With the crank and rods sorted, all that’s left to round out the rotating assembly are the pistons. Hypereutectic pistons work extremely well in naturally aspirated applications, and feature very low expansion rates when subjected to heat. This allows for tighter piston-to-wall clearance and reduces piston slap. For nitrous and forced-induction applications, or even high-compression naturally aspirated motors, forged pistons go a long way in enhancing durability. While hypereutectic pistons are much more brittle than forgings, and tend to crack in the face of extreme cylinder pressure or detonation, forged slugs are much more forgiving. Hypereutectic pistons can be had for as little as $200, while forgings run $500 or more.
It’s important to keep in mind that there is a right and wrong way to prioritize parts selection within the rotating assembly. Since the pistons bear the brunt of the abuse inside an engine, and are therefore the most likely components of the rotating assembly to fail, it’s not practical to spend big bucks on a forged crank and rods only to top everything off with hypereutectic pistons. A more effective allocation of any budget would be investing in high-quality forged pistons and rods, and matching them up with a cast crankshaft. Of course, if you can afford it, an all-forged rotating assembly is the ultimate in durability.
Everyone dreads machine work because even the most hard-core do-it-yourselfers can’t afford to have a boring bar or honing machine in their garage. That means you have to pay someone else to do it for you, and hope that it comes out right. While there’s no surefire way to ensure quality machine work outside of hiring a shop with the best reputation in your neck of the woods, knowing what each process entails goes a long way in stretching your budget.
At a minimum, most engine blocks will require, boring, honing, and deck resurfacing. Depending on how badly the main caps are worn, a block may also need to be align-bored and honed. Furthermore, rusty or excessively greasy blocks can benefit from hot tank cleaning, and all rotating assemblies must be balanced prior to installation. Rates vary widely from shop to shop, but it’s not uncommon to shell out over $1,000 for these services.
Granted, quality machine work costs money, but the procedures themselves are easy to understand. Boring is typically performed on a machine like the Rottler FA, which has several cutters that rotate around a boring bar. As the bar moves down the bore, it slowly removes material until the machinist stops it when it’s within a few thousandths of an inch of the final overbore size. From there, the block is honed on machines like the Sunnen CK-10, which not just smooths out the bore surface, but also machines superfine peaks and valleys into the bore surface. Quality honing is essential for proper oil control and ring seal. Generally, race motors use a superslick finish to reduce friction at the expense of oil consumption, while street motors prefer a rougher surface for superior oil control. The align-boring and honing process is similar to boring and honing the cylinder bores, but instead it’s performed on the main caps.
Decking involves resurfacing the deck of the block with a cutting head on a machine like the Sunnen HBS-2100. Decking provides a smooth surface for the head gaskets to seal upon. It also reduces the deck height to tighten up quench clearance for improved horsepower and detonation resistance. With all the block machining complete, the last step is balancing the rotating assembly. The goal is to make sure that the rotating mass and reciprocating mass are equal, which usually requires removing material from the crank counterweights. It may seem like a frivolous expense, but balancing is not optional. A properly balanced rotating assembly is essential in enhancing bearing life and ensuring smoothing performance.