Valve Spring Installation - The Rights Of Spring Time

The valvespring appears to be no more complicated than the spring that swings your screen door shut behind you. Granted, the concept is pretty simple, but in the context of the engine's valvetrain, you'd better pay attention to the valvesprings' fine points or they'll bite you a lot harder than that screen door smacking your backside.

Getting Sprung
Starting at the beginning, the spring selection has to match the application. This means that the spring's rate and installed load has to be matched to that required by the chosen camshaft. A light valvetrain or lower rpm will reduce the spring requirement, while a heavyweight valvetrain or more rpm will require more spring with the same camshaft. What we're after here is the spring's ability to control the motion of the valve and its actuation mechanism, while not overdoing the spring load to the point that the life of the camshaft and lifters is unduly compromised. You have to strike a balance. Unless you've garnered a wealth of experience in selecting valvesprings through trial and error, it's best to consult the camshaft manufacturers' recommendations, either through their catalog or on their tech line if you're running an unusual combination or have special requirements.

The minimum information you will need is the installed height at which the springs will be run, and the camshaft and valvetrain components used. For stock or mildly modified engines, most manufacturers have already done the homework. They have specifications on the stock installed height and a recommended spring to suit a given cam. If your engine's been modified with longer valves, different retainers, or machined spring seats on the heads, etc., these changes have to be taken into account when the spring selection is made. A valvespring micrometer makes checking the installed height easy and precise.

Commonly, street performance springs with cams of just over .500-inch lift will be a single spring with a flat wound metal damper. The factory used this type of spring in performance applications. The spring with damper is commonly mistaken for a dual valve spring, but it isn't. The damper works as a friction brake, helping to control spring surge through the friction between the main spring and damper. As spring requirements increase, more load is needed. There are only two ways to do it within the constraints of a given working height: Use a thicker spring wire or a higher quality material. Thicker means heavier, which causes problems in valvesprings, since one of the main masses the spring has to control at high rpm is its own. Further, the thicker the wire, the less clearance to coil bind, meaning that you can only go so thick before you preclude the use of high cam lifts; ironically, the main reason we need a thicker spring in the first place. Trick springs are available in high-tech materials, which can provide a higher spring load from the same wire diameter as a conventional spring, but the most common and economical approach is to go with a dual or even triple valve spring.

Dual valve springs simply add another smaller diameter spring within the outer, typically with a surge damper in between. Triples, once popular in very fast classes of drag racing, add a third, smaller, spring inside the second. With multiple springs, special retainers are matched which have register steps to properly locate each of the springs as installed. Multiple spring assemblies have a much smaller I.D. than a single spring, requiring the spring seat in the head to be machined to match. Also, since conventional umbrella-type valve stem seals don't fit within the tighter confines of the multiple spring, the guide boss is typically machined to .500 inch or .530 inch to take the more compact positive P/C-type Teflon seal.

Note, however, that P/C-type Teflon seals may be too efficient at scraping oil on the exhaust side, which never sees the engine vacuum that tends to draw in oil past the seal. For this reason, the seal is often left off on the exhaust guide, primarily to minimize the risk of running the exhaust stem too dry and seizing the valve under high load. The penalty is a puff of smoke at start-up.

Related to the valvesprings is the retainer-to-guide clearance. With modest valve lifts this is not an issue, but with high lift, the retainer can actually crash into the top of the guide boss, or at least squash the valve stem seal. When you're in the range of lifts where this becomes an issue, you'll probably be machining the guide boss and spring seat for a dual spring and positive seal. Check for retainer clearance by installing a valve with a checking spring, then rig a dial indicator to determine how far open the valve can be pushed before the retainer makes contact. Mopar Performance recommends a minimum clearance of .050 inch. Make an allowance for the thickness of the stem seal used, as they will take up a portion of the clearance space. The guide boss can easily be shortened by most machine shops when the related machining is done for dual springs or positive valve seals.

