Big-block Chevys used both mechanical (solid-lifter) and hydraulic camshafts well into the late 1980s when factory roller cams began to appear. Flattappet hydraulic cams are the bread-andbutter cams of Chevrolet production engines. They are used in all standard Mark IV engines. Solid-lifter mechanical cams were reserved for high-performance applications, but since the late 1980s hydraulic roller cams have taken their place.
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Performance-minded builders almost exclusively turn to the aftermarket for either catalog grinds or purpose-ground cams for specific applications. Hence the only reason to use a factory cam or a restoration cam would be a muscle car (or other) restoration effort or a racing series such as NHRA’s Stock Eliminator. See the chart on page 105, which lists all the primary Mark IV camshafts and more recent factory grinds of note. It’s difficult to find original NOS (new old stock) factory cams and thus major cam manufacturers such as Crane Cams and Comp cams offer replacement cams ground to original specs.
While you might get away with reusing a factory cam in rare circumstances, no self-respecting restorer is likely to try it, although in some cases you can have a cam reground if you absolutely must have an original cam in your engine. The difficulty would be in finding a salvageable cam and verifying that it is indeed the correct grind for your application. With the availability of original-spec replacements it’s just not wise or cost effective to consider a regrind even when you’re determined to duplicate original factory cam timing.
Most big-block cams are interchangeable even across generation lines between Mark IV, Gen V, and Gen VI engine blocks. Two exceptions include cams used in 1965–1966 396 blocks and late-model TBI and PFI engines. Cams for 1965–1966 396 engines have a 3/16-inch-wide by 7/64-inch-deep oiling groove around the rear journal.
They only work in 1965–1967 blocks. If you want to use any other cam in these blocks you must have the groove cut in the rear journal, which most cam manufacturers can do for you.
Cams designed for later TBI and PFI engines do not have a mechanical fuel pump drive lobe and are thus incompatible with engines using a mechanical fuel pump unless an electric fuel pump is substituted. Some cams cannot be used in Gen V and VI applications because of the non-adjustable net lash valvetrain system used on these cylinder heads. Special GM rocker studs (PN 10198929) and nuts (PN 10198930) can be used to allow cams with lifts up .520 inch. Similar components are also available from all major cam manufacturers.
For cams with higher lift, 3/8-inch aftermarket studs are available to convert the heads to a fully adjustable system with standard Mark IV rockers. You can also drill and re-tap the stud bosses for stronger 7/16-inch studs if necessary, but all of this seems a bit much when most builders are likely to have switched to a more compatible cylinder head in the first place. Since roller cams can easily be reused, it’s possible to find yourself in the unique situation of installing a used marine cam designed for reverse rotation or perhaps a cam designed for the late model 8.1L BigBlock engine. The 8.1L cams utilize a different firing order and it is not the same as the popular 4/7 swap used on some aftermarket racing cams. The 4/7 swap is a popular firing order change made to the camshaft. It swaps cylinders 4 and 7 in the firing order, the theory being that it evens out the firing pulses and the forces directed against the crankshaft throws and mains on those cylinders. It typically produces a 7- to 10-hp gain. The benefit is well recognized and cam manufacturers now offer 4/7 swap cams as available grinds in their catalogs.
The following valve lifters and accessories are applicable to all big-blocks. Small-block lifters are identical and can be used if necessary. Note that the major aftermarket camshaft manufacturers offer retrofit kits to install hydraulic roller cams on earlier non-roller Mark IV engines. These kits provide lifters with tie bars necessary for proper lifter alignment. They are straightforward bolt-in kits that bring the benefits of hydraulic roller cams to early-model engines. That includes Comp Cams’ four-pattern cams that provide different timing for outboard and inboard cylinders to compensate for differences in runner length.
Used pushrods are generally a bad idea unless you’ve always owned them and you know their history. Nonetheless it is important to understand big-block pushrod usage to be sure you’re using the correct pushrods for your application. The chart on pages 108 and 109 provides all the factory pushrod details you need to know. Exhaust pushrods are always longer than intake pushrods and the lengths remain pretty standard across the Mark IV lineup, but lengths vary among Gen V and VI engines and tall-deck engines. A few base engines used 5/16-inchdiameter pushrods and matching guideplates, but they’re seldom seen and often discarded. The aftermarket does not support them.
Standard big-block engines come with 3/8-inch-diameter, .080-inch wall pushrods, and guideplates with matching guide slots. Performance engine’s use 7/16-inchdiameter, .080-inch wall pushrods and appropriately matched guideplates. Most are made from 1010 steel except for extra-high-performance versions used on L88s and other top-end engines. These are made from 4230 chrome-moly steel. It’s unlikely that you’ll find many original NOS sets so you’ll be looking for aftermarket pushrods of similar dimensions.
Note that Gen V and VI engines with roller cams use different-length pushrods in both 3/8- and 7/16-inch versions. They also are made from 1010 or 4130 chromemoly steel depending on the application. Tall-deck 572 pushrods are appropriately longer. (See the chart on pages 108 and 109, which lists most of the pushrods shown in Chevrolet Performance catalogs.) The aftermarket offers broad pushrod coverage and the valuable ability to order special lengths once you’ve finished your machine work and established a true exact length for your application.
