Building An Affordable Big Block Chevy Engine - Budget Rat

How to Build an Affordable Big-Block Chevy Engine

Tony Nausieda Aug 1, 2006 0 Comment(s)

Sticking to a reasonable budget when you're planning out your big-block Chevy can be an exercise in frustration. A trick set of BBC aftermarket heads and a quality rod/piston combination will easily approach three grand. It's numbers like this that send guys running back to the small-block camp, but there's an alternative: Save a ton of green by starting with a good core motor. The better the core, the more stock parts you can recondition--and most of the stock BBC equipment will suffice nicely for a naturally aspirated street mill. The concept with this build is to disassemble, clean, and recondition as many stock parts as possible and use affordable, proven aftermarket parts when the stock ones truly compromise our goal of high performance. Our strategy here is exactly what most of you set out to do: build a reliable combination decent enough to turn respectable times at the track--without requiring a vacuum canister, a car trailer, or a second mortgage.

The Right Core
To keep things on the cheap, we found a 454 that didn't need to be bored; in fact, it still held the original standard-bore cast pistons with discernable cross-hatching on the cylinder walls. Even better, the stamped suffix ID'd it as a '70 390-horse variant out of a Caprice, otherwise known as an RPO LS5. Fitted with closed-chamber, oval-port heads, 10.25:1 compression, and a steel crank, it was well worth the $1,000 asking price, we figured. Days later the original intake, distributor, exhaust manifolds, and smog stuff netted an astounding $734 on eBay, slashing our initial investment to a laughable $266. Really. If you're having trouble locating a musclecar-era BBC, you can build the near-equivalent of this motor using a smog-era 454 Gen IV short-block and a set of closed-chamber, oval-port heads. Any of the large oval-port castings from '65 to '70 will have small, 98-101cc chambers to achieve approximately 9:1 compression with smog-era 454 pistons, and these heads are still quite plentiful and cheap.

Autopsy
With the engine apart, we found the crank had been cut 0.010 inch under on the mains and rods. Scarred main bearings and excessive wear on a few rod bearings told the story of a rebuild gone wrong. With parts in hand, we headed to the experts at Johnson Machine Service (JMS) in Monrovia, California, to formulate a game plan for reconditioning. Owner/operator Mike Johnson gave us mostly good news: The block, pistons, and crank were all usable, and the heads weren't cracked, although everything was a little tired. As expected, the big end of each rod was out-of-round and needed to be resized; one was replaced because it showed evidence of overheating. The JMS advantage became clear when Johnson plucked an identical specimen from a huge barrel of core BBC rods. Years of churning out high-performance big-blocks has created a huge stock of cores, so every-thing is available in-house if you'd rather not put forth the effort to search for the parts yourself.

The Rebirth
While the block baked in the hot tank, Howard Allen polished the stock steel crank journals to create a pristine surface finish, checking them with a mic to ensure proper bearing clearances. After separating and cleaning the stock piston/ rod combo, Robert Hendrix ran the rods through a strengthening shot-peening process and lightly blasted the pistons with glass-bead media. Hendrix resized the toughened rods until the Sunnen bore gauge couldn't discern any out- of-round on their big ends. Will Didier assembled each piston/rod set, using a Carrillo rod heater to expand the pin bore for minimal distortion and galling. Since we essentially reused the entire stock rotating assembly (except for bearings and rings) we didn't balance it, hoping that GM's factory spec would work within the confines of a 5,500-rpm redline.

With the short-block renewed, we tore into the cylinder heads. We caught a lucky break, salvaging the stock iron valve guides after a quick knurling operation. Since we saved some cash on this step, we went ahead and had Pete Hillemeyer machine the guides for Teflon PC-type seals that improve oil control. Hillemeyer went to work on the ever-accurate Serdi machine, cutting the three-angle profile into the seats. Sal Alcaraz fixtured the finished castings and cleaned up the deck surfaces with a 0.020-inch cut. We didn't cc the chambers to check how this changed compression, but the change in compression should be minimal. Finally, the stock 2.06/1.72-inch valves were checked for straightness, and any severely pitted valves were replaced with unscathed candidates from JMS' ample inventory of cores. Hendrix ground each valve's seating surface until it had perfect concentricity with its stem, and your author wrapped up the headwork by hand-lapping the valves to the seats.

We'd had our fill of stock parts, so it was time to order the performance equip-ment that would separate our motor from the stock LS5 tune. A modern cam profile, intake, carb, ignition, and oiling system are all you need to extract tire-frying power from any BBC, especially the 454-cube variant. Crane supplied a hydraulic cam and lifter kit--the Powermax 286 keeps duration mild (226/236 at 0.050 inch) and is designed with a wide lobe sepa- ration (112 degrees) for a smooth idle, but it boasts 0.544/0.556 inch of lift to move plenty of air through the combustion chamber. Crane's Mark Campbell spec'd a mild dual valvespring, installed with 0.050-inch-tall retainers to keep the seat pressure around 110 pounds for longevity. Keep in mind that cutting the valve seats changes the installed height of the valve- springs, so please check this spec when you're assembling the heads. Crane's 3/8-inch hardened pushrods were more than adequate to actuate the stamped steel rocker arms--you read that right--using this mild hydraulic cam, stamped long-slot rocker arms suffice. Just be sure to use factory-style valve covers with welded drippers to keep the pivot balls well oiled. Trans-Dapt offers awesome-looking factory-style, non-dripper BBC valve covers if you opt to run roller rockers.

Induction came courtesy of a World Products Merlin dual-plane intake and a Holley 850 vacuum-secondary carb (the OE carb spec'd for GM 502 crate engines). The venerable MSD Ready-to-Run distributor, 8.5mm Super Conductor wires, and NGK plugs comprise the simple yet robust ignition system. Oil is pumped to all the right places via a complete, tried-and-true Milodon oiling system.

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Driving out the 454's freeze plugs revealed a good bit of rust scale in the cylinders--not uncommon in an engine that's been sitting for years. We did our best to scrape the scale from the water jackets before the block and heads hit the cleaning tank.

With decades of racing and engine-building experience under his belt, Howard Allen knows his way around a crankshaft. Our stock steel crank only needed a polish to dress the journals.

Aftermarket valves were beyond the scope of this project. Robert Hendrix marked the hammered sealing surfaces of our stock valves with machinist blue prior to grinding. He ground the valves until a continuous band of metal was machined off the surface, guaranteeing concentricity to the valve stem (and good sealing on the seat).

Using a high-dollar Serdi machine, Pete Hillemeyer masterfully cut the iron seats on our stock oval-port castings. Installing hardened inserts wasn't part of our plan; instead we dropped a lead additive to every fifth tank of gas to prevent seat recession.

Back at home, we scrubbed the oil galleys with diluted Simple Green, using Milodon's tough engine brushes to loosen the stubborn deposits.

Wiping out the bores is especially important if you've honed the block--honing dust can wreak havoc on a set of rings.

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