Small block Chevy engines long ago became mainstays in both thetraditional and high performance marketplace. So many of themhave been rebuilt over the years, and so much has been writtenabout the rebuilding process, it would appear that nothing moreneed be said.
But the small block engine has changed over the years and so haveconsumer preferences and orientation. Consequently, many longstanding rebuilding techniques are due for a change, or at leastsome refinement. That is, assuming a "high performance rebuild"is more than a standard short block with high performance headsand a high lift cam.
Points to keep in mind are that many customers for high performanceengines are much more informed than their counterparts of 10 or20 years ago. As opposed to a still wet-behind-the-ears teenager,the current high performance customer will likely be 30 to 50years old with some racing background and basic knowledge of propermachining practices.
It’s also probable that he or she has been down the road before,may have had a bad experience with a previous shop and is a bitgun shy. The flood of questions gushing forth from the mouthsof many high performance customers today is often a consequenceof previous experience; they’re looking for some verbal reassurancebefore spending money.
Being the most popular engine in the world, the small block Chevypresents a number of high performance opportunities. Originallyintroduced with a displacement of 265 cubic inches, the smallblock Chevy has grown over the years, ultimately reaching 400cubic inches. Two economy versions, one displacing 262 cubic inches,the other a "whopping" 267 cubic inches were also produced,but these are entirely unsuitable for performance use.
|Small Block Specifications|
Since the 1950s, all bore/stroke combinations have been rebuiltin high performance form. However, at this late date, blocks with4.00" bores constitute the lion’s share of the performancebusiness. There’s also a sizable demand for 4-1/8" bore blocksfrom which 400+ CID small blocks are built.
As with any rebuild, one that will produce a high performanceengine starts with the block. In the overall scheme of things,only two types of cylinder blocks exist – those with two-boltmain caps and those with four. But over the years, dip stick positionhas changed, rear main seal configuration has been updated, anda variety of alloys have been used.
Dip stick position isn’t much of an issue, unless you’ve orderedthe wrong oil pan. Then you wind up with the dipstick on one side,the notch in the oil pan on the other, and an engine with a severeoil leak. Alloy content is a somewhat different matter. Thousandsof high performance small blocks based on a standard alloy blockcasting have run successfully for years. But for maximum strengthand longevity, a "high tin" block is preferable.
A block’s alloy content is denoted by two figures cast into thefront face, just above the main bearing bore, in the area normallyconcealed by the timing cover. Many production small blocks havethe numbers "010," "020" or both cast intotheir front face, just above the main bearing bore. If both numbersare present, one above the other, it indicates that the blockalloy contains 10% tin and 20% nickel. A single number, eithera "010" or "020" represents the amount ofnickel and indicates negligible amounts of tin.
No numbers, other than the casting numbers that are typicallyfound beneath the timing cover, translates to only minor amountsof tin and nickel being present in the block alloy. (Tin and nickelare two metals that are commonly alloyed with cast iron to improvedurability, hardness and heat dissipation.)
Although a "010"/"020" block is most desirable,it’s not always possible to find one that’s suitable for highperformance use. Alloy composition aside, cylinder wall thicknessis the overriding consideration in block selection, and one withno tin or nickel and thick cylinder walls is generally preferableto a high-nickel block with thin walls. Truck and older ChevyII blocks are reputed to have thicker than average cylinder walls,but there are no guarantees; sonic testing is the only way tobe certain that wall thickness is adequate.
Beginning with the 1986 model year, Chevrolet began producingblocks with a one-piece rear main seal. There’s enough differencebetween 1985 and earlier and 1986 and later blocks that oil pansand crankshafts are not interchangeable unless an adapter is fittedto the block. Most commonly, a crankshaft and oil pan designedfor the older-style, two-piece seal is installed in a late modelblock with one-piece seal. Adapters allowing this to occur areavailable from a variety of aftermarket companies and throughGM Performance Parts as p/n 10051118.
The introduction of hydraulic roller lifters for the 1987 modelyear brought about other cylinder block changes. To accommodateoriginal equipment hydraulic rollers – which are of a differentdesign than aftermarket types – the tops of the lifter bores wereraised and machined flat. The tapped bosses were also added inthe lifter valley so the sheet metal "spider" that holdsthe lifter link bars in place could be attached.
Standard hydraulic or mechanical lifters can be installed in a"hydraulic roller" block, but original equipment rollerlifters cannot be installed in a "non-hydraulic roller"block. "Hydraulic roller" blocks also have a tappedhole on either side of the camshaft hole for attaching the retainingplate that’s installed to prevent the camshaft from "walking"forward.
Another variation that can ruin an otherwise well-planned enginebuilding party is main bearing diameter. Beginning with the 1968model year, main journal diameter was increased from 2.30"to 2.45". On the other hand, all 4-1/8"-bore productionblocks are machined for a 2.65" main journal diameter. Consequently,it’s advisable to verify main journal, bearing and bearing saddlediameters to assure proper fit.
