Cam timing events are transmitted to the lifter, pushrod, rocker arm, and then the valve. You need strong rocker arms for particular applications, so the valves are precisely actuated. Rocker arms are installed on all overhead-valve engines and transmit lifter movement to the valves. Because the rocker arm is essentially a lever, the length of the rocker (its pushrod contact point to pivot), increases valve lift relative to cam lobe lift. In other words, the rocker’s length provides a ratio-multiplier that magnifies the lift of the cam lobe to create increased valve lift.
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Rocker Arm Ratio
Determining gross valve lift involves considering both the camshaft lobe lift and the rocker arm ratio. The ratio of the rocker arm (length of fulcrum) increases the effective valve lift beyond just the camshaft’s lobe lift. Here’s the formula:
Gross Valve Lift = cam lobe lift x rocker arm ratio
For example, if a camshaft’s intake lobe lift is .365 inch and a rocker arm ratio of 1.6:1 is used, the formula works out like this:
Gross Valve Lift = .365 x 1.6 .584-inch gross lift
If you’re considering changing the rocker arm ratio and you know what the gross lift is and what your current rockerarm ratio is, here’s a simple formula to calculate current lobe lift:
Current Lobe Lift = current gross valve lift ÷ current rocker arm ratio
For example, if your current gross valve lift is .600 inch and you’re currently using 1.5:1 rockers:
Current Lobe Lift = .600 ÷ 1.5
.400-inch lobe lift
Now that you know what the cam lobe lift is, multiply that by the rocker arm ratio that you plan to use. That gives you the new gross lift.
New Gross Valve Lift = lobe lift x new ratio
For example, let’s say that you’re considering moving from a 1.5:1 rocker to a 1.6:1 rocker, using the same camshaft. The formula works out like this:
New Gross Valve Lift = .400 x 1.6
.640-inch new gross lift
As a rule, each ratio increase (in whole numbers) adds about .035 inch of increased valve lift. Changing from 1.5:1 rockers to 1.6:1 rockers increases valve lift by about .035 inch. This provides increased valve lift and quicker valve opening and closing rates. Although this sounds great, before you rush to a higher rocker arm ratio, be aware of the following:
- Increasing rocker arm ratio decreases piston-to-valve clearance, so you need to check this.
- Valvespring retainer-to-valveguide clearance must be checked, as this clearance decreases.
- Check for valvespring bind at full lift.
- Longer pushrods are likely required with high-ratio arms, so make a careful pushrod length determination. You are measuring for precise pushrod length anyway.
As you know, rocker arm ratio refers to the distance between the centerline of the rocker arm pivot to the center of the pushrod cup. The higher the ratio, the shorter this distance becomes. Unfortunately, most published OEM rocker arm ratios cannot be trusted. Due to mass-production tolerances, the listed ratios are theoretical.
For instance, a set of OEM rockers that are listed as having a 1.5:1 ratio might actually involve a range of, say, 1.49 to 1.54:1. For high-precision fulcrum lengths and superior consistency, the performance aftermarket is the way to go. High-quality aftermarket performance rockers are produced with a much higher level of precision and consistency. Increasing rocker arm ratio is a neat way to increase power. You get the effect of a high-lift cam without the need to increase duration. The valve is opened faster, lifted farther off its seat, and closed later.
Stud-mounted rocker arms allow the rockers to pivot around the axis of the stud; shaft-mounted rockers maintain a constant rocker alignment. Studmounted rockers require some type of guiding or alignment to prevent the pushrods from walking around. Depending on the cylinder head design, closer tolerance pushrod slots in the head or steel guideplates take care of this.
If your head requires guideplates, you must use hardened pushrods to prevent pushrod galling. Most guideplates are stamped from steel and may have sharp burrs around the pushrod slots. Be sure to inspect each guideplate and deburr any sharp edges before installing them.
