Engine Assembly Basics -- Cam, Valvetrain, Top-End

In the last issue of Engine Masters, we set out to start a series of articles that gets right to the core of engine building by showing the tricks, techniques and tools required to successfully build an engine. The aim is to arm the reader who dreams about putting together a high performance powerplant with the information needed to get it done. How? The goal is to answer the question of "How to do it" as much as "What to do." We covered what goes into building a short-block, and how it is done, in the last issue. This time around, we turn our attention to the cam, valvetrain, and top-end.

Really, this is a pretty broad subject, and the details vary depending upon the caliber of the build, and the equipment being used. The idea is to focus on the ideas that are universal to any engine buildup. Some of the detail may be more than you need to know for a very basic assembly, but knowing more is never a bad thing. We encourage feedback from our readers, so if you have an area of engine building you'd like explored in detail, contact me at steve.dulcich@sorc.com.

Install and Degree the Cam
At the most basic level, only a few things need to be ensured while installing a camshaft. The camshaft must be clean and properly lubed, and once in, it must spin freely in the block. The lube used will depend on the type of cam. Flat-tappet camshafts require a special high-load break-in lube on the lobes, which is usually packaged with the camshaft. The bearing journals should be lubed with an assembly oil or regular motor oil. Roller camshafts, either mechanical or hydraulic, normally do not need special cam lube on the lobes, and can be installed with oil on the lobes and journals.

Installing the camshaft can be as simple as sliding the stick into the bore, and making sure the timing dots are in alignment at TDC when the timing set is bolted in place. Most mild to stock rebuilds, and a surprising number of high performance engines, are put together this way. Taking such an approach is like flying blind, since the camshaft's installed centerline will not be known unless it is actually checked by the process commonly referred to as "degreeing-in." Unfortunately, inaccurately degreeing-in the camshaft is one of the most common engine assembly blunders, often making matters worse than just lining up the dots. If the cam is going to be degreed-in, it has to be done with the utmost in accuracy. Tools for degreeing a cam are relatively inexpensive; degree wheels can be purchased from most cam manufacturers, while some such as COMP Cams offer kits with everything you'll need.

Lifters Logistics
Lifters basically come in four types: flat-tappet and roller, available in solid or hydraulic versions depending on the camshaft type and design. Always use the appropriate lifters for the type of camshaft being used. Flat-tappet lifters must rotate in the bores, while rollers must maintain their alignment to keep the roller wheel in line with the lobe and avoid destruction. Typically, solid rollers and retrofit hydraulic rollers use a link bar arrangement to pair up the lifters and hold them in alignment. OEM hydraulic rollers will typically use alignment yokes and a spring-steel spider for alignment. The essentials for lifter installation are cleanliness, proper lubrication and free movement.

Valvetrain Variables
Setting up the valvetrain can be one of the most complex and time-consuming aspects of a high-end engine build. javascript:preview_story_session()Often, professional engine builders will need to extensively blueprint the valvetrain geometry in the course of a custom buildup. Within the scope of this story, we are addressing the more basic considerations, dealing with traditional valvetrains. OEM valvetrain layouts came in a variety of styles, including stud-mounted, shaft-mounted, and pedestal valve-gear. A variety of aftermarket systems cross over from the OEM configuration to an entirely different style of valvetrain. With such a wide variety of systems the permutations are nearly endless. The key rules are to use the correct length pushrods, ensure proper motion and clearance, and maintain as close to central contact on the valve tip as possible with a minimum of lateral sweep. There are three relevant geometric points in a valvetrain, the position of the rotational center of the rocker, and the contact points where the rocker picks up the pushrod and valve tip.

In a stud-mounted system, two of these variables depend on pushrod length, since the actual pivot point of the rocker will change with the pushrod length. With stud-mounted rockers, the pushrod length will affect the position of the rocker pad or roller across the valve tip. In a shaft or fixed-pedestal system, the valve tip position, as well as the rocker's pivot point, is fixed. With these systems, the geometry is essentially set by the rocker design and the valve length. Pushrod length in these installations comes down to achieving the required range or position in the rocker's adjuster. In custom installations, the pushrod length will often need to be determined by mocking up the valvetrain with checking pushrods, available from Powerhouse Tools and others.

Some Things About Springs
As the seriousness of the performance effort increases, the more critical valvesprings become. In lower rpm performance engines, which may never see over 5,500 rpm, the most basic level of care in valvespring selection and installation will generally get you by. As rpms increase, springs become one of the most critical factors in wringing the performance potential from a combination. Things to become familiar with are the seat load, installed height, open load, spring rate, and coil bind clearance. Let's take them one at a time.The seat load is the amount of force the spring exerts with the valve closed. This affects the valve's potential to bounce on closing, which can cost serious power at high rpms. Springs are rated for load at a given height, referred to as the installed height. The more a spring is compressed, the greater the force, so the installed height directly affects the seat load; it's critical to know the installed height to have known amount of seat load.

Open load is the amount of force exerted by the spring when the cam is at peak lift, which is the point when the spring is compressed the furthest. The open load has to be enough to keep the lifter in contact with the cam as it goes "over the nose," yet not so high that it is beyond what the cam, lifters, and valvetrain can withstand for acceptable wear.