It’s also advisable to disregard model year when determining blockcharacteristics. Considering that new cars are typically introducedin September or October of the previous calendar year, it’s notat all unusual for a casting date to disagree with the model yearof the vehicle in which it was originally installed. Prior engineswaps can also confuse the issue, so accurate measurements shouldalways be made.
For the 1992 model year, Chevrolet introduced a Second Generationsmall block known as the LT1. (Installed in 1992 and later Corvettesand 1993 and later Camaros, Firebirds and 1994-’96 GM "B"and "D" bodied full-sized cars). Within the Second Generationfamily, most major components are interchangeable. However, a265 CID version of the engine was also produced, (the base Capriceengine) so don’t be surprised if you come across an LT1 blockwith 3-3/4" cylinder dimensions. With the LT1′s reverse flowcooling system, neither the block nor heads are interchangeablewith a First Generation small block.
Irrespective of the block selected, a performance rebuild shouldinclude align honing. Many machinists either overlook or disregardthe importance of align honing. But every critical block dimensionis taken off main bearing saddle alignment, so align boring and/orhoning should be the first machining operation and it must bedone accurately.
When a block is align honed, you absolutely must have the oilpump installed and the bearing caps tightened to the requiredtorque, using the same type of fasteners (either studs or bolts)that will be installed when the engine is assembled. This is criticalbecause when you tighten the main cap bolts or studs, or the oilpump bolt, it distorts the cap.
It is obviously possible to build a high performance engine andforego align honing. But if the engine is "hammered"very often, or if the owner installs a nitrous oxide system, youmay very well end up with an unhappy customer.
Of course, the best choice for a high performance engine is aforged crankshaft, but these aren’t readily available at low cost.In truth, small block Chevy cast cranks are more than adequatefor most high performance applications. From 1969 until 1986,when Chevrolet converted to a one-piece rear main seal, c/n 3932442was installed in virtually every 350 small block not equippedwith a forged crank.
But the casting number doesn’t tell the whole story. The samecrank casting is used as the basis for 305 crankshafts. Althougha 305 crank can physically be bolted into a 350 block, it’s bestto avoid doing do. The 305′s lighter reciprocating assembly weighttranslates to a considerable difference in the balance factor.
Unless a 305 crankshaft is completely rebalanced with the appropriatebob weight, it will cause severe vibration if installed in a 350.If there’s any question as to a crank’s identity, it should bechecked so it can be used in the appropriate engine assembly.It’s also advisable to check any cast crankshaft for cracks. Asa general rule, a crankshaft should pass magnaflux inspectionbefore it’s installed in a high performance engine.
Pistons and rings
The best deal in town on small block pistons can be found in theKeith Black and Speed-Pro catalogs. Both companies offer hypereutecticpistons which are ideal for high performance street (and somerace) engines. These pistons are typically cheaper than theirforged counterparts and are actually better suited for long termoperation in a high performance street engine.
The hypereutectic material is extremely hard and has a very lowexpansion rate so it can stand considerable abuse. Since it isinstalled with .001" to .002" piston-to-wall clearance,it can handle the abuse over a long period of time without theclatter associated with most forged pistons. Both flat top anddomed varieties are available so just about any compression ratiocan be achieved.
Most seasoned performance and race engine builders have very strongopinions regarding brand and type of piston ring and the requiredcylinder wall finish. However, for long term durability in anytype of engine, a Total Seal ring set with a plasma moly top ring,Gapless™ second and stainless steel low tension oil ringis tough to beat.
Cylinder wall preparation can also be a hotly debated topic withvarious preferences for honing stones, and final surfacing proceduresinvolving specific plateau finishing specifications, etc. However,at many shops, the standard cylinder preparation for the ringcombination cited above includes boring the block to within .005"of desired finished bore size then traveling the rest of the waywith a hone. The typical procedure involves removing the first.0035" with 220 grit (500 series) stones, then removing another.001" with the 280 grit stones (600 series). A final finishis then achieved by removing the last .0005" with 400 grit(800 series) stones.Although some engine builders use a super-slickcylinder wall finish, many others do the final hone with 400 stones,which knocks the peaks off the ridges left by the coarser stones.Many rebuilders feel this type of finish is best for quick ringseating and long term ring seal.For optimum sealing, rings shouldbe fit to the individual cylinders and end gaps filed to fit.In lieu of manufacturers’ recommendations otherwise, the top ringshould be given .020" to .022" end gap with forged pistonsand .026" to .028" with hypereutectic pistons.A 5/64",5/64", 3/16" ring configuration is often preferred forstreet and recreational marine engines. (Wider rings deliver betterlong-term durability.) Although a 1/16", 1/16", 3/16"ring combination will provide improved ring seal at high rpm,such considerations are unwarranted in a street or recreationalmarine engine because the engine doesn’t spend enough time inthe tachometer’s "Twilight Zone" to justify the trade-offof reduced ring life. Another consideration is that with a 1/16",1/16", 3/16" ring package, oil consumption tends tobe higher than with wider rings.
