Tech Articles

Build a Better Boss (302 Ford, That Is)

(Image/Jim Smart)

Ford’s incredible 1969-70 Trans Am BOSS 302. Not the 2012-13 Coyote DOHC BOSS, but instead the genuine pushrod original from an era when Ford did the unthinkable repeatedly—and won! Ford’s classic 1969-70 BOSS 302 engine was a bold step forward after a miserable season in 1968.

The odds of you building a vintage Ford BOSS 302 engine are slim because Ford built so few and so many are already spoken for. However, it’s always fun to think about building one or even erecting one from scratch, which you can do with a Ford Performance BOSS block (M-6010-BOSS302) and Cleveland heads. You may also cruise the online auctions for a vintage 5.0L roller block (F1AZ-6010-B). There are plenty of them lying around used and new-old-stock. 

Ford has always been a car company that dared to go where no automaker had ever gone before. That tradition dates back to the company’s founding in 1903. Think of Ford as a free-spirited car company with a rich history, driven by ego, that took big risks much as it is doing today with electric vehicles.

In the 1960s, Ford was an outspoken car company, much as Pontiac Division was within General Motors. Pontiac was the outlaw division. It didn’t conform to corporate protocol and invented the muscle car in its 1964 GTO. It had to sneak the GTO in under the radar of upper management. Ford took on the world with the GT40 supercar. Ford went to Le Mans and beat Ferrari not once, but four times, settling an age-old dispute over who had had the fastest cars in the world. Henry Ford II wasn’t going to have it any other way. Enzo Ferrari, and others, were handed their backsides.

Ford’s great success with its powerful V8 engines is legendary. The brute 427 FE Series big block that won Le Mans also reigned supreme in limited production 1964 Thunderbolt Fairlanes in NHRA competition. The darned thing was rabid dog fast. The 306-horse 289 cubic inch High Performance small-block V-8 in Shelby GT350s took on Corvette and the rest and managed to spank America’s sports car in SCCA competition.

With such great success under its seat belt, Ford decided to dabble with the 289 and stroked 302 to see what could be done to further its performance image. For the 1968 Trans Am season, Ford took the 302 cubic inch “Tunnel Port” out there in search of victory. Because the 302 Tunnel Port called for high revs in the 9,000 rpm range to make real power, racers scattered these engines all over racetracks across the country and left feeling defeated. The 1968 Trans Am season was a disaster.

Enter Ford’s development of an all-new 351ci engine with poly-angle valve heads and tight-fisted wedge heads destined for a 1970 introduction. Because the new 351 engine had the same bore spacing as the 289/302, it seemed logical for Ford to adapt this high-performance cylinder head to the small-block Ford V8. The 302 Tunnel Port already had a terrific four-bolt main block and steel crank along with beefed up “C3AE” 289 rods and special pistons designed for the Tunnel Port heads. Ford engineers took the new 351 cylinder head and modified it to work with the Tunnel Port block.

The result of this engineering effort was the BOSS 302 engine, which became the Trans Am mill of choice for 1969-70 and beyond in SCCA competition. To conceive the BOSS 302, Ford had to develop a special forged piston with a significant dome that would reach deep into the 351C head’s 64 – 67cc chambers. TRW developed these pistons and suppled Ford with them for both racing and production engines.

Because the BOSS 302’s heads sported monster-sized intake ports, these engines had a high-revving demeanor with peak torque and horsepower coming in around 7,000 rpm instead of the Tunnel Port’s outrageous nine grand. The result was a powerful 290-horse factory screamer with peak street power coming in around 6,000 rpm. These elements made the BOSS 302 a unique mill compared to what the rest of Detroit had to offer at the time. 

We will admit to you our results on the dyno were disappointing—304 horsepower and 288 lb.-ft. of torque. We learned on the first few pulls that we had too much carburetor with our 750 cfm Holley 4160 and too little camshaft. Those of you who understand the BOSS 302 know Ford fitted these engines with a 780 cfm Holley but not necessarily because that’s what they needed. It was what the SCCA mandated for street homologation. Truth is, a factory original Boss 302 is happiest with a 600 to 650 cfm Holley. The BOSS 302’s biggest shortcoming is port sizing, which is something Ford has rarely been able to get right. The BOSS 302 engine could use better cylinder heads with right-size port and valve sizing in a set of Trick Flow cylinder heads and a more compatible roller mechanical camshaft. We tried this formula to see what our basically stock BOSS 302 would do. We got our answer. 

