When installed in the 2.0L motor the pistons produce a compression ratio of 11.7:1. This is a little higher compression than what we wanted; however, we decided to modify the pistons for more valve clearance. Note: This isn't necessary for a really inexpensive build but the SR responds well to tight cam lobe centers when used with a proper header. Without enlarging the reliefs the intake cam can be advanced 5 degrees and the exhaust retarded 5 degrees safely with most cams but we wanted to be able to run tighter if necessary. Howard Watanabe of Technosquare used a piston-milling fixture on a mill to enlarge our valve pockets by 1mm and make them 1mm deeper. Since this is a cast piston we didn't want to go overboard and weaken the piston too much. This and some planned headwork will drop our compression to around 11.4:1 when we're done. Howard then balanced each piston to within one gram by removing material around the pin boss to equalize the weights.
Next, we shipped the pistons off to Swain Technology for coating with their PC-9 dry film lubricant coating on the skirts and their Gold Coat thermal barrier coating on the crowns. Swain Tech's Gold Coat thermal barrier coating is three layers instead of the typical one layer, consisting of a proprietary combination of different ceramics. Gold Coat reduces heat transmission to the underlying metal by 20 to 40 percent. This protects the piston from detonation-induced heat and reduces thermal expansion, allowing a tighter piston-to-wall clearance. Aluminum starts to lose strength at around 350 degrees and our cast piston can use the help. By reflecting this heat back into the combustion chamber, the coating helps promote a better burn, harnessing this bit of otherwise wasted heat. Swain Tech's Gold Coat transfers heat across through the surface coating but not past it to the underlying metal. This promotes even heating across the piston crown, thus helping reduce the potential of detonation-causing hot spots and promoting an even burn for good flame travel. Swain coatings bond to metal at a molecular level and will not ordinarily ever crack or flake off.
Swain applied their PC-9 coating to the skirts of our pistons. PC-9 uses molybdenum disulfide and tungsten disulfide in a heat-conducting polymer resin matrix to make a tough, lubricious surface. The ability to conduct heat helps the pistons stay cool in use. PC-9 coating will help improve piston skirt and cylinder wall wear and reduce thrust load-induced friction.
With the Swain coatings we can keep the stock bore, saving the expense of machining the block. Howard simply deglazed the bores with a finishing hone. With the PC-9 we'll be running an extremely tight piston-to-wall of 0.001 of an inch. This should help reduce oil consumption and improve ring seal as well as reduce noise even with our worn bore. By saving on machining, we gained the advantages of coating-a good trade-off.
The stock SR rods are very strong and safe for extended use at up to 8,000 rpm and for a low-buck motor, they're perfectly adequate. For our goal of a bulletproof inexpensive build, however, we chose some rods from Eagle. Eagle ESP rods are forged from 4340-steel alloy with a high nickel and chromium content. The nickel and chromium give the steel a high-tensile strength, high toughness and a much improved fatigue life over high-carbon and chrome-moly steels. A good feature of Eagle Rods is that they're forged in two pieces with the cap forged separately. This allows the rod to have the metal's grain flow in a circle around the big end bore. With this forging method there's less distortion of the rod's big end under load and superior strength of the part, especially fatigue strength.