Making informed decisions in the selection of pistons can pay dividends.
Cast pistons are great-just so long as you are not trying to build the ultimate race engine, which is what most of us are trying to do! To keep costs down, many classes call for cast pistons. A poor choice here can cost power and, in a worst-case scenario, an engine. Here is what you need to know to avoid such pitfalls and gain an edge over the competition.
Let's start the ball rolling with claimer pistons. KB's claimer offerings are essentially made of the same material found in their tough, hypereutectic high-performance pistons. The only difference is that claimer pistons don't go through a final and expensive heat treatment process. What you're left with is still, in the world of cast pistons, a stout piece. Also, the piston geometry is made to favor high-performance, sustained wide-open-throttle operation. This is a definite plus because many cast pistons are not made for these conditions, as they're designed to run quietly and have a long life span. However, that does not preclude some being much better for race applications than others, and that is just what we will look at now.
Although mandated by many class rules, most cast pistons are not made for racing. Knowing
Compression is definitely a budget racer's best friend, and you need to make every effort to maximize it. Many rule books call for pistons with four equal-sized valve cutouts, so you should look for valve cutouts of the minimum size to get the job done. The ultimate situation here is to get pistons with no valve cutouts and machine-in your own. This tends to get a little more expensive, unless you have your own mill or access to one.
The form of the valve pockets can also influence the amount of cubic centimeters contained within. Some valve pockets are cut with one continuous pass across the piston that gives the clearance for two valves. Others are cut with two individual cutouts to clear the valves. The two individual cutouts typically save a couple of cubic centimeters in the piston, and this is the preferred style. If the rules call for four valve cutouts in the piston crown but do not specify that they have to be of equal size, then you're in good shape. To meet the regulations, cut two very small valve pockets on the redundant side and two the requisite size to clear the valves. This, in effect, leaves you with a piston equal to a two-valve cutout design.
A KB claimer piston can be identified by the word "claim" at the position indicated.
At this point, we must decide whether or not the valve pockets are too small to accommodate a given cam and valvetrain combination. Cam manufacturers usually recommend clearance of about a hundred thousandths of an inch. It should be noted, however, that this is conservative and is intended to cover the more klutzy drivers, who miss shifts and ignore tach red lines. In reality, we can safely cut that figure by 25 percent. If the driver treats the equipment as he should, those clearances can be cut to fifty thousandths. Here, it never hurts to clay the pistons and establish exactly what the clearance is. If the valve clearance is more than the required minimum, there is an option to cut that clearance rather than start from scratch with a new set of shallow cutout pistons. It is easier to skim the piston crown to reduce the depth of the cutouts and then cut the block to reduce the valve pocket depth and achieve the desired piston/deck height. Going this route has another advantage in that it puts the top ring closer to the piston crown. This now brings us to the subject of minimizing piston-to-head clearance.
Thin sections of the piston outboard in the valve pockets can overheat. This can cause det
Here are the two styles of valve cutouts you are likely to see. Because of the few cubic c
Piston-to-valve clearances should be checked on the two axes shown here. Cam companies ten