Rocker arms with higher than stock ratios locate the pushrod cup closer to the rocker arm’s central pivot point. During test fitting, closely check for pushrodto-guideplate-slot clearance at maximum lift. Although aftermarket performance guideplates usually have longer slots for this reason, but you still need to check this clearance. If you lengthen (deepen) the pushrod slots, be careful to only make the slots deeper. Don’t oversize their width.
Also check for pushrod clearance in the heads; a high-ratio arm moves the pushrods from the original intended location in the head. If you need additional clearance at the cylinder head’s pushrod slots, you can do this with a hand-held grinder (with the head off the block). Don’t get too carried away. A clearance of around .010 inch is adequate. Removing too much material can be dangerous and may open-up a hole in an intake runner.
When installing guideplates, with the rocker studs finger tightened in place and with rockers and pushrods installed, you should be able to wiggle the guideplate from front to rear (in relation to the engine) in order to center the pushrods in the width of the guideplate slots (guideplate mounting holes usually allow a bit of movement and adjustment). Once the slots are centered to their pushrods, fully tighten the plates (using the rocker studs).
A stock OEM rocker arm (in most cases) rubs on the valve tip and pivots on a ball fulcrum, causing friction. A fullroller rocker arm has a needle bearing trunnion (the pivot) and a roller bearing at the rocker nose that contacts the valve tip. This virtually eliminates frictional losses. Less friction results in quicker response, freer revving, and potentially more horsepower.
Aftermarket roller rockers are also available with a roller bearing at the nose but an OEM-type pivot, and some OEM rockers are available with a needle bearing trunnion pivot but a non-roller tip. A full-roller rocker is a good choice for any application. However, using full-roller rockers becomes more important when dealing with small-diameter valvestems and/or high lift and spring pressures.
If you run 5/16-inch-diameter or smaller valvestems with high lift and high spring pressures, having a roller bearing at the nose dramatically reduces the chance of valve deflection. Also, because of the larger arc imposed on the rocker due to a high-lift cam, roller tips greatly reduce the chance for valve tip damage. If cost isn’t a factor, go to full-roller rockers.
Shaft-mounted rocker designs are available for most applications (depending on cylinder head design). They pivot on a common shaft instead of individually on rocker studs or bolts. The advantage of a shaft mount involves improved valvetrain stability. When shaft mounted, the rockers can’t individually move around the axis of the valve, so rocker-to-valve and rocker-topushrod stability and consistent contact is improved.
A shaft-mount design may have one common shaft that provides the pivot axis for the entire bank of rockers on a cylinder head. Other setups have each cylinder’s pair of rockers (intake and exhaust) pivot on a shaft that holds one pair of rockers (so each pair of cylinder rockers have their own shaft). Especially for high-RPM and heavy-loading applications, shaft mounts offer increased stability because the rockers can only pivot on their shafts and can’t move laterally across the valve tips.
Break-In and Adjustment for Roller Rockers
Granted, there’s no frictional issues with a full-roller rocker at the valve and at the pivot/trunnion, but make sure you apply moly lube to the pushrod-to-rocker contact (to each upper end of the pushrod and to the contact, cup, or ball at the rocker). Rocker manufacturers include a high-pressure lube in the kit or they recommend a specific lube.
It’s highly advisable to soak all roller rockers in a tub of clean, 30-weight engine oil for at least 1/2 hour before installing (I usually soak mine overnight). This allows oil to penetrate into the needle bearings and bearing axles.
Adjust valve lash according to the cam manufacturer’s instructions where solid lifters are used (whether roller or flat-tappet). For hydraulic lifter setups with roller rockers, place the cam on its base circle (where the lifter is at its lowest point) and gently adjust the rocker to achieve zero lash, then tighten the adjuster by 1/2 turn (this provides a small amount of preload to the hydraulic lifter).
