|Engine Build Head Gasket and Cylinder Heads Install -|
We installed and degreed the camshaft in the last segment of the engine series. In this portion of the assembly, we’ll look at cylinder heads and head installation. There are some words of caution here: If the engine has a roller camshaft (that requires considerable valve spring pressure), or has a set of stiffer than normal high performance valve springs, you should exercise extreme caution if you decide to perform the assembly at home. We don’t recommend it. Springs under tension are extremely dangerous, and we’ll explain it further in another portion of the article. Aside from physically assembling the cylinder heads, installation is very, very straightforward. If you can tighten bolts and know how to operate a torque wrench, then it’s a piece of cake. Check it out:
Virtually all engine machine shops or cylinder head manufacturers will assemble the heads for you (as is the case with the engine build series cylinder heads). There’s good reason for this: They have the tooling and the expertise. But what follows is a look at how the assembly is accomplished:
When the machine shop rebuilds or blueprints your cylinder heads, the process of grinding the valves and grinding the seats will effectively sink the valves deeper into the cylinder head. What this does is change the installed height of the valve spring. Valve spring installed height is the distance from the base of the valve spring pocket on the cylinder head to the outer spring lip on the valve spring retainer.
Measurements to determine installed valve spring installed height can be made a number of ways, but by far the easiest is with Crane Cams’ height micrometer. What it does is take the place of the valve spring in the cylinder head. The retainers and valve locks are installed, then the tool is tightened until it reaches installed height (basically, as far as it will go). Compare this dimension to the spring manufacturer’s specification.
So how do you adjust installed height? Small adjustments are made by way of shims placed under the valve spring. Note that aluminum heads all require at least one hardened shim in order to prevent the spring from damaging the soft aluminum in the spring pocket (what happens is the sharp, hard end of the spring erodes the softer aluminum cylinder head material). When you add shims, the installed height is reduced. If the installed height needs to be increased, there are two choices available: 1) increase the depth of the spring pocket, or 2) install valves with longer stems. Of the two, the use of valves with longer stems is much preferred. The reason is that cutting into the head to increase the spring pocket depth can weaken the head or inadvertently cut into a water jacket.
On a street-driven engine, valve stem seals are absolutely critical. Any oil that burns within the cylinder reduces engine power, fouls spark plugs and as we all know, creates considerable tail pipe emissions. In order to prevent oil from being drawn into the cylinders past the valve guides (as the piston goes down in the bore, it creates a vacuum) some form of valve stem seal is required. Quality high-performance seals most often have a spring that holds the seal in place (by surrounding the seal). The sealing material is often Teflon or another low-friction material. Some valve stem seals mandate that the valve guide OD (and sometimes the height) is machined to a specific size to accept the seal. Others are designed to install over the stock valve guide. In either case, the valve stem and the inside of the seal should be well lubricated. Many seals are supplied with a plastic installation tool that facilitates the installation. The seal must be carefully slid over the valve stem lock (keeper) grooves. The tool is placed over it and then lightly tapped in place over the guide with a small hammer.
Remember when we mentioned the dangers of installing valve springs? Here’s why: Today’s high-performance valve springs should be installed by an experienced machine shop. The reason for this is the increased valve spring pressures found today mandate specialized tools (for example, a heavy-duty valve spring compressor, often air powered). Springs are under hundreds upon hundreds of pounds of pressure when compressed. If you make one miniscule error, the spring can physically launch off the cylinder head at what amounts to critically high speeds (some equate it to a bullet). That’s why a pro should install them.
Nonetheless, before the springs are installed, the head must be clean. Grit can enter the combustion chamber by way of the guides. Lubricate the valve stems with assembly lube and add the appropriate shims. As the spring is installed, it should only be compressed sufficiently so that you can install the valve stem locks (keepers). To keep the locks in place during the installation, it’s a good idea to coat them lightly with engine assembly lube or even a small amount of grease.