Once the “C8FE” Tunnel Port block is cleaned up, first order of business is to sonic check cylinder walls and cut the #5 main cap for a one-piece rear main seal. Did you know you can convert a small-block Ford two-piece rear main seal to a one-piece? (Image/Jim Smart)
We begin this BOSS 302 build with machining a rare BOSS block for a one-piece rear main seal (left), which is a nice stealthy improvement no one will see. (Image/Jim Smart)
The 1982-up style one-piece rear main seal will work on a BOSS crank. Although engine builders tend to lube the seal lip, never lube the seal lip. It goes on dry. (Image/Jim Smart)
Main saddle tolerances are checked with a dial indicator. (Image/Jim Smart)
Before you is a genuine “C8FE” Tunnel Port block—the real deal void of any damage. Early production 1969 BOSS 302 Mustangs were also produced with C8FE blocks before mass production got underway in the spring of 1969. (Image/Jim Smart)
This freshly-sleeved BOSS block is undergoing a complete battery of machine work—honing, decking, and align honing. All threads are being chased clean. Screw-in oil galley plugs will be installed in front for durability. (Image/Jim Smart)
Once decking and honing are complete, cylinder bores are cleaned up on top for easier assembly and to remove any potential hot spots. (Image/Jim Smart)
When you build a vintage BOSS 302 engine, you do have choices. You may opt for the original flat-tappet mechanical camshaft or go with an aggressive roller mechanical cam. You may also opt for a mild custom ground roller mechanical. We’re using moly lube on the camlobes for proper break-in and engine assembly lube on the journals. (Image/Jim Smart)
What makes the BOSS 302 block different than a standard 302 block is screw-in freeze plugs, wider four-bolt mains, and thicker main webs. (Image/Jim Smart)
Because we were unable to find a BOSS 302 steel crankshaft, we opted for a steel crank from Scat with a 3.000 inch stroke. Crankshaft endplay is checked and comes within 0.004 to 0.008 inch. (Image/Jim Smart)
Crower Sportsman connecting rods and 4.000-inch standard bore Speed Pro forged pistons from Summit Racing have been dynamic balanced and are ready for service. Bores have been match-honed to the slugs. (Image/Jim Smart)
Piston ring end gaps are checked. Check ring end gaps even with pre-gapped rings. (Image/Jim Smart)
This is our Scat 3.000 inch stroke steel crank, which is a drop-in replacement for the factory BOSS crank. (Image/Jim Smart)
What makes the BOSS pistons different from standard 289/302 pistons is the raised dome designed to fill the 61 to 67cc chambers. This is very similar to the Ford 351C-4V piston with the raised dome. The BOSS 302’s original TRW forged pistons were prone to failure. (Image/Jim Smart)
Down under, connecting rod side clearances and crank endplay have been checked. Because we’ve gone with aftermarket rods and crank, clearances must be checked for 360 degrees, confirming crank counterweights clear the block and piston skirts. (Image/Jim Smart)
Before the cam can be degreed and confirmed, true top-dead-center must be checked. This is done with a dial indicator, which is zeroed at the block deck first. The piston is brought to top-dead-center, then, the dial indicator moved to the dome. The crank is turned slowly each way and the dial indicator watched for movement. Watch the dial indicator as the crank is turned each direction until piston movement is observed. Note the number of degrees the crank is turned until there’s movement. Then, put the crank smack in the middle. (Image/Jim Smart)
Once true TDC is confirmed, use a degree wheel to observe valve timing events and record your findings. Compare your findings to the manufacturer’s cam card. We’re running a Crane flat tappet 27-C9ZZ-A mechanical BOSS cam. (Image/Jim Smart)
Because the BOSS 302 is a high-revving mill, it calls for a windage tray to keep oil where it belongs in the sump. The BOSS 302 was originally equipped with a factory windage tray and deep sump pan. We looked to MPG Head Service for the 302 windage tray. In the Summit Racing inventory is the Scott Drake baffled oil pan for all small block Fords. (Image/Jim Smart)
