Theres been allot of questions on piston design lately with topics ranging on clearances, piston slap and knock retard from excessive piston clearances, alloys, piston types,,,,etc,,, etc. Also there have been allot of new hemi piston offerings from many companies lately so I thought I put together a little tech discussion on pistons to help educate people looking at aftermarket pistons so they can make educated choices. Not to insult the more knowledgeable but just to get the novice up to speed I’ll start pretty basic and hopefully the thread will get some interesting tech questions going. As most of you already know there are two main types of pistons, cast and forged. Cast pistons are used on the stock hemi engine and are used for low cost and the low thermal expansion characteristics of cast pistons allow them to be installed with tight clearances of around 0.0015†so there is limited engine noise. The down side of cast pistons are that they are fragile and more failure prone than most forged pistons. Almost all cast pistons these days are from hyper eutectic alloy which means that the silicon content is higher than the eutectic point, which is the point where the aluminum silicon ratios yield the lowest melting point. But we aren’t here to discuss stock pistons are we? Being power hungry aberrant car crazies were mainly concerned with forged pistons: In performance automotive use there are two main types of aluminum alloys that are commonly used. 4032 and 2618. They both have strengths and weaknesses in performance automotive applications so I’ll cover those. 4032 is a forgeable alloy that’s normally used in low to medium performance range forged pistons. Due to it’s silicon content it has a very low thermal expansion coefficient and subsequently can be installed with tight clearances of around 0.0025-0.003†in a NA application on a hemi diameter piston. Because it can be installed tighter than other forgeable aluminum alloys it is a good alloy choice for light street performance use because the tighter clearances will have less engine noise and therefore less ghost noise tripping our very sensitive knock sensors. The high silicone content also wears better than other forgeable pistons alloys. 4032 alloy pistons are normally cheaper to manufacture also due to the alloy being less costly and more easily machined. So in a lower performance application the piston will last longer without wearing out. The down side is the silicone will crystallize over time and it’s also less ductile so it has a tendency to break rather and deflect under heavy loading so it’s normally not the recommended alloy for boost NOS or high power NA engines. Typically the extended life design limit of 4032 is around the 500 flywheel HP range. This is due to crystallization and weakening of the alloy over time. On applications over about 500 crank HP on a V8 2618 is generally preferred because it’s more ductile nature allows the piston to flex rather than brittle fail. The thermal expansion rate of 2618 is still low for forgeable alloys but higher than 4032. Typical installation clearances on a 4†bore would be in the 0.0035-0.004†on a round style piston and 0.004-0.0045†for a box or X style piston. The cam profile of the piston is also instrumental in driving this clearance so always use the manufacturers recommendations though. This is normally at a given dimension from the bottom of the skirt very close to the bottom of the skirt but will change some with manufactures. I know of one neophite who actually used feeler gauge at the crown of the piston to determine the piston to cylinder clearances. I know that’s hard to believe that kind of stupidity, but no supprise he measured something stupid like 0.025†of clearance. In almost all cases this is incorrect because the piston is built with a cam shape and for example you may have the proper 0.0043†of clearance at 0.500†up from bottom of skirt perpendicular to the pin but after the cam from the skirt and then at the crown typically there is 1 to as much as 2 degrees of taper to make sure there isn’t contact with the wall from the heat on the top of the piston causing increased expansion on the crown. On the custom pistons I spec normally it’s 1 degree for NA to 1-1/2 to 2 degrees on a heavy NOS or forced induction application because of the higher temperatures on the crown. As for the pistons cam design I’ve been subtly tweaking that in on the hemis so to achieve a 1-1/2 to 2†wear pattern on the skirts of the pistons. It’s a balancing act of spreading the wear surface of the piston over as much of the piston as possible to get better wear characteristics and less ghost knock from piston noise but if you go too far you risk hanging a piston and scuffing it. Same type of balance act on the piston to cylinder clearance. Too much and it’s noisy too little and the piston scuffs and makes a mess. When in doubt a little noise is preferable to a wasted engine. Anyone who has ever seen a badly scuffed piston will