Even if you’ve already measured and checked for proper pushrod length, it’s a good idea to perform a final check. With the rocker removed, paint the valvestem tip with a black marker. Install the rocker, adjust lash, and rotate the crank 2 full turns. Remove the rocker and check the witness mark on the valve tip. It should be at the center of the valve. If the mark is biased toward the intake side, the pushrod is too short. If the mark is biased toward the exhaust side, the pushrod is too long.
Rocker Arm Materials
Rocker arms may be stamped steel, ductile iron, investment cast steel, aluminum forgings, extrusions, machined billet aluminum, or powdered metal. These material choices are for OEM as well as performance aftermarket arms.
If a 100-point restoration is the goal, you likely want to use whatever type was originally installed (likely stamped steel, ductile iron, cast aluminum, or steel forgings). If you have a late-model LS engine, you may opt to stick with PM rockers. However, for upgrades with power and durability in mind, you want to reduce friction and increase rocker strength, and you want rockers with correct and consistent arm ratio. This means upgrading to high-quality aftermarket rockers that are made of investment-cast steel, forged steel, forged aluminum, extruded aluminum, or billet aluminum.
Because steel is generally stronger than aluminum, and depending on how radical the build is and the anticipated spring loads and high-RPM use, steel may be a better choice in certain applications. But there are various opinions. The best thing to do is to talk to the rocker arm manufacturer for a recommendation.
Both types of materials are used in high-output racing engines. Sometimes it boils down to race sanctioning body rules or simply on how much you want to spend. From an expense standpoint, from less money to more, ductile iron, cast aluminum, forged aluminum, and steel rockers are your choices.
PM rockers are in common use by many OEMs (the GM LS engines are an example). Although the OEM trunnion/ bearing design is poor (see page 142 for more details), the PM construction is surprisingly good. Generally, PM rockers are okay up to about 400 hp. Beyond that, I don’t even consider them.
Overhead cam (OHC) engines don’t have pushrods that connect lifter movement to rocker arms. Instead, cam “followers” provide a direct leverage connection from the valve tips to the lifters or cam lobes to the valve tips (depending on design). For a high-performance or race application, aftermarket upgraded followers, which have roller contact (instead of friction/sliding contact) at both the valves and lifters, are available. Just as in roller rocker upgrades on overhead valve (OHV) applications, roller cam followers greatly reduce frictional losses and allow for higher and more consistent engine revs.
Valve Cover Clearance
Nice big, beefy aluminum roller rockers may provide the performance and durability that you desire, but remember that dimensionally larger rockers may not clear OEM valve covers. Since most custom engine builds are likely fitted with aftermarket valve covers (which are readily available in taller heights), this really isn’t an obstacle, but consider this clearance in order to decide if you need taller covers.
A simple way to determine this is to place a cover onto the head, with no bolts. While a buddy slowly rotates the crank by hand, place your hand on top of the valve cover. If rockers begin to contact the cover, you can feel the cover bump up. If so, you need taller valve covers. Before you invest in a pair of spiffy valve covers, test rocker clearance using a spare cover with the same height of the ones that you’re considering.
Upgrading OEM LS Rocker Arm Trunnions
The GM Generation-3 and -4 LS engine platform provides a quantum leap beyond the earlier small-block Chevy engine. Overall, it’s a great design with plenty of horsepower potential and good durability. However, a notable weakness involves the rocker arm. The majority of OEM LS-platform engines have PM rocker arms, which are surprisingly strong and well suited for street operation. The downside involves the trunnion and needle bearing system.
Under high loads and high engine speeds, the cageless bearing outer housings can work their way out of their bores, allowing the needle bearings to work their way out of the bore and scatter into the engine. There’s no guarantee that this failure will occur, but especially with an upgrade to a high-lift cam and stiffer springs, the potential for the failure increases. If you find one or more mystery needle bearing during a routine oil change, it is clear evidence that at least one rocker arm is losing its trunnion bearings.
If you plan to retain the factory PM rockers, a simple and inexpensive fix is to perform a trunnion bearing upgrade using a readily available kit.
