Just to qualify my forthcoming comparison; I don't have any first-hand, "inside" engineering knowledge or information regarding the OEM JD 340s piston design as to "what" or "why" they did the things they did. However, as an engineer myself, I do know engines and I do know piston design, so the differences I've observed between these two parts are consistant with generally accepted piston design parameters that are still used to this very day in countless applications everywhere. In other words, physics don't change over time and remains a constant- technology does both however.
In short, if I had a 340s sled that needed pistons, I'd run these things all day long myself in my stuff and not even think about them ever again after installation. Pretty nice piece for a cast piston...
Overall Piston Weight:
1. The OEM slug is a bit lighter than the aftermarket piece, especially in the crown thickness, crown height, pin boss, piston wall thickness and the underside of the piston crown. It's understood that a OEM piston would need to be manufactured as light as possible to meet the durability requirement envenlope of its design, while meeting the bean counters needs of minimal cost per unit. Additionally, "lighter" often means "faster", due to less reciprocating mass and increased fuel efficency is also a positive by-product.
2. The aftermarket piston is a bit heavier in all of these areas listed above that can be a negative or positive, depending on how you look at it. Personally, given the nature of these old engines, their intended use these days and the riding style of the guys that ride them now-a-days, I think all these new traits are a definite positive!
2a. The thicker crown & height will be more resistant to burn-through and forgiving to the guy who has his fuel mixtures off a bit, poor fuel quality, crappy pre-mix oil, air leaks, etc.
2b. The heavier piston pin boss will better sustain higher RPM for a longer period without fatigue for the racer guys who like to wing the RPM up on these things, plus they won't readily deform to detonation as easily as the OEM parts, due to the issues in 2a.
2c. The thicker piston wall would likely not "collapse" as easily due to det events or high RPM use, which should make them quieter in operation and provide a tighter ring seal during rod angle transition.
2d. The thicker crown underside should be more thermo-dynamically stable since it's so much thicker, which would also be beneficial in those areas listed in 2c.
*Summary- The small amount of weight penalty increase with these pistons (which is only approx. 2.3g) compared to OEM, is miniscule to the obvious benefits listed above, where I'll take those features any day to gain increased durability/reliability of a "daily rider" vintage sled.
Top Ring Land Location:
1. The OEM slug has a much thinner "top-down" ring land surface measurement, compared to the aftermarket piston does- which is great for fuel efficency, compression ratio and reduced crevice volume; however this can be a negative trait, because as a result, this area becomes very "thin" and easily burned-down.
OEM Measurement- .065" (edge of piston crown to top of installed top ring, that includes ring side clearance- often slang called "top-down" measurement in the performance industry)
Aftermarket Measurement: .167"
Difference: .102"
2. Typically, the reason piston manufacturers place the top ring so close to the combustion chamber is reduce what is called "crevice volume", which is any area in the combustion chamber where unburned fuel can puddle and not burn effectively to produce work, or power. Most naturally aspirated engines often have their top rings packed as close to the crown as possible to reduce CV, since the combustion temperatures are relatively low, compared to forced induction engines, nitrous applications, etc. Reducing CV improves combustion, improves compression ratio and reduces fuel consumtion at the detriment of making the piston more likely to burn the crown edge or the ring itself if the combustion temps get too high; e.g. incorrect AFR mixtures, bad gas or oil, leaky crankcases, dirty carbs, etc.
2a. In the case of this aftermarket piston, the top-down measurement is substantially lower on the piston crown, therefore, its greater ability to withstand higher combustion/operational temps, (which is good for a fanner engine) and less likely to burn. The piston lasts longer and the rings last longer as a result. The only "negative" to this is that the CV is greater, which would in theroy, make the fuel consumtion go up. How much is a mystery, but I seriously doubt it's anything you could ever measure after a day of riding...
2b. As an added bonus, this piston won't give up any compression due to the lower top ring location and increased CV compared to the stocker, because the piston crwon is slightly taller than OEM, effectively placing the piston crown further up into the head quench area, making up that otherwise "lost" bit of compression.
*Summary: A win-win!
Wheww, my typing hands are tired.... Sorry so long, but I just LOVE this engine design stuff! Read on....
PJ