Spring rate is a rating on how "stiff" the spring is, or how much load is gained as the spring compresses, normally given in pounds per inch. A spring rated at 350 lbs/inch will gain 350 lbs for a theoretical inch of compression. How much the spring will gain for any amount of valve lift can be calculated by multiplying the lift by the spring rate. A 0.700-inch lift cam with a 600-lbs/inch rate spring will gain 0.700-in x 600 lbs/inch for a gain of 420 lbs. Add the amount gained to the seat load, and the open load is the result. For instance, if in our example the seat load was 200 lbs, gaining 420 lbs in opening will give 620 lbs open. Most spring suppliers publish spring rate specs. The coil bind height is the height of the spring when it's compressed down solid. Most spring manufacturers give specs on coil bind height. By measuring the actual installed height of a spring on a cylinder head, and subtracting the amount of valve lift, the compressed height of the spring at full lift can be calculated. Comparing this figure to the published coil bind height will give the clearance to coil bind. As an example, let's say we have a spring with an installed height of 1.800 inches. If the valve lift is .600 inch, the spring is compressed to 1.200 inches at full lift (1.800 in. - 0.600 in. = 1.200 in.). If the manufacturer's catalog lists the coil bind height at 1.100 inches, it's safe to say there is 0.100-inch coil bind clearance left (1.200 in. - 1.100 in. = 0.100 in.).

How much spring load is the right amount? It depends upon the radicalness of the cam profile; the type of cam - solid, hydraulic, or roller; the valvetrain weight; and the engine's rpm range. Typically, top experienced engine builders spec their own springs, but for the average enthusiast, it's best left to the cam manufacturer.

Critical Clearance: V-To-P
One of the most important checks to make when building an engine is the valve-to-piston clearance. As cam duration and compression ratios go up, this figure becomes more critical. Some make the mistake of presuming that the valve-to-piston clearance is a function of the cam's lift specification, but in reality it is more related to the duration and lobe separation angle. The piston is well out of reach of the valve by the time the valve reaches maximum lift, usually at around 102-112 degrees after TDC. The critical factor is what's happening during overlap, when the piston is very near TDC, the exhaust valve is closing, and the intake valve is opening. The exhaust valve will normally get closest to the piston somewhere in the vicinity of 15 to 5 degrees before TDC, while the intake valve will be closest in about the same range after TDC.The most common methods of checking valve-to-piston clearance are directly reading the travel to contact with a dial indicator or making a check using clay on the piston tops. Both will provide a good indication. The dial indicator system will give a more precise number, but does not reveal potential radial clearance problems of the valve's edge to the position of the valve pocket machining. The clay system will show exactly where the valve is in relation to the piston and valve notches, but it is more difficult to get an exact clearance number.

Bolting It Together
By now the short-block should have the cam installed and degreed, and the cylinder heads should be standing by fully assembled, with the compatibility and clearance of all of the valvetrain components verified. The piston-to-head clearance should have been verified during the short-block mock-up stage, and the appropriate head gaskets sourced for the desired quench clearance. All that is left is to bolt together the final long block by installing the cylinder heads, valvetrain, and intake manifold. These procedures are relatively straightforward if all of the checks have been made beforehand.

Livin' For Lash
What separates a hydraulic lifter from a solid is the addition of an internal hydraulically operated plunger within the hydraulic lifter's body. The way the valvetrain is set up will depend upon whether the lifters are hydraulic or solid. Hydraulics generally run with pre-load, while solids must be set up with lash. Let's first consider a hydraulic. It's helpful to understand how the hydraulic mechanism works and what it does. Oil pressure enters the lifter through an orifice in the lifter body, and flows through another orifice into the hollow body of the lifter plunger. A one way check valve at the bottom of the plunger allows oil to fill the cavity below until all the valvetrain clearance is gone, effectuating the hydraulic self-adjustment to zero lash. When the cam rotates into the lift cycle, the check valve at the base of the plunger closes under the pressure imparted by the valve spring, preventing the oil from being squeezed back out as the valve opens. At the top of some hydraulic lifter plungers is a metering valve or plate, which supplies oil to the pushrods for valvetrain oiling.With the valvetrain installed (or adjusted), the pushrod compresses the plunger within its range of travel, and the hydraulic mechanism automatically zeros the lash. How far down the lifter plunger has been displaced at its base setting is called the lifter pre-load. The recommended pre-load with hydraulic lifters is usually in the range of 0.020 to 0.040 inch. Many stock valve-trains are non-adjustable, although many parts and processes involved in an engine build can alter the factory pre-load. In these cases, the solution is custom-length pushrods, or making the change to adjustable rockers.With adjustable rockers, setting the pre-load is simply a matter of setting the lobe being adjusted to the base circle, and tightening the adjuster until the clearance in the valvetrain is just taken out (zero clearance). Then turn the adjuster in 1/2 to 3/4 of a turn and lock the adjuster down.Solid lifters have no self-adjusting hydraulic mechanism, and need to run with clearance in the valvetrain. The lash specification is given on the cam card for a solid cam. The adjustment is made with the lobe being adjusted set on the base circle. A feeler gauge of a thickness matching the lash spec is inserted between the rocker and the valve tip, and then the adjuster is taken to zero lash and locked down. When the feeler gauge is removed, the lash will be set at the thickness of the feeler gauge. Lash is usually set cold when the engine is built, and then re-adjusted once the cam is run-in with the engine hot. The hot setting will be more true to the engine conditions in operation.