Here, the heads are bolted...
Here, the heads are bolted in place and the valvetrain is installed. Then we carefully run the engine by hand through two complete power cycles (four turns of the crank). This leaves enough of an impression in the clay to get a measurement.
Piston technology began to change in the '70s, with close attention paid to fuel economy and cleaner emissions. Heavy pistons consume fuel. They also contribute to emissions because it takes more fuel to move them. As a result, Detroit began designing lighter pistons with less skirt. Less weight, less skirt to create friction. The up side to all of this is greater efficiency with less power lost, and less fuel burned.
Manufacturers have managed to shave a lot of weight out of pistons because aluminum technology has improved a lot since the '60s. however, not much has changed when it comes to cast pistons. If you're rebuilding an old FE Ford or Y-block using cast pistons, expect to see virtually the same kind of piston Ford installed to begin with. If you are stepping up to hypereutectic or forged, expect to see significant changes in piston design.
Piston design has always varied because engine building needs vary. Piston crown design effects what's going to happen on the upstroke. Anticipated compression ratio effects piston requirements. It is a common misconception that compression ratio is effected by combustion chamber size alone, piston crown, deck height, and compression height also contribute to compression. When you are shopping pistons, it is important to remember these issues.
When the heads come off, we...
When the heads come off, we can see in the putty how deep the valves entered the valve reliefs. We cut the putty in half to see how much valve clearance there is. We want at least .050-inch clearance between the valves and the piston.
Another advance in piston design and manufacturing is high-temperature coatings that protect pistons. This is where the space age meets the automobile. There are coatings available that protect the piston crown from temperature extremes if you are going to push the engine hard. These coatings also protect the piston skirt from wear issues, reducing friction considerably. See your piston manufacturer for more details.
Riding In Cars With Pistons
Anytime you are shopping pistons, there are all kinds of factors to consider: chamber size, valve sizing, how much stroke. This all plays into the relationship your pistons will have with the rest of the engine.
You have to check clearances to make sure everything is going to work well together. It's always a good idea to do a dry run before engine assembly is complete. Valve-to-piston clearances must be checked, especially if you're working with a custom application.
Checking valve-to-piston clearances calls for a mock-up assembly of the short-block and heads. You can do this during engine assembly, or with the pistons without the rings before assembly. The benefit to an initial mock-up is not risking ring damage if you have to disassemble the engine because the pistons weren't right for the application.
Another way we check clearances and timing is with a degree wheel. When we "degree" an assembled short-block, we are checking valve timing events, valve lift, piston timing, and piston deck/compression height. In fact, this is something you should check before checking valve-to-piston clearances. When we are doing this, we want to check true top-dead-center (TDC) with a TDC indicator. This, coupled with the degree wheel, teaches us the absolute truth about camshaft specifications and crankshaft indexing.
Five Golden Rings
We tend to glance over piston rings because they just aren't that terribly interesting. But in the interest of reliable operation and performance, piston rings need as much attention as the rest of the engine. Think of piston rings as seals between the combustion chamber and the crankcase. The two compression rings seal the combustion chamber. Each of the compression rings is made out of a different material, based on what each ring is exposed to.
What do pistons have to do...
What do pistons have to do with degreeing a camshaft? Plenty, because piston timing has everything to do with proper valve timing. consider what happens when the piston reaches the top of the cylinder with either of the valves open. Valve and piston timing have everything to do with each other and safe engine operation. We check the piston for correct top-dead-center (TDC) while checking valve timing events.
Piston-ring shopping mandates...
Piston-ring shopping mandates your close attention to detail. Ring selection is rooted in how you intend to use the engine. Cast-iron and ductile-iron rings are best for street use. Chrome rings are more for the high-revving race crowd. Molybdenum-impregnated rings are well suited to street and racing applications, and they offer longevity.