Comp Cams, as an example, offers a high-quality trunnion bearing upgrade kit (PN 13702-KIT). This kit, applicable to LS1/LS2/LS3/LS6/LS7 OEM rockers (as well as iron-block versions such as LQ9, etc.), includes new trunnions, new caged and prelubed bearing assemblies, circlips, and rocker arm mounting sockethead cap screws (along with two thick, installation-assist washers). The new trunnions have extended tips with circlip grooves. Instead of the bearing housings relying on a press fit, the additional circlips serve to prevent possible bearing assembly walk-out.
These bearings aren’t going anywhere, so the rocker bearing issue is no longer a concern. The new trunnions and bearings are also made of high-quality materials and are designed to withstand high RPM and high spring pressures. An upgrade kit replaces the cageless OEM needle bearing assemblies and PM trunnions with very high strength, premium 8620 steel-alloy trunnions and fully caged roller bearings for vastly improved durability.
The trunnion-bearing swap procedure can be handled using a bench vise or hydraulic press with appropriate-size inserts or with a specialty tool designed for use with a bench vise. An example is Summit Racing’s tool (P/N SME-906011), which makes this job easy.
Trunnion Upgrade Procedure
Before beginning, clean the rockers with solvent or in a hot tank. After removing the original bearings and trunnions, you have the option of additional surface finishing, by having the rockers tumbled in a media tumbler or by a light blasting with glass bead, soda, walnut shell, etc. Avoid dwelling the blasting pressure in the trunnion bores, valve contact tips, or pushrod cups, and do not run a reamer through the trunnion bores. Do not sand blast; it is far too aggressive.
Place the tool vise fixture onto one jaw of a bench vise. Place the OEM rocker into the fixture’s V-saddle.
Align the (black) presser to the OEM bearing, with the counterbored end of the presser facing the bearing. Tighten the bench vise to press-out both OEM bearings and the trunnion (needles fall out). Inspect and clean the rockers as needed.
Place the presser onto the short end of the Summit kit’s mandrel. Slide a new bearing onto the long end of the mandrel (make sure that the stamped symbols on the bearing faces the mandrel’s flange). Slip the aluminum alignment sleeve onto the long end of the mandrel, seated against the bearing.
Slide the mandrel assembly into the rocker arm bearing bore (the aluminum alignment sleeve enters the bore first to align to the rocker body).
Using a bench vise, press the bearing into the rocker body until the mandrel’s flange bottoms-out onto the rocker body. Perform this single-bearing installation to all 16 rockers before proceeding with trunnion and opposite-side bearing installation.
Place the rocker arm body onto the Summit vise fixture, with the previously installed bearing facing the fixture.
Slide the new trunnion halfway into the bearing bore (leaving the outside end of the trunnion exposed to align the remaining bearing). Apply engine oil to the outside of the bearing. Slip the remaining bearing onto the exposed trunnion (make sure that the stamped symbols on the bearing cage face outward).
Place the kit’s thick steel washer onto the end of the trunnion (against the bearing face). Tighten the bench vise until the washer bottoms-out against the rocker body
Verify that the trunnion rotates freely, and that a slight amount of endplay (side to side) exists, in the range of .004 to .008 inch. If you don’t have any endplay, adjust the bearing depth accordingly. Using snap-ring pliers, install one circlip to each end of the trunnion. Verify that the clip is fully seated in the trunnion clip groove.
Install the upgraded rocker arms to the cylinder heads using the new socket head cap screws supplied in the Comp Cams kit. Be sure to rotate the trunnion so that the flat recess side of the trunnion’s bolt hole faces upward (the socket head cap screw must seat against this recessed flat side of the hole). Tighten the cap screw to 22 ft-lbs, following the OEM installation procedure.
Install the new upgrade kit using a press or rocker arm vise tool. It’s critical to smoothly draw the OEM bearings out and draw the new bearings in. Do not try to bang the old trunnions out or attempt to install the new bearings with a hammer.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks