There's a lot of good, bad, and conflicting info out there when it comes to these pistons.
After a ton of research I think I've got something valuable to give back to the community. So, let me put all my knowledge of these pistons in one place for everybody.
Hopefully some of you can make good use of it.
I work for a Federal-Mogul piston facility. We don't make the Speed Pro Jeep pistons; those come from India. But it's the same process, metal, and engineering.
The main differences in the 802 and the 825 pistons is simply, as stated, "ring thickness". Actually, the 825's are US inch "SAE" spec, and the 802's are metric spec. With thicker SAE rings, the 825's are slightly lighter, due to the wider ring grooves. Here are those specs:
H802CP pistons: 564 grams, rings: 1.5mm / 4mm (.059" / .157")
H825CP pistons: 568 grams, rings: 5/64" / 3/16" rings = .078" / .187" = 1.98mm / 4.75mm
You can look up both of those spec sheets here (unfortunately the 677's don't have a spec sheet there):
http://www.fme-cat.com/Default.aspx
As far as cc's and dishing (what oletshot was referring to): the definitive answer to this is that these hypereutectic pistons can have a
MINIMUM crown thickness of 0.220", at their thinnest point.
If you look inside the piston, the design of the mold leaves a "checkered" pattern in the center core area, with raised cross hatch. It is from the very bottom (thinnest part) of this pattern that you need to measure to.
I've put a picture of what I'm talking about below.
This figure is F-M's tech center consensus and was taken directly from Federal-Mogul Technical Document #1104, available on their site here:
http://www.fme-cat.com/livedocs/1104.pdf
I have an untouched set of the H802CP's with stock bore size. I measured the crown thickness.
My long-reach mic had too large of a nose, so I used a depth gauge with a pointed tip to get inside the checkering.
It was 2.410" from the skirt to the thinnest crown pit. The bowl was .117" deep at the center. Measuring the piston's overall length gave 2.772".
Subtracting 2.772 - 2.410 - 0.117 gave me a minimum crown thickness of .245".
The other 5 pistons were all within .001" on each measurement.
So, it looks like the bottoms of the bowls on my set can go .025" deeper, for a total of .142" deep.
But that isn't true for every set. You have to measure to be sure, or risk blowing chunks.
The reason F-M / Speed Pro techs can't give you a straight answer is because these can vary. They may all be within tolerance, and in a stock application these might work just fine.
But in most of our cases this area is critical when balancing cost, performance, and risk.
Although the piston specs for both of these state a 0.116" deep dish, and mine all measured .116-.117, the "deck" or "crown" thickness underneath can also vary.
These are made on a multi-section molding die that does NOT have tight tolerances. They use a ceramic spray parting agent which over time can build up on the die core and thin out the crown. They're supposed to clean them out every so often, but ...ya know... Plus the temperature that the mold is poured at (water or air cooled), the temperature and chemistry of the metal, and a dozen other factors contribute to the varying thickness of the crown and the depth of the dish. The speed at which the metal is cooled increases the hardness, but also contributes to some shrinkage, thus adding to the variation.
I cc'ed the dishes with a proper buret. I got between 13 and 14 mL's as cast on this set. That's a far shot from the 15.1 and 15.5 that I've read on here and other places.
When dishing these, close attention needs to be paid to the distance between the dish and the outer bore edge. There isn't much area left to make the bowl much wider. This distance can be "cleaned up" by a radius mill, but the limiting factor there is the strength of the top ring land. If you take too much out, you weaken the piston crown land area, and it may be subject to cracking and breaking off between the dish and the ring groove.
Two illustrations below show what I'm talking about. I took as many measurements as I could and modeled it fast in QuickCad to get the dimensions that I couldn't take, like the ring-groove to bowl distance.
The blue area is the cross hatch which I measured mine to be around .015". The pink is the oil slot.
I can't find a definitive answer anywhere out there as to how far you can go, but the land depth is already quite thin (.200"), and the inside of the top ring groove is already pretty close to the dish (.160" by my calculations). So, a stock bore set of these shouldn't be widened any closer to the edge. Now, If you have overbore pistons, technically you can enlarge the bowl diameter about the same amount as your overbore size because the ring grooves move the same radius distance from center as the overbore.
Since I'm using standard bore pistons, and can only go .025" deeper. I'll be widening by dish just enough to make a nice machined radius and will mill a little further into the quench area to match the porting on the head's combustion chamber. With that said, the MOST I calculate I could get out of my set is 18cc.
Some guys will say you can go larger, and I'm sure some of you have successfully done so. But I'll play it conservative.
Bit of advice: Because of the variations, I think that after machining the pistons, doing a weight match again is a must.
The bottom line is this: The 4.2 rod route is a touchy one because of the limitations of these pistons. If you dish, you can't go very far and have to take a lot out of your head, plus there is lot of time and money involved. If you don't dish and don't port, you have to run your quench to the point of being in detonation-city. So, maybe that set of 4.0 rods and custom pistons doesn't look so bad any more.
Hopefully 