Measure Up
Getting a handle on the installed height is easy during the assembly phase of the heads. Use a height micrometer, which gives a highly accurate reading. The machine work on the valves and seats must be completed first, since this will affect the final position of the valve. The other components can also alter the installed height, so the check should be made with the retainers and keepers that will be used in the finished engine. In fact, the original steel Mopar retainers themselves typically vary by .015-.020 inches as measured where they sit from the highest to the lowest, depending on minute variations of the factory machining.

This variation is not highly critical in mild applications, but in more demanding applications, it doesn't hurt to check each retainer and select a consistent set-if you've got a bin of the stockers to choose from. Generally, quality aftermarket retainers are more accurate, but there's no guarantee that every valve seat was machined to exactly the same height, or even that the factory spring seats are consistent in height. Also, the measured installed height will generally grow .010-.020 inch after the engine is run, as the keepers wedge up tighter to the valve stem and retainer. This is more the case with stock-angle 7-degree retainers and locks, and less of an issue with aftermarket 10-degree locks such as Comp Cam's Super Locks and matching 10-degree retainers.

The tolerances can stack up against you if the installed height is checked on only one of the valves. Say one valve seat is machined slightly lower, adding .010 inch, a spring seat dug a little deeper at the factory adding .015 inch, and a looser retainer adding another .010 inch. Now let's say the one spring you checked was at 1.875 inches instead of the recommended 1.850 inches, and you figure that's close enough. That one unlucky valvespring could well be installed .060 inch too high going in, and may be as much as .080 inch off by the time the keepers wedge-up in running. That's enough to cost serious spring load. If the springs are on the marginal side, it can add up to a difficult-to-diagnose problem, even though you'd swear the installed height was checked.

Tightening up the installed height is easy with shims under the springs, but remember to get shims that match the outside diameter required by the spring, and have an inside diameter which securely registers with the machined boss surrounding the valve guide. Mopar Performance has an assortment of shims available for production-machined Mopar engines, while aftermarket companies such as Competition Cams carry spring shims with IDs to match custom machined spring seats. If more installed height is required, special retainers as well as keepers are available which add .050 inch or .100 inch to the installed height, but clearance to the rocker may become a problem with some parts combinations. To a small degree, the spring seat on the head may also be machined to gain installed height, but the factory discourages the practice, as thinning the material in this part of the head may cause reliability problems.

Getting measurements of the installed height is just sound engine-building practice, and isn't difficult to do even if the engine is already assembled in the car. Minimally, a dial caliper can be used with the spring still in place. Measure from the top of the retainer down to the machined flat area of the spring seat, or shim, if they've been installed. Subtract the thickness of the retainer (usually .100 inch for stock Mopar steel retainers), and you have the installed height. This will get you in the ballpark and is useful for diagnosing a valvespring problem and discovering if the installed height is way off. A more accurate check involves removing the spring and then checking the installed height with a valve spring micrometer. To remove the valvespring, run a compressed-air line into the cylinder (rockers removed) to hold the valves shut, then use an on-the-car spring compressor such as those available from Competition Cams to remove the spring. Now the height mic can be installed, and the installed height verified and corrected with shims if required.

How's That Rate?
Now we know how to check the installed height, but what spring load do we have? Check the manufacturer's catalog, which usually lists the spring load at the installed height and a second value with the spring compressed, typically another .500 inch. This is useful when comparing springs from various manufacturers. Competition Cams goes one step further in their catalog, with tables listing the spring loads at increments of .050 inch. Their tables are accurate when compared to our tests of a number of their springs. These tables provide valuable information to the engine builder who's considering how varying the installed height will affect the seat load. They're also useful for zeroing in on what the true load will be at maximum lift, or to use the jargon, "over the nose."

