,
To test whether or not you’ve lifted the head, you’ll need to do a cooling system pressure test. Remove the radiator cap and attach a cooling system pressure tester to the radiator, you can buy a cheap one from places like harbor freight or failing that, modify an old radiator cap with a fitting so you can pressurize the cooling system with an air pump or some source of regulated pressurized air. If your pump doesn’t have a reliable, built-in pressure gauge, tee one in on the line. Pressurize the system until the gauge shows 15-20psi. Clamp the hose with locking pliers and check the pressure. If it drops rapidly or over the next few minutes you have a problem. If you did lift the head, you’ll need to inspect it to make sure it isn’t warped or cracked as well. If so you might be able to salvage the head by surfacing it. You will probably have to surface and check the block deck as well.
Forged pistons generally come in low silicon 2618 alloy (not the breast implant stuff) or high silicon 4032 alloy. Low silicon forged pistons are the toughest but expand more under heat, requiring looser piston to wall clearances. This will create some piston slap, especially when the engine is cold, and will also produce more cylinder, piston and ring wear because the pistons will move around in their bores a lot more. Low silicon pistons are good for drag racing cars or handling lots of boost, but not for a lot of miles. High silicon pistons are not quite as strong or ductile, but they are more durable for a wider range of temperatures and will expand less under heat. They can run tighter piston to wall clearances, reducing ring flex and cylinder rock for better sealing and less wear on the cylinder walls. High silicon pistons are good for high mileage endurance racing or street cars. Check with the piston manufacturer for the recommended piston to wall clearances as they depend on the design, material, end use, and cam shape. Turbocharged motors should run slightly looser clearances due to the heat.
Don’t skimp on your engine’s internals. Depending on your piston choice and the condition of the cylinder bores, you might not need to bore the block out to install an OEM sized forged piston but you definitely should have a machine shop measure each piston and hone each bore to create accurate and equal piston to wall clearances within 0.0001-0.0002” of each other. You probably will have to have the cylinders honed as forged pistons require a bigger piston to wall clearance than stock cast parts.
