This is an intriguing comment. Why should it matter where the piston head land sits? Why did placing the piston head inside the body instead of inside the cap make piston action more precise? I can see how it might have helped alignment during piston travel. But decreasing the piston head diameter made Mk10 lockup less crisp, IMO.
I believe that the reason for the greater precision in the piston travel was indeed a matter of alignment. This is because the MK10 design allowed for the ambient chamber and the HP piston land to be machined out of a single piece of brass, ensuring that the surfaces that the piston head o-ring and piston stem o-ring operated on were perfectly parallel. Apparently, at the time, SP was limited in diameter by this machining process; I don't know why. In the MK5, because the ambient chamber and HP chamber were threaded together, there apparently was enough inaccuracy in the threading that there were slight alignment issues between those parts. However, in later designs, they returned to the MK5 approach, presumably with either more precise threading or a lack of concern for any alignment issues. By that time (MK15/20/25) they were using bushings to further lower the tolerance between piston shaft and the o-ring journal, reducing extrusion.
But, IMO, the MK10 is not as good a design. The smaller, more aerodynamic piston certainly flowed more air on a bench than the MK5. But in real life, I have found that there is more rise in IP at higher tank pressures in the MK10 than there is in the MK5. In essence, the MK10 is less balanced than