agree with that. Some have probably heard reference to Round style piston and Box style piston. The round style is the more conventional piston that most are used to seeing. It’s a good design that works well and can normally be fit about 0.0005†tighter than the box forgings. Then why do most stroker hemis use the more race oriented box forgings since they have to go in a bit looser and cost $20/piston more? That a little known secret that’s normally only known to the few engine builders that pioneered stroker hemis and had to figure out how to make the big (over 3.795†) strokes work without interference back before the everyone can build em kits were readily available. It comes down to clearance, where the box forgings have extra reinforcing ribbing and are used in race engines for the extra strength in the pin bosses so shorter, lighter wrist pins can be used at extremely high power levels and endurance applications. But in the hemi there normally employed because the shorter wrist pin is needed to clear the toner ring on the #8 piston. Also, the round style piston itself will hit.Yep those expensive forgings $20/piston over round style are for the most part there to make clearance for the toner ring. The one exception to that rule is the top guided hemi builds (like the 5.7/372 and 6.1/392 that’s top guided) those require the use of a box piston for the extra reinforcement to properly center the rod from the piston end rather than the more conventional crank end. Which is one of the basic uses for a box piston.
WOW! Great discussion thread Stu... Will try and moderate this heavily to keep the off topic out... Thanks for a very informative thread.
Good point, there are allot of new piston options out so this may help people understand the benefits or differrent parts.
Great write up! This great information now that there are several different stroker motor kits available. What did you mean by "top guided" for the 6.1/392. I just haven't heard that term before. What do you do with/to the piston oil squirters in your stroker builds?
Most rod piston combos are located from the crank end of the rod so they have like 0.015-0.020" of rod side clearance and thats what makes the rod perpendicular to the bore and centered up on the piston. On a top guided engine it's the other way around. The big end of the rod is much narrower and floats on the rod journal so there is like 1/8" of side clearance and the rod is centered up by the piston and a tight piston to small end clearance. Theory is this allows the piston to operate more squarely in the bore and have a better ring seal. Additionally the tighter clearance on the pin allows a lighter pin to be use, yet stay strong. Since the big end of the rod is narrower the weight of the big end can be reduced also. It makes a very fast responsive engine as you know. On strokes under about 3.850" I keep the squirters and just bend the ends to clear but on the bigger strokes I have to plug the holes because there isn't room and the body of the squirter will hit the connecting rods.
Great writeup Stu. I had forged pistons on my old supercharged jeep and it clattered like crazy when cold. I always thought something was wrong with it until I learned more about the looser tolerances of forged setups.
Excellent write up Stu, I guess for a novice like myself it all depends on the actual build and I'd hope if your considering doing a motor swap or stroker that you dont go cheap on parts because you will most likely end up paying more in the end to repair broken parts. Wonder if you could breakdown which pistons would be best for certain applications? Maybe your next topic will be about springs or pushrods???
The belief is that the common failure we see with the stock pistons is due to the narrow top ring lands coupled with tight ring gaps. Can you describe the mechanics of the failure for us? I'm under the impression that when you exceed some chamber temperature the rings expand enough to close the ring gap. This causes the ring to seize against the cylinder wall. The ring groove is now trying to drive a seized ring up and down the cylinder. Since the top land is the weak point, the top of the piston breaks off on the down stroke. Is this correct? Surely chamber temps are important to piston health (and rings, and valves,). Are there some common temperature operating ranges? Maybe specific to our engines? People wonder what EGTs they should be seeing. Are there some "rule of thumb" numbers that you should be looking to stay within for EGT? At the runner? and/or at the collector?
This is what I keep hearing. Chrystler had to put the rings that close to the top due to something about the smog and carbon buildup. I've been told that when the rings heat up and expand, they will go towards the path of least resistance which in our case is the small space left on top.
Yes, but I think it's not that the ring gets too wide for the groove but that the diameter (after the gap closes) gets to large for the cylinder.