For most street applications 3/8-inch pushrods almost always provide adequate strength and they are lighter and easier on the valvetrain. If you’re using a cam with more than .600-inch lift and stronger valvesprings it’s a good idea to step up to 7/16-inch pushrods to provide additional insurance against valvetrain damage. Examine the pushrods carefully. Some are made from two pieces with ball ends while others have a hardened end. The hardened end goes up and fits in the guideplate. The other end is not hardened and the guideplate wears on it. Most aftermarket pushrods are fully hardened so this is not a problem.
Also note that stock pushrod guideplates do not fit early Bowtie heads because the raised intake ports and the reinforcement ribs on the aluminum heads interfere. Some builders grind the guideplates or the reinforcement bosses for clearance while others simply raise the guideplate by adding a pair of hardened washers underneath them. The problem with that is a reduction in thread engagement by the rocker arm stud. Aftermarket studs are generally a bit longer if you are considering this modification. This problem was remedied on later versions of these heads.
Chevrolet Performance provides guideplates in 3/8- and 7/16-inch versions. Each uses the same-size rocker stud and all the guideplates are hardened. Guideplates generally don’t wear out so used versions picked up cheaply at a swap meet often serve your purpose well. Inspect them carefully to make certain the guide slots are undamaged and that the mounting holes are not wallowed out. The smaller 3/8-inch ones are particularly easy to find since most performance enthusiasts replace them with 7/16-inch aftermarket versions and leave them lying around the shop. Ask around and you may find a buddy who will just give them to you to get rid of them. If you want new ones choose from a dealer or go directly to the aftermarket through Summit Racing, JEGS, or other retailers. A couple of words about rocker studs: All Mark IV rocker studs are 7/16-inch coarse thread on the bottom. The upper threads are either 3/8 or 7/16 National Fine to accept factory-style nuts or aftermarket locking nuts. Gen V and VI stud bosses are drilled for 3/8 coarse threads and have to be enlarged if you want to use 7/16 studs.
Valvesprings are generally specific to the camshaft being used so it’s best to stick with the cam manufacturer’s recommendations. If you’re using factory heads and a factory-spec camshaft you can use the springs shown in the chart below. Just be sure to check carefully for coil bind and at least .06-inch to retain guide (or seal) clearance at full lift. It is also important to provide a proper location for the spring in the spring pocket. Aluminum heads require steel spring cup inserts to protect the head and you must remember to accommodate their thickness and diameter when selecting and setting up valvesprings.
Used valvesprings are out of the question; how would you identify them accurately anyway? Check the cam specs carefully to see what your application requires. In most cases you’re better off choosing a complete cam and kit from the aftermarket. This allows you to match the correct springs, retainers, locks, and seals for your application.
Big-block rocker arms all have a 1.7:1 ratio. They do not directly interchange between Mark IV, Gen V, and Gen VI versions, which use a non-adjustable net lash system. This was a problem when Genseries engines were first introduced, but changes have been made to allow greater freedom of choice in rocker arm assemblies, depending on the cylinder head you are using. The simple solution for late-model production heads is to discard the net lash system and replace it with standard rockers and screw-in studs. This is a direct swap with 3/8-inch studs. If you want stronger 7/16- inch studs have your machine shop drill and tap the stud bosses for the larger studs. Aftermarket manufacturers also offer a special stud with compatible 3/8-inch threads on the bottom and larger 7/16- inch threads on top. It works well for many applications, but should be avoided if high valvespring pressures or higher rocker ratios are employed. Production rockers still work with cams up to .520- inch lift, but why bother when the aftermarket offers so many affordable choices? If you want to retain factory rockers chose from the following selections.
Chevrolet Performance steel rocker arms are designed for 454- and 502-ci big-blocks. These rocker arm kits include one rocker arm and grooved high-performance ball per unit. The aluminum roller rocker arms include bearings and fulcrums with an extra-wide design for broader load distribution. All rockers have a 1.7:1 ratio and the trunnion mounts require 7/16-inch studs. These high-performance rockers interchange freely on all Mark IV heads and easily fit most aftermarket replacement heads. The roller-tips are ground from 8620 steel and they ride on precision-machined 4130 alloy steel axles. Production valve covers require aftermarket spacers to use these rocker covers.
Early 1965–1966 big-blocks used a 3/4-inch-wide Morse-type timing chain connecting a steel crank gear with aluminum cam drive sprocket sporting nylon teeth to ensure quiet operation. When properly maintained, they held up relatively well, but lack of attention often rewarded owners with broken teeth and potential engine carnage. In 1967 the chain drive was narrowed to 5/8 inch and it continued in service throughout the Mark IV engine’s tenure. A factory double-roller chain incorporated on many Chevy and GMC pickups was found to be durable, but somewhat noisier.
Gen V and VI engines were revised with an even narrower single-roller offering reduced friction and greater durability. Each of these drives is interchangeable on all big-blocks. No sense looking for earlier pieces unless you’re determined to use an NOS drive with nylon teeth to duplicate an original piece. Chevrolet Performance’s timing chains and sprockets offer top performance and dependable service, or you can opt for any of the replacements offered by cam manufacturers. For racing applications, you’ll likely bypass the options and look directly to an aftermarket gear-drive or belt-drive system.
Written by John Baechtel and Posted with Permission of CarTechBooks