You have two choices when it’s time to rework heads. If you’re building a weekend driver, you really don’t need hardened exhaust valve seats. If you’re going racing or going to drive daily, you’re going to need either hardened exhaust valve seats or stainless steel valves. It is more costly to cut and insert exhaust valve seats. (Image/Jim Smart)
Although a valve job seems simple in scope, it isn’t. Once you have replaced the guides and cut seats, there’s more to think about. Guides need to be cut for Viton valve seals. Spring pressures, installed height, and coil bind need to be checked. Record your numbers and double-check your work. Although builders reuse valves, this is discouraged, especially if you’re going for high revs. (Image/Jim Smart)
Cylinder head decks are milled for trueness. (Image/Jim Smart)
Viton valve seals are general protocol these days because they’re so effective and they last. (Image/Jim Smart)
Before fully assembling the heads, check valve-to-valve clearances, which should be 0.060 to 0.100 inch minimum. (Image/Jim Smart)
Deck surfaces have been inspected and cleaned in preparation for cylinder head seating. We’ve opted for the Fel-Pro Print-O-Seal head gasket. Apply Permatex’s The Right Stuff where the head gaskets meet the intake. (Image/Jim Smart)
Ryan Peart of JGM Performance Engineering torques cylinder heads per the Ford Shop Manual—65 to 72 ft.-lbs. following the suggested pattern from your Ford Shop Manual. (Image/Jim Smart)
JGM Performance Engineering opted for a Fluidmpr harmonic damper. Because these pre-1982 small block Fords are externally balanced with a 28 ounce offset balance, they call for balance weights at the flywheel and at the harmonic dampener. Remember this when you’re ordering these components. (Image/Jim Smart)
Crane 1.6:1 full roller rockers reduce internal friction and free up power. (Image/Jim Smart)
Generally accepted valve lash is 0.025 inch hot depending upon how you will use the engine—street or track. For street, you can go tighter at 0.022 inch hot. (Image/Jim Smart)
For break-in and dyno pulls, we’re running SAE 30 weight with a zinc additive. Whenever you’re running a flat tappet cam, always use a zinc additive or a diesel grade oil with zinc. Fire the engine and let it run at 2,500 rpm for 30 minutes before the first pull. The first pull is about ring and bearing seating. (Image/Jim Smart)
MSD’s Digital E-Curve distributor drops in place of the factory Autolite dual-point and with three connections. You can set a timing curve with a rotary dial. No springs or stop bushings to deal with. The digital module lets you select from 20 different centrifugal advance curves and five vacuum curves. There’s also a built-in rev limiter to keep you out of trouble. (Image/Jim Smart)
We managed to find a Ford “C9FE” dual-plane BOSS intake manifold, which will deliver both low to mid-range torque and high rpm horsepower. The irony of the BOSS head/manifold combination is port sizing tends to be too large even at 7,000 rpm. You still don’t get the velocity needed for good power. (Image/Jim Smart)
Although we will get pushback on this, we found 750 cfm to be too much carburetor for this application. Port size and the limitations of a stock camshaft grind limited horsepower to 304 at 5,500 rpm and 299 at 4,500 rpm. We needed more cam just for starters along with carburetor resizing. (Image./Jim Smart)

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5 Comments

  1. Ragtop Man says:

    Wait… 1.6 rockers? Did you get them at a Chevy garage sale? Factory was1.73 IIRC. What cam specs and static CR? Something is very wrong with this, a blueprinted stocker should be in the 375-400 HP range.

  2. Pingback: Building Classic Ford 351 Cleveland Power

  3. Boss 302 is a Windsor engine; uses Windsor cam lobe profile & 1.6 Windsor rockers.

    • The factory Boss 302 did not have a deep sump pan although it did have a baffle in the sump, and yes it has 1.73 ratio rockers from Ford.

  4. Great article is there a video available ?

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