The next step is to measure the bore. In a machine shop, this will usually be done with a dial-bore gauge-a large, spring-loaded internal caliper with ball-bearing tips and a dial that can quickly determine the bore diameter. The machinist should measure prior to machining, and several times during the process as well. The critical measurements will be taken during the final hone, as this is where the piston-to-wall clearance will be determined. With the piston's actual diameter in mind, the machinist will carefully hone each bore to its final dimension, removing just enough material to obtain the desired clearance. So if the piston actually measures 4.026 inch, and the desired piston-to-wall clearance is 0.004 inch, the bore should be honed to 4.030 inch exactly. The actual piston-to-wall clearance will be recommended by the manufacturer and will vary depending on the type of piston; for example, forged pistons generally require more piston-to-wall clearance than hypereutectic because the forged units will experience more thermal expansion as they reach normal operating temp.
Fitting Rings to PistonsThis might seem like a simple operation-merely slip the rings on the pistons and you're done. But the reality is that attention to detail with regard to rings can mean a better quality seal when the pressure's on. You'll need to refer to each manufacturer's recommendations for each of these specifications since they may vary. Besides setting the ring endgap, the two critical dimensions are ring side and back clearance. Side clearance is the gap between the top or bottom of the ring to the ring groove while the back clearance prevents the ring from protruding out from the piston wall. Higher quality piston manufacturers spend more time ensuring that the machined ring grooves are parallel to improve ring seal while high-end ring companies spend loads of money ensuring that ring faces are as parallel as possible. Really anal engine builders prefer to lap their own rings on a perfectly flat surface using 1,000-grit wet/dry paper.
Ring endgap It's common knowledge that setting proper ring endgaps will improve cylinder sealing and therefore performance. Typical replacement and performance ring packages are designed for a given bore size, offering a sizeable endgap (especially for the top ring) to prevent butting the ends, which causes all sorts of mechanical grief. For the car crafter, most performance ring manufacturers offer 0.005-inch oversize ring packages that allow you to custom set your top and second-ring endgaps.
This task is not difficult, but can be time consuming. If your budget is tight, several companies offer manual ring grinders like one from powerhouseproducts.com (PN POW105050 $65.00). To measure the endgap, square the ring in the bore and measure the gap with a feeler gauge. Keep track of the number of turns on the ring grinder and work slowly until you establish the proper endgap. Then you can use the turn count to cut the remaining seven rings. Always trim just one side of the ring and make sure to keep that edge perpendicular. Don't forget to deburr the end once the gap is set. Always file toward the id of the ring to prevent peeling away the ring face material.
A good general rule for ring endgap is 0.0040 inch per inch of bore for a normally aspirated street engine on pump gas. Nitrous, super- or turbo-charged engines generally want more endgap. The other major move now is to open up the endgap for the second-ring package to prevent pressure buildup between the second and top rings that could unload the top ring.