At 6,000 rpm, valves slam again their seats 3,000 times per minute. Exhaust valves not only reciprocate vigorously at half the speed of the crankshaft, they’re subjected to combustion temperatures of approximately 1,800 to 2,000 degrees F. Lifters, pushrods, rocker arms, and valvesprings take a like amount of punishment without the levels of heat valves experience. Of all your engine’s components, valvetrain components are the most likely to fail. Many a broken valvespring or failed keeper has brought down the mightiest of engines. This is why your attention to this area is vital.
A valvetrain’s greatest ally is stability. Valvetrain systems must have matched components for stable operation. This is why camshaft manufacturers have increasingly gone to camshaft kits in recent years to help you choose the right combination of components. Camshaft companies make it easy to package your valvetrain system. Packaging a valvetrain depends on how you want your engine to perform.
This Tech Tip is From the Full Book, FORD 351 CLEVELAND ENGINES: HOW TO BUILD FOR MAX PERFORMANCE. For a comprehensive guide on this entire subject you can visit this link:
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If you’re building a street engine that will be operating in the 2,500- to 5,500- rpm range, camshaft specifications need to have conservative torque because you want the engine to make good low-end torque for the street.
Lifters, pushrods, rocker arms, and springs need to follow suit. Run a spring that is too soft for your camshaft profile and your engine experiences valve fl oat (not enough spring pressure to close the valves quickly) at high RPM. Likewise, run a spring that’s too stiff and you can wipe the cam lobes from excessive pressure against the lobe. This is why running matched components is so important.
Rocker Arms and Pushrods
The rocker arm and pushrod transfer the cam lobe’s energy to the valvestem. Think of the rocker arm as the camshaft’s messenger because the rocker arm multiplies lift, which makes the valve open farther than the camshaft’s lobe lift. Rocker arm types range from stock cast affairs all the way up to extruded and forged pieces with roller bearings and tips. Forged or extruded roller rocker arms are quite costly, which generally leaves them out of a budget engine program. However, this doesn’t mean you have to settle for stock cast or stamped-steel pieces either.
Stock cast or stamped-steel rocker arms don’t perform well under the heavy demands of radical camshaft profiles. An aggressive camshaft profile breaks a stock rocker arm. This is why it is always best to err toward heavy-duty whenever you’re building an engine. Stamped-steel, ballstud, roller-tip rocker arms are a good first step toward valvetrain durability whenever you opt for an aggressive camshaft. The roller tip reduces the stress you experience with stock rocker arms. The thing is, when you increase lift and valvespring pressure, a stamped-steel or cast roller-tip rocker arm doesn’t always stand up to the test, especially when spring pressures climb to more than 350 pounds. Even the best stamped-steel, roller-tip rocker arm will fail when over stressed.
When lift and spring pressures go skyward, you want a roller-pivot, roller-tip, forged rocker arm for your budget engine build. Going that extra mile with a super-durable rocker arm ensures longer engine life, especially if you’re going to drive it daily. For the weekend racer, stepping up to a better rocker arm is like writing a life insurance policy because marginal rocker arms do not stand up to the high-revving task. Rollerpivot, roller-tip rocker arms also ensure valvetrain precision and accuracy when the revs get high.
Crane Cams, Comp Cams, or Ford Racing are good sources for rocker arms and pushrods. These companies all have a lot of valuable experience with valvetrain components and offer wide selections. Always run the same brand of rocker arm and camshaft.
When it comes to valvetrain adjustment, Clevelands have flexibility in adjustable aftermarket studs where adjustable studs were not originally used. Early small-block Fords (1962–1967) had adjustable, ball/stud-mounted rocker arms. From 1968 to 1977, they received no-adjust, positive-stop rocker arm studs which are undesirable for the performance buff. From 1978 to present, the rocker arm stud was replaced with a newdesign, stamped-steel rocker arm, fulcrum, and bolt that mount atop a boss. Clevelands differ from other small-block Fords in that they never had a studmounted rocker on common examples. Only the Boss 351 and 351 High Output had stud-mounted adjustable rockers.
The 335-series small-block engine family (351C, 351M, 400M) had two types of rocker arm arrangements from the factory. The Boss 351 (1971) and 351 High Output (1972) had adjustable, stud-mounted rocker arms. The rest had bolt-fulcrum style, no-adjust, stampedsteel rocker arms as found on small-block Fords 1978-up.
If you’re building a Cleveland wiith a bolt-fulcrum rocker, there is no valvelash adjustment. However, bolt-fulcrum rocker arms sometimes require pushrods of varying lengths to get into proper adjustment. If you have a noisy rocker arm, confirm lifter status before doing anything else. A collapsed lifter causes excessive rocker arm noise. According to Ford you are allowed 5 to 55 seconds of lifter leakdown time. A damaged or excessively worn rocker arm can make noise, as does a loose fulcrum-retaining bolt. Ford suggests a .060-inch-longer pushrod to help take up excessive clearance.
