How To Build A Better Boss 302 Engine
JGM Performance Engineering Shows How To Get More Horsepower From Ford's Trans-Am Powerplant
From the June, 2007 issue of Mustang Monthly
By Jim Smart
Photography by Jim Smart
Sometimes we stumble onto engine projects in the darnedest ways. In this case, we have a Boss 302 that has been sitting around for more than a decade in at least four different machine shops, as well as in my garage. It's a good example of how not to plan and execute an engine project. The engine has been consuming more than its fair share of space lately, so it became a priority.
Our Boss 302's cylinder walls...
Our Boss 302's cylinder walls needed finish-honing to accommodate standard-bore Speed Pro forged pistons. Jim Grubbs gave our bore a crosshatch pattern for good oil control and proper ring seating.
Always hone lifter bores for...
Always hone lifter bores for the same reason you hone cylinder walls: oil control and smooth operation. At home, you can use a brake cylinder hone on lifter bores.
It belongs to Ron Bramlett of Mustangs Plus in Stockton, California. He initially wanted to build this engine for one of his race cars. After one of my visits to Mustangs Plus, Ron drove me and his Boss 302 engine to Los Angeles. We dropped it off at a local machine shop, which sleeved the block and did much of the machine work. I eventually hauled it to other machine shops where there were several false starts but no progress.
Most of the delays were rooted in hard-to-find parts. A Boss 302 engine isn't the same as the average 289/302. It has a four-bolt main block, a steel crank with a 3-inch stroke, and C3AE-style 289 rod forgings with broad shoulders and larger 3/8-inch bolts. Although the Boss 302 uses 351C head castings, they have different cooling passages. It also uses a unique dual-plane, high-rise intake manifold, which was an issue for us because we didn't have one.
Bores are chamfered to enable...
Bores are chamfered to enable smooth piston/ring installation. Our block has been line-honed and decked for perfect fit top and bottom.
Does your machine shop use...
Does your machine shop use a torque plate for cylinder honing? If not, find another machine shop. Torque plates are bolted on top of the block to simulate cylinder-head installation to get cylinder bores true to what they'll be with the heads installed. Honing the bores without a torque plate is trouble because when you install the heads, cylinder bores will distort, creating ring-seating issues and abnormal wear.
Ryan sets the Scat crank with...
Ryan sets the Scat crank with a 3-inch stroke-that's right, 302 ci. We could've stroked this Boss 302, but we wanted to retain its high-revving qualities. Ryan set the bearings and splashed on engine-assembly lube. Once the crank is set, Eric snugs the main caps. He'll torque them in thirds: No. 3 cap, No. 1 cap, No. 5 cap, then Nos. 2 and 4 caps. With each torquing, he'll check the crank for freedom of movement. He should be able to turn it with his fingers right up to final torque. If it becomes too tight, clearances are in error.
When I told Jim Grubbs of JGM Performance Engineering about the engine, he offered to build it. He thoroughly inspected the C8FE Boss block and D0ZE head castings. Everything checked out. Our block had been successfully sleeved and remained leak tight. Jim measured the bores and pistons, concluding that the block needed needlepoint machining and blueprinting. Our Crower Sportsman rods were ready for inspection and massaging, and Ron's Scat crank with 3-inch stroke was sent to the balancer along with pistons, rings, bearings, rods, a 28-ounce Centerforce flywheel, and a Fluidampr harmonic balancer.
When you build an engine, whether it's a stocker or a high-revving screamer, blueprinting should always be done. Professional engine builders teach us that it's important to look at your engine and see things you've never seen before. Never install parts right out of the box; always inspect and massage them as necessary.
We have an early Boss 302...
We have an early Boss 302 block-technically, a 302 Tunnel Port block cast December 5, 1968 and used for a production '69 Boss 302 Mustang. Based on the VIN stamped on the back of the block, 192845, it was originally produced for 9F02G192845. According to the Boss 302 Registry, the car survives in Medford, Oregon.
Boss 302 blocks have screw-in...
