with a 1.63 rod ratio. Using a 3.250-inch stroke, a 5.315-inch connect-ing rod, and a 4.125-inch piston, we arrive at 347 ci with the 1.63 rod ratio-a 347 done the right way.