The latest trend in oil rings is low tension. The oil rings arethe most significant contributors to ring drag, so reducing tensionsignificantly lowers internal friction. In a low tension oil ring,improved ring conformability (the ability of the ring to stayin contact with the cylinder wall) is achieved by manufacturingthe oil rails from material with reduced radial thickness. Somecompanies are also experimenting with rails that are .015"thick rather than .024" in thickness.
From the time the small block was introduced, Chevrolet has offereda variety of cylinder heads. Most of the pre-emissions era highperformance heads have 64 cc combustion chambers. Note that thisis a nominal engineering dimension; in real life, most "64cc chambers" actually measure 67 or 68 cc. Head milling isusually required to achieve a combustion chamber that actuallymeasures 64 cc.
For a typical, lower-cost performance engine, 186, 462 or 492castings are the most commonly used heads. These are the tried-and-true"double-hump" castings of the type originally installedon fuel injected Corvette and ’60s era Z/28 engines. Nothing haschanged much in this area of small block Chevy high performance.However, amongst owners of late model fuel injected engines, Corvettealuminum heads have taken the spotlight.
In stock form, the Corvette aluminum head (c/n 10088113, p/n 10185087)has good air flow characteristics which are sufficient to supportthe needs of an engine producing a maximum of about 330 hp. Properlyported, however, these heads are suitable for 400+ hp engines.Another consideration is that these heads were designed for useon fuel-injected engines. As such, they have no heat riser passagesto bring heat to the bottom of the intake manifold, which cancause cold start problems if an engine is equipped with a carburetor.
Strange as it may seem, there is quite a demand for CNC-portedCorvette aluminum heads for installation on street-driven smallblock engines. In fact, some shops specializing in late modelperformance engines install CNC-ported heads on virtually everyengine they sell. With a price of more than $1,200 per pair, CNCmodifications are obviously targeted at the high end of the market.But the strong demand for this type of porting indicates the diversenature of consumers who spend money on small block Chevy rebuildingservices.
Along with aluminum heads usually goes a tuned port or LT1 aluminumintake manifold. For all intents and purposes, an intake manifoldshould be an intake manifold and the procedures used for installationshould be the same. But that doesn’t seem to hold true for latemodel fuel injection manifolds. Every time one of these manifoldsis removed from an engine, the cylinder head mating surfaces shouldbe checked for warpage and angularity. For some reason, thesemanifolds are extremely prone to distort, thereby causing sealingproblems.Many engine builders who specialize in tuned port andLT1 engines will not install an intake manifold unless its conditionhas been verified. They’ve been burned too many times by oil consumptionproblems caused by internal vacuum leaks which allow manifoldvacuum to pull oil in between the manifold and head surfaces.
Prior to the advent of electronic engine controls, a high performanceengine just had to have the type of camshaft that rattled thefenders and scared small children. These types of cams are notcompatible with a stock ECM (electronic control module, also knownas a powertrain control module and vehicle control module, dependingon year and model). Consequently, a more conservative approachis required to ensure reasonable idle quality and driveability- while remaining emissions legal.
Emissions legality has become a major consideration in performanceengine building. While acceptable exhaust emissions and high performancemay seem mutually exclusive, they can co-habitate successfullyin the same engine. The key to this harmony is proper camshaftselection and as luck would have it, newer designs are much moreappropriate for current performance requirements.
The best choice is an hydraulic roller camshaft, which is thereason that since 1987, they have been factory installed in anever increasing number of small blocks. Roller profiles are capableof opening valves at a much faster rate and lifting them higherthan a flat tappet cam, and this is precisely the requirementfor not only keeping emissions in check, but for achieving maximumpower while maintaining compatibility with computerized enginecontrols.
The faster opening rate and higher lift translates to more effectiveuse of duration, so cams with comparatively short duration (whichkeeps the computer happy) produce excellent horsepower and torqueover a wide rpm range. The most aggressive production hydraulicroller cam is the one installed in 1994 and later Camaro and CorvetteLT1s. It features intake and exhaust durations of 203 and 208degrees respectively for intake and exhaust (measured at .050"lift) and raises the intake valves .450" and the exhaustvalves .460". Aftermarket performance cam-
shafts with similar duration will have up to .500" lift.These lift specs make checking retainer-to-valve guide clearanceessential. They also require that proper valve springs be selectedso that the possibility of coil bind is eliminated.
Another consideration is camshaft retention. Late model blockswhich were originally equipped for an hydraulic roller cam incorporatea retainer on the front of the block to prevent the cam from walkingforward. When retrofitting an hydraulic roller cam in an olderblock, some form of retainer must be added. Most performance camshaftmanufacturers offer such a component.