Boss 302 blocks have screw-in core plugs, also known as freeze plugs. According to Jim, never try to remove them. Screw-in plugs coupled with four-bolt main caps are what separate a Boss 302 block from other small-block Fords.
We're doing something sacrilegious...
We're doing something sacrilegious by converting an early Boss 302 block to a one-piece rear main seal. Ford used a two-piece rear main seal in all of its engines until 1982. Installed properly, it works well but eventually leaks. Ford and the rest of Detroit went to one-piece versions to eliminate that problem. Converting your small-block to a one-piece seal is easy when it's performed during a rebuild.
Going to a one-piece rear...
Going to a one-piece rear main seal involves cutting the block with the same equipment used for boring cylinders. Jim cuts the lip out of the main cap used to retain a two-piece seal. Once the lip is cut, the No. 5 main cap will accommodate a one-piece seal. The lip must be machined off the crank as well. This is an easy task for any machine shop.
Ryan checks crank endplay...
Ryan checks crank endplay with a dial indicator. It should be between 0.004 and 0.008-inch, with a maximum of 0.012-inch. This is especially important with a manual transmission where thrust loads vary.
Always install screw-in oil-galley...
Always install screw-in oil-galley plugs. Ford used press-in plugs originally. This is a good engine life insurance policy.
Power and reliability come...
Power and reliability come from getting all the tolerances correct. Piston-ring end gapping, for example, determines cylinder sealing and internal friction. The better the cylinders are sealed, the more heat energy can be put to work.
A mock-up can save a lot of grief when it's time for assembly and fire up. Mocking-up an engine early on establishes dimensions, enabling you to correct irregularities before final assembly.
During our mock-up, we learned...
During our mock-up, we learned that our Boss 302 block exhibited incorrect deck height and deck warpage. Eric Peart sets up the block for milling to correct warpage and piston deck height.
Mock-up includes installing...
Mock-up includes installing Nos. 1, 3, and 5 main bearings with lubrication.
Jim checks piston deck height...
Jim checks piston deck height with a dial indicator during mock-up. It determines piston depth in the bore at top dead center. Most machine shops do one bore. You should check all of them. Ideally, you want identical deck height on all bores. Record it on each cylinder.
A mock-up involves temporarily assembling the engine-crankshaft, bearings, pistons, and connecting rods-to measure tolerances. It should also include installing the cam and heads to check piston-to-valve clearances. To be absolutely certain of every aspect of an engine build, you need to check everything, including:
Short-block preassembly one cylinder bore at a time to check deck and compression height. Do this without installing piston rings. Rod and crank-to-block clearances.Minimum clearance allowed is 0.060-inch. Parts grow when they get hot. Piston-to-crank counterweight clearances; a 0.060-inch minimum. Crankshaft endplay-0.004 to 0.008-inch. Maximum limit is 0.012-inch. Connecting-rod side clearances should be 0.010 to 0.020-inch. Maximum allowable is 0.023-inch. Check again during final assembly. Piston-to-cylinder wall clearances. Piston ring and groove dimensions, including ring-side clearances. CC valve reliefs to check valve-relief volume. Ring end-gap clearances. Main- and rod-bearing clearances. Thrust-bearing and width clearances. Install the oil pump to determine pump-to-crankshaft counterweight clearances; no less than 0.060-inch. Oil pump rotor-to-housing clearances. Check for smooth operation by hand once the pump cavity has been oiled. Piston-to-valve clearances on all cylinderbores. Valve stem-to-guide clearances and movement. Valves should glide smoothly through guides. Check stem and guide widths. Spring compression and pressure. When installing heads for mock-up, use a head gasket but don't torque. Measure (cc) chambers to determine volume. Record the numbers. Intake manifold for proper fit without a gasket.
What is Blueprinting?
Blueprinting is engine building with strict discipline and attention to detail. True blueprinting is methodical and challenging. It gets in the way of getting the job done quickly, which is why most don't bother with it.
Blueprinting includes removing...
