Of all of the processes involved in assembling a high-performance engine, properly setting up the valvetrain is perhaps the most critical. Failure to pay attention to the fine points can spell disaster, so it pays to fully understand what to look for as it relates to the engine's requirements. While an engine using mostly OEM parts can typically be bolted together with minimal fuss, as the list of aftermarket and race parts gets longer, it becomes increasingly vital that the system is validated by checking, measuring, and dialing-in the entire system, as required. We stopped in at Andy Mitchell's Outlaw Racing Engines in Upland, California, to look at the principle techniques and tools to do the job on a 509ci big-block Chevy we'll be covering in a future story. Nevertheless, the same ideas apply to all types of engines.
It's a fundamental truism that if the valves hit the pistons in an operating engine, the stage is set for an engine failure. Most stock engines leave the factory with generous valve-to-piston clearance, but in a high-performance engine, clearance here can become a scarce commodity. Modifications such as higher compression pistons, more aggressive camshafts, higher rocker ratios, milling, decking, or even the cylinder head design itself can put a crunch on the available clearance. An experienced engine builder considers the depth of the valve clearance notches in the pistons along with all of the above mentioned factors to select parts that will provide the required clearance. Often, when custom pistons are being ordered, the valve notch depth is specified to assure adequate clearance with the engine's parts combination.
When it is all said and done, the only way to know exactly how much piston-to-valve clearance there is in an assembled engine is to directly measure it. There are a couple of different techniques to accomplish this: measuring with a dial indicator at the valve tip, or checking the clearance with the use of clay impressions. Of these two techniques, the clay method is the more foolproof, and it also provides the engine builder with a look at radial clearance to the valve relief, something that the indicator method will not reveal. To check valve-to-piston clearance using the clay method, slugs of modeling clay are strategically placed in the valve pocket of the piston. The cylinder head and valvetrain is assembled, and the crank is rotated through two full revolutions in order to go through both valve events. Next, the cylinder head is removed, and the clearance can be determined by measuring the minimum thickness of the clay impressions.
Spring Load, Installed Height, And Coil Bind
The workhorse of an engine's valvetrain is the valvespring. When functioning properly, the spring's force ensures that the valvetrain is controlled to actuate the valves as intended by the camshaft lobe design. As rpm and the camshaft's specifications become more aggressive, the spring's requirement to maintain valvetrain control becomes more critical. Generally, more spring force is needed to overcome the inertia and negative acceleration of the mechanical valvetrain parts and the valves. Increased spring loads increase the strain on the valvetrain components, as well as the camshaft and lifters, requiring a fine balance between the supplied force and the mechanical limits of the parts.
With the clay loaded into...
With the clay loaded into position, the head is bolted in place, the valvetrain is set up with a checking spring in place and adjusted, and the crank is spun through two revolutions to allow for the full four-stroke cycle.
The cylinder head is removed,...
The cylinder head is removed, and if everything was done correctly, the valve will have made an impression in the clay corresponding to the minimum clearance. For inspection, cut the clay segments with a razor blade to expose the compressed cross section.
The simplest way to check...
The simplest way to check piston-to-valve clearance is by the clay method. Strategically placed lumps of clay are arranged in the valve pocket to take an impression of the valve. Lubricating oil prevents the clay from sticking to the valve.