The rocker arm is a fulcrum. Its job is to turn the cam lobe's circular rotation into an up and down movement. This is able to be done because the rocker has a pivot point or fulcrum that allows the cam lobe to open the valve.
 Rocker arm ratio
The rocker arm's pivot point is not in the center of the rocker; instead it is offset more towards the pushrod. The amount of offset is expressed as a ratio (1.5, 1.6, 1.7, etc.).
The rocker arm ratio multiplied by the cam lobe lift equals the net valve lift.
The stock small-block Chevy rocker arm has a 1.5:1 ratio. These rockers are called 1.5 ratio rockers or just 1.5 rockers for short. This means that the rocker arm tip at the valve end of the rocker moves 1.5 times the amount of movement seen at the pushrod end of the rocker- which is also the same amount as the camshaft's lobe lift.
Let's say your camshaft has 0.350" lobe lift. With a 1.5 rocker, the cam's lobe lift becomes 0.525" lift at the valve (0.350 x 1.5 = 0.525).
The same 0.350" cam lobe using 1.6 rockers would give you 0.560" lift (0.350 x 1.6 = 0.560), w/o having to remove and replace the cam. With the Chevy factory hydraulic roller cam and lifter set up, this will give you more lift before running into problems w/the dogbone retainers losing contact w/the stock lifters.
Changing the rocker ratio will also change the off-the-seat duration seen at the valve a small amount. However, seat-to-seat duration will not change.
 Types or rockers
There are several different materials rocker arms may be made of: hardened steel, stainless steel, aluminum- both extruded and cast. There are several different designs: pedestal mount, stud mount, shaft mount. There are different pivot point: Ball and stud (like a SBC), shaft, or a roller trunnion. There are different tips: Self aligning, non self aligning, roller (also in SA and non SA designs).
 Stamped steel
A mainstay of OEM engines. They're cheap to produce and perform adequately, although in serious competition and high performance applications they leave a lot to be desired. The ratio is often less than advertised, the ball pivot like used by Chevy and Pontiac are prone to wear and friction.
 Roller tip
The roller tip only rockers are mostly a gimmick. Their only real advantage comes not from the roller tip (which by one estimate would need to be a 2" diameter roller to duplicate a stamped steel rocker), but from the more accurate ratio; stock stamped steel rockers often have less than advertised ratios and this means less than advertised lift, and a good chance of less peak performance.
That said, the 1.52:1 advertised roller tip rockers are a lot more uniform and they can increase performance, primarily due to the increase in ratio- NOT because of the roller tip.
 Roller trunnion
 Full roller
 Self aligning vs. non self aligning
 Installing screw in rocker arm studs
When screw in studs are going to be installed, the stud boss will usually need to be machined shorter to make up for the thickness of the guide plate (if used) and for the hex portion of the stud and the radius between the hex and the stud. In the case of the early style SBC, 0.400" is usually a safe amount to remove, but this can vary some depending on the exact parts used.
 Rocker arm adjustment nuts
Polylocs are the preferred method to hold the rocker arm adjustment. The stud needs to have a flat or even surface for the center locking allen nut to tighten against.
If you're using stock lock nuts, they use a disrupted thread on the top side of the nut (arrows, right), so getting the nut to start on the stud is nearly- if not completely- impossible, unless the nut is correctly orientated on the stud (flat side down, as said). Replace these lock nuts if they've been used much; the self locking function loosens w/use.
Self locking rocker nuts can do a number on aftermarket studs, so use polylocs when possible. In some cases polylocs cause clearance problems w/the valve cover so check first. Using a thick or doubled gasket will sometimes give enough clearance. Using polylocs on stock studs will not always work- the polylocs need an even surface for the set screw to register on, often stock studs are uneven on the top.
 Converting a non-adjustable valve train
Old and new engines have both used net lash or non adjustable rocker arm arrangements. The latest Chevy engines are now among these; for years the Chevy V8 enjoyed the benefits of an adjustable stud-and-rocker setup but unfortunately that's gone by the wayside.
The aftermarket has seen to it that there's the wherewithal available to get adjustability back into these engines.
 LS series engines
The LS series Chevy engines are becoming the new "go-to" swap engine, rapidly gaining ground on the original Gen 1 SBC. One stumbling block to doing cam swaps and modifying these engines for high performance use is the non adjustability of the net lash system now used by Chevy.
The original rocker arms have a roller trunnion and a non roller tip (shown above at Roller trunnion). They're not a bad rocker arm, in fact there are kits to allow them to be used in an adjustable setup, and there are replacement roller trunnion/bearings for them that allow the stock rocker to be used more reliably in high performance applications.
If an aftermarket roller trunnion isn't enough, Crane (right) and others have made rocker arm/adjustable valve train conversion parts available.
 Lewis tool