Blueprinting includes removing ragged edges from virtually everything, including cam bearings. Removing sharp edges makes assembly easier without scoring.
Our Boss block has been sleeved....
Our Boss block has been sleeved. Blueprinting includes a block pressure check and bore cleanup, ensuring sleeves are secure and leak tight.
Use a micrometer and triple...
Use a micrometer and triple check all dimensions. We don't recommend Plasti-Gage.
Think of your V-8 engine as eight individual engines on a common crankshaft. Each cylinder bore has its own personality, making a significant contribution along a crank it shares with seven others. Each is important to power and smooth operation. When blueprinting:
Do a detailed inspection of every part. Dress and deburr everything. Anything with a sharp edge needs to be smooth. Mark all bores and inspect crosshatch pattern. Check decks, mains, and saddles for irregularities. Disassemble and inspect the oil pump, checking side clearances and the pressure relief valve for proper operation. Always index the inner and outer rotor during reassembly. Pressure relief-valve piston movement should be smooth and without resistance. Apply Loc-Tite to bolts and use a torque wrench. Blueprint all moving parts (pistons, rings,bearings, rods, crankshaft, flywheel, balancer) before balancing. Blueprinting often involves removing metal, which changes the weight. Inspect each connecting rod for bending and other irregularities, and check each end for proper indexing. Record the inside diameter of both ends. Toss or machine anything out of spec. All edges should be deburred. Make sure crankshaft counterweights are knife-edged for improved crankcase aero-dynamics. Polish rough surfaces smooth. Ground ragged edges and polish them smooth to prevent stress cracking. Polish radiuses and journals. Oil holes are chamfered for improved oil distribution. Check that the crankshaft is snoot-dressed to remove rough edges. Examine Woodruff key for rough edges and dress as necessary. Chase threads. Polish rough edges on pistons to reduce friction and prevent stress cracking. Ring grooves are massaged to improve travel. Skirts are dressed to remove ragged edges. A thermal coating is applied to piston crowns to minimize heat issues. Make sure riston rings are properly end-gapped and dressed to remove rough edges. Each set must be match-fitted. Rod and main bearings should be inspected and deburred, then checked and match-fit to each rod and main saddle. Ditto for journals. Check rear main journal seal grooves for rough edges; clean them for proper seal fit. Check all saddles for imperfections. Dress main bearing cap mating edges with a file to improve fit. Do the same with rods and caps. Massage all oil passages to remove rough edges. Passages are opened at main-bearing saddles for improved oil flow at the bearings. Dynamic balancing is detailed to less than 1 gram difference per reciprocating unit. Consider the crank pulley if the engine is externally balanced. Cam sprocket is dressed for a smooth fit. It should glide onto the camshaft. Dress sprocket teeth; smooth fit and operation mean longer life. Use a one-piece fuel pump eccentric. Taper and dress oil drainback holes. Buy new valves and guides. Valves should glide smoothly through guides. Remember: 0.0010 to 0.0027-inch clearance; max is 0.0045-inch. Dress valvesprings to remove ragged edges. Dress combustion chambers. Ragged edges and rough surfaces create hot spots, which cause detonation. Measure (cc) the chambers to determine volume(compression). Measure (cc) the valve reliefs or piston dome rise(compression). Five-angle or full-radius valve job improves flow and valve cooling. Use nickel-hardened exhaust valve seats on old iron heads. Port-match for improved flow. Don't over-torque intake manifold bolts.
After close inspection and...
After close inspection and assembly of each piston/rod assembly, Ryan assembles the short block. Billet piston ring compressors make light work of it. The cylinder walls are lubricated with engine assembly lube. Whether or not the pistons and rings are lubricated is a personal decision. Wall lubrication is sufficient.
During mock-up, we checked...
During mock-up, we checked connecting-rod side clearances. During final assembly, we check all tolerances again.
Ryan performs the final head...
Ryan performs the final head assembly using Viton valve seals. Although we've seen more than our share of Teflon valve seals, he tells us the new Viton seal outperforms and outlasts anything they've used to date.
