Updated: Jan 11, 2022
for these engines, you have the choice of cast replacement pistons, or custom made forged pistons. there's a lot of speculation with engine building, that forged pistons are better in every way, over the latter, and that is simply not true. forged pistons are a must, if you are building an engine with forced induction, or turning the engine over 5500 rpm constantly, or building a stroker with offset ground crank and long rods with higher wrist pin location (as an example). which 98 percent of people are not, with their nailheads, and under most applications, their engine doesn't even make power spun up higher than that. otherwise the cons of forged pistons, outweighs the pros, on a street engine. everything with engine building is a trade off, in one direction or another. race engine building is as far away from street engine building as boats are to airplanes (exaggerating phrase).
forged pistons require 4x plus the piston to wall clearance to compensate for expansion. many variables come into play such as piston construction and bore size. forged pistons are more dense than cast. denser aluminum is heavier by volume, and expands more from heat via volume. so to make a forged piston of the same design as a cast piston, it will weigh significantly more. to save weight in design, forged pistons typically have a shorter skirt (which also cuts down on friction.) as well as narrower rings which also cut down on weight.
on an engine that's going to be raced, or driven excessively hard, or a blower motor, under those conditions, then yes, the forged piston is better for that application. that being said, just because the piston is stronger, DOES NOT mean it encourages longer engine life in most applications. this is where the misconception comes in. people tell me they need a forged piston to handle detonation. a forged piston is not the answer or cure for not tuning your engine properly. that is downright silly. if you tear down an engine that's received heavy detonation (common with boat engines as their timing covers typically don't have timing marks and so the owner simply never sets the timing. also, there is NO SUCH THING as "setting the timing by ear.") you commonly find the block is riddled with cracks on the deck (heads as well) , typically stemming away from head bolt holes and rings are shattered to pieces. throwing forged pistons in there isn't a solution for that kind of abuse, which is essentially the same as beating your engine with a hammer. because of the excessive piston to wall clearance, and common shorter skirts, there's premature and accelerated cylinder wall wear, overall. as it is, around 75% of a street engines wear and tear is right when it gets started up, prior to operating temp... there's excessive oil burning/consumption, and shorter ring life from the piston rocking back and forth prior to reaching operating operating temp, as well as the obviously piston slap noise when cold. the average person building one of these engines, wants it to last forever. they want it to run quiet, and they don't want it to burn oil. they want it to be reliable enough to handle burn outs and long road trips. or maybe they only take the car out on sundays to go to the local cars and coffee event? either way, the cast pistons that are available, are the most practical fit, for probably 98% of the guys that are building these engines or restoring old cars in general. i have yet to have reported cast piston failure on any engine we have ever built here. the original cast pistons lasted 60 plus years, more often than not and are rarely broken when i tear down cores that are completely worn out, otherwise. unless engine was completely abused from lack of oil, extreme lean conditions, or heavy detonation, or overheating. you can also get the compression above 10 to 1 with a cast piston, no problem with decking of the block with a cast piston. bear in mind, none of these engines were 10 to 1 from the factory without a very rare export kit or power pack. the highest compression nailhead was the 59 dynaflow 401, which was rated 10.50. that engine was only 9.73 to 1. all those late 10.25 rated engines were only 9.5. most the 322s were 9 to 1 or less.