5.0 Mustang & Super FordsHow To Chassis Suspension
Valvespring Installed Height
Properly Installing Valvesprings To A Consistent Height
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In order for valvesprings to properly and consistently close an engine's valves, they must be installed at a consistent installed height--the distance from the bottom of the valvespring retainer to the bottom of the valvespring, with the spring installed on the cylinder head. Installing new springs at the height specified by the manufacturer usually ensures that the springs develop the proper seat and open pressures without going into coil-bind (the coils stacking solidly during valve lift). However, because even springs in the same batch vary slightly, some race engine builders install the springs to achieve a specific seat and open pressure, even if the exact installed height varies slightly from spring to spring.
When valves and seats are ground during the cylinder-head reconditioning process, the valve stem sticks through the head farther, thereby increasing installed height and reducing spring pressure. Normal wear also decreases the spring rate. Lowering the installed height can compensate for any decrease in spring pressure, so long as you don't approach coil-bind.
The procedure for figuring out and correcting installed height is similar whether you seek to install all the springs at a given exact height, vary the heights slightly to compensate for variations in spring tension, or compensate for sunken valve seats. It was explained to us by the experts at Jim Grubbs Motorsports.
Checking and Correcting Installed Height
First, install the retainer with keys on the valve stem to be checked. Retainer thickness varies somewhat, so from this point onward always keep the same retainer together with the same valve as a matched set. Measure the distance from the bottom of the retainer to the spring pocket floor. If the measured distance is greater than the desired installed height, add enough valvespring shims to reduce installed height to the proper dimension. When shooting for a uniform installed height (rather than a specific pressure), try to get all heights within 0.005 inch of each other. Shims vary slightly due to their nominal thickness, so if you have enough shims and mic them all, you can get pretty darn close!
Spring-locating cups are more stable than plain flat shims. Mandatory on aluminum heads to keep the spring from galling the head, cups can also eliminate the need to excessively stack standard flat shims, as well as help locate and hold the spring in place.
If the measured distance is less than the desired installed height, there are several solutions:
* Machine the spring pocket deeper. On stock heads, there's the risk of cutting through to the internal coolant passages. Special stepped cutters are available to minimize the problem. Even if you don't run into water, deepening the pocket on a fully prepped head limits the amount of material the head porter can remove from the port roof without excessively thinning the casting.
* Install longer valves. This is a good solution if you plan for it ahead of time when spec'ing out the engine combo, but it's expensive if you've already bought a set of valves. Longer valves alter the valvetrain geometry, so you'll probably need custom pushrods too.
* Install offset valve stem keys or retainers. The aftermarket offers 0.050- and 0.100-inch offset keys and retainers for adjusting installed height. These usually eliminate the need to deepen the pocket or (if the initial installed measurement was much greater than spec) stack an excessive number of shims. But raising the retainer may cause rocker-arm interference, and there's no room for a lash cap.
Uncorrected coil-bind quickly destroys a valvetrain. The dimension at which the spring's coils all stack solidly should be at least 0.050 inch greater than the cam's total valve lift, minus any lash if you're running a solid cam. Each spring component of multispring assemblies should be checked separately, and then all the components should be checked together as an assembly. The previous installed-height fixes are also quick and dirty fixes for a coil-bind condition, but when they're used for this purpose the final spring pressures will end up below the desired spec. The proper fix for a coil-bind problem is to find a suitable valvespring that accommodates the valve lift generated by the camshaft.
Anytime you change the cam, valves, rocker arms, or retainers, check for interference between the valvespring retainer and the valve guide tops (or PC-style seals, if so equipped). The distance from the retainer bottom to the top of the guide (or seal) should be at least 0.050 inch greater than the cam's rated valve lift (minus the lash if using a solid cam). If the valve stem seal height is the cause of insufficient clearance, you can often substitute a different seal type or brand. Sometimes the guide itself can be shortened without any serious problems. Otherwise, you're looking at a combination of offset retainers and keys and more shims and/or longer valves--with all their attendant drawbacks.