The Extra Mile
Modern shop speak teaches us to degree a camshaft during an engine build to ensure the camshaft is all the manufacturer says it is. That's not the only reason for degreeing a camshaft. It also ensures valve timing events as they relate to piston timing. Most engine builders stop at the No. 1 cylinder when degreeing a cam. If you want to be sure, degree on all eight cylinders.
To understand the Boss 302 cylinder head, you have to go back to 1968 and the 302 Tunnel Port small-block. The 302's tunnel port head had huge ports with a pushrod traveling through them. Although it flowed well at high rpm, torque didn't come on until well above 8,000. The problem was, the engines weren't designed to rev that high. During the 1968 Trans-Am competition, valvetrains failed, not to mention bottom ends, making 1968 one of the worst years for Ford in SCCA competition.
Desperate for solutions, Ford engineers looked to the upcoming canted-valve 351 Cleveland head being developed at the time. It had the same cylinder head bolt pattern and bore spacing as the 289/302, making it a bolt-in for the 302 with minor water jacket modifications and revised pistons. Ford engineers learned the poly-angle valve head delivered better torque at lower rpm ranges.
We haven't tried anything unique with our D0ZE Boss 302 heads. They haven't been ported or fitted with larger valves. Jim installed hardened exhaust valve seats and did a radius valve job to improve airflow without adversely affecting low- to mid-range torque. This approach, coupled with state-of-the-art valve seals, makes the JGM Boss head adequate for street and strip use.
Once cylinder head assembly...
Once cylinder head assembly is complete, Ryan performs a vacuum check on all ports to determine valve seat and seal performance.
In The Dyno Room
Nothing compares to the dyno to determine whether or not you've chosen the right parts. When we planned this Boss 302 engine a decade ago, we were going to experiment with cylinder heads, camshafts, and intake manifolds. When we decided to blow dust off this Boss 302, we were faced with several critical decisions. Because we wanted to build a streetable Boss, we decided to go with our dusty collection of parts. The cam chosen then was the Crane 27-C9ZZ-A mechanical flat-tappet version with 0.502-inch lift and 292-degree duration-essentially a stock Boss 302 camshaft with a pinch more lift.
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 know Boss 302s know Ford fitted these engines with a 780-cfm Holley but not because that's what they needed. It was what the SCCA mandated for street homologation. The truth is, a box-stock Boss 302 is happiest with a 600 to 650-cfm Holley four-barrel. Unfortunately, we didn't have one.
Jim chose a Fel-Pro gasket...
Jim chose a Fel-Pro gasket set with Print-O-Seal technology. Print-O-Seal head gaskets keep coolant where it belongs.
Jim decided to reduce jet sizing from 0.72 (primaries) and 0.76 (secondaries) to 0.68/0.72. As the engine warmed and rings seated, performance improved considerably, but it was never optimum due to carburetor size. Large-cfm Holleys are too much for stock Boss 302 engines, which is why they struggle in street trim. They lack low-end torque due to a poor combination of large-port heads, excessive carburetor size, and insufficient camshaft.
Jet swaps and carb swaps, along with ignition-timing adjustments, can make significant differences in performance and driveability. We're convinced our Boss 302 would have 10-20 more lb-ft of torque and 10-15 more horsepower with a properly jetted 650-cfm Holley.
We've opted for a MPG windage...
We've opted for a MPG windage tray, which we had to modify to work with a Milodon pickup. The MPG windage tray bolts on once clearance is made for the pickup. We suggest the new one-piece oil pan gasket from Fel-Pro for all small-block Fords.
This is our Boss 302 aluminum...
This is our Boss 302 aluminum intake manifold from SEMO Classic Mustang. There was a hairline crack in one of the runners, which had to be welded. Ryan media-blasted the weld, making the repair invisible.
You have to love Crane's roller...
You have to love Crane's roller technology for vintage Ford high-performance V-8s. Here we have a 1.6:1-ratio roller-tip rocker, which reduces internal friction and allows for a more aggressive cam profile.