Many go with an adjustable studmounted rocker arm with mechanicalor hydraulic tappets. There’s endless discussion about how to adjust valve lash on hydraulic-lifter Clevelands. However, adjustment is nothing more than common sense. Hydraulic-lifter pistons have a very limited amount of travel or preload: .020 to .060 inch. On top, that means approximately 1/4 to 3/4 turn at the rocker arm. When adjusting valve lash, you want the lifters smack on the cam lobes’ heels (valves closed) on compression stroke. But honestly, cam manufacturers offer even more precise advice: Slowly turn the crank until each valve closes completely, then, make your adjustment.
Turn the pushrod with your fingertips while tightening the rocker arm adjustment nut. When the pushrod becomes ever so slightly resistant to your fingertips, turn the adjustment nut 1/4 to 1/2 turn. Though Ford suggests 3/4 turn with some applications, this is too much. If you’re running poly locks, tighten the Allen screw lock. Do this in the engine’s firing order, one cylinder at a time.
You don’t know if you are successful until you fi re the engine and it is at operating temperature. If there’s significant rocker arm noise, there’s too much lash and you need to go a little tighter. Some aftermarket rocker arms, such as the Comp Cams Pro Magnum or Ultra Pro Magnum, make a soft clicking sound, which makes a Cleveland sound more like it has mechanical tappets, but is of no consequence.
With roller or fl at mechanical tappets, valve-lash adjustment is simple. As with hydraulic tappets, follow the firing order with both valves just closed. Valve lash between rocker arm tip and valvestem is .022-inch cold for both intake and exhaust. When you start the engine, you should hear uniform rocker arm chatter. Any loud clicking is excessive valve lash.
Proper valve adjustment is crucial to both performance and durability. A valve that doesn’t seat from over-tightening ultimately burns and fails, not to mention misfires and rough operation. Valves need contact with the seat not only for the obvious (compression) but also for heat transfer to the seat and water jacket. The .022 inch the rocker arm has at the valvestem gives you the necessary allowance for safe operation.
Proper pushrod length is a very serious consideration for any engine builder. It can mean the difference between long engine life and having to pull heads in a few thousand miles. A pushrod that’s too long pushes the rocker arm tip undercenter, causing excessive side loading to the valvestem and guide. Likewise, a pushrod that’s too short does the same thing on the opposite side. A pushrod checker (found at your favorite speed shop) helps you make the right decision for not much money.
When checking pushrod length, the rocker arm tip should be close to center on the valvestem tip and dead-center with the valve fully open. Remember, the rocker arm tip is going to walk across the valvestem tip when you come up on the high side of the cam lobe.
Take a black felt-tip marker and darken the valvestem tip. Then install the rocker arm and pushrod. Hand crank the engine and watch the valve pass through one full opening and closing. Get down along side the rocker arm and valvespring and watch the rocker arm travel. Then inspect the black marking for a wear pattern. This shows you exactly where the rocker arm tip has traveled across the valvestem tip.
The pattern should be centered on the valvestem tip. If it runs too much toward the outside of the valvestem tip, the pushrod is too short. If it runs toward the inside of the valvestem tip, the pushrod is too long.
Valves and Springs
Valves and springs play an important role in power and reliability. Weak spots in either area can rob you of power or lead to engine failure. This is why choosing the right valves and springs is so important. Most cam manufacturers offer a variety of valvespring combinations designed to work well with the camshaft you have chosen. In fact the best way to shop for and buy a cam is to purchase a camshaft kit, which includes valvesprings, retainers, and keepers matched to your camshaft profile. You match a cam and springs because you want a compatible spring for the profile. More radical cams call for stiffer springs. Milder camshaft grinds need less valvespring pressure. Too much spring pressure can wipe the cam lobes. Too little can cause valve fl oat (valve seating doesn’t keep up with the revs) at high revs.
Choosing the right valvespring is strictly a matter of following a camshaft grinder’s recommendations. Most springs are applicable to hydraulic or mechanical lifters. Some are specific only to roller camshafts. Crane Cams, for example, offers dozens of different valvespring types.
When you opt for a camshaft kit, there’s comfort in knowing the manufacturer has matched the springs to the camshaft profile. Most of the homework has been done for you. There won’t be concern about coil binding, or too much or too little spring pressure.
The best time to check for coil bind is when you’re degreeing the camshaft. Do this before permanently bolting on the heads. Most cam grinders tell you the recommended installed spring height and seat pressure. Correct installed height and spring pressure are achieved with the use of shims as necessary. Coil bind is checked by measuring coil spacing with the valve at maximum lift. There should be no less than .060 inch between the coils at full valve lift. Retainer to valve guide clearance at full lift is the same; no less than .060 inch. This clearance is vital because coil bind or retainer contact with the head causes valvetrain failure. The .060 inch you give it allows for thermal expansion of metal parts and any camshaft aggressiveness and spring movement at high revs.
Another consideration is piston-to valve-clearance whenever you’re installing a camshaft with greater lift and duration. The most common practice is to press modeling clay into the piston valve reliefs, then temporarily install the head and valvetrain, then turn the crank two complete revolutions. If you feel any resistance, back off and remove the head. Chances are at this point you have valveto- piston contact. Forcing the crank bends the valve and/or damages the piston. If no resistance is felt in two full turns of the crankshaft, remove the head and examine the clay. Slice the clay at the valve relief and check the thickness of the clay. This is your valve to piston clearance, which should be no less than .060 inch.
Written by George Reid and Republished with Permission of CarTech Inc