While the catalog info is useful in selecting a spring to work with a given installed height, once the springs are in hand, it pays to verify the published load. This is done with a spring tester. Though the professional Rimac spring testers are kind of expensive for the enthusiast, most performance machine shops have spring testing equipment and can quickly give you numbers on your spring. It's most important to know the load at the installed height, the over the nose load, and the height at which the spring reaches coil bind. Diehard do-it-yourselfers can get good results from lower-cost hydraulic pressure gauge-type testers, which work in conjunction with a vice or press.

Testing springs becomes even more important with used, bargain basement, or swap meet springs of unknown origin. Again, the most important three things we need to find out are the seat load (at the installed height), the over the nose load, and the coil-bind height. Compare these specs with those the cam manufacturer recommends for your camshaft and rpm range to determine if the spring will work.

A good example is a 360 engine we built on a shoestring budget. The installed height ended up at 1.750 inches, while the unidentified springs were installed without making any load check. The engine carried a moderate hydraulic street cam and the stock valvetrain. The engine revved freely to 6200 rpm without any noticeable hijinx from the valves, but when we switched to stronger but heavier aftermarket roller rockers, the valves floated at 5700 rpm. It would have been easy to blame the rockers, but some investigation revealed that the springs were designed to provide 100 pounds of seat load at 1.650 inches. At the actual installed height of 1.750 inches, the load was only about 70 pounds. It was amazing that the engine pulled the rpm it did, and shows just how good the stock Mopar valvetrain is at high rpm. But the problem wasn't the rockers, it was the springs.

Bound Up And Hog Tied
Of all the checks done on a set of springs, perhaps none is as critical as checking for coil-bind clearance. Coil bind occurs when the spring is compressed to the point that the coils stack up solid and won't squeeze down anymore. If the cam's still trying to push them further down, it ain't gonna happen-something is going to give. If you're lucky, the valvetrain will fail immediately, piercing a pushrod through the rocker cup or bending all 16 pushrods. Most likely, though, the camshaft will be instantly destroyed. Sorry, dude, it's teardown time; the cam and lifters are junk.

Avoiding coil bind is easy enough, it just takes a little ciphering. Most manufacturers list the coil-bind height of all of the springs they sell. If you don't know what the springs, compress it in a vice or arbor press and measure the coil-bind height directly. Say we have 1.2 inches when the coils are stacked up solid. That's the coil-bind height. We will already know the installed height. Let's say it's a 440 with an installed height measured at 1.850 inches. Take the lift at the valve (which can be measured directly with a dial indicator or known from the cam specs and rocker ratio) and subtract it from the installed height. Let's say in this example we have a roller cam with .555-inch lift. The installed height of 1.850 inches minus the lift at the valve of .555 inch gives us 1.295 inches, the height of the spring at full lift. But we still have .095 inch left to coil bind-the height of the spring at full lift minus the coil bind height (1.295 minus 1.200 equals .095 inch). The minimum clearance depends on who you ask: Competition Cams recommends a minimum coil bind clearance of .060 inch, while Mopar Performance calls for a more conservative .100 inch. In our example, with .095 inch, we feel safe.

Now let's say we change from a set of 1.5:1 rockers to a set of aftermarket 1.6:1 rockers. Theoretical lift goes up to .592 inch, and coil bind clearance goes down to .058 inch; way below what Mopar Performance considers safe, and even slightly below Comp's minimum. We're cutting it close here-maybe too close. The lift number was derived mathematically by the ratio change, but it's the real lift at the valve that's important. Just because the rockers are rated at 1.6:1 doesn't necessarily make it so. It's common for aftermarket high-ratio rockers to deliver more, sometimes substantially more, than their rated ratio. Let's say we dusted off our dial indicator, and with the valvetrain set up found the rocker actually delivered .625 inch at the valve. Now our coil bind clearance is down to .025 inch, too tight by almost anyone's standard, and we've got problems. If the installed height of the spring was 1.3 inches, anything above .55O-inch lift would stack the spring up solid, and instant destruction would result. Make the checks, do the math, select the right spring at the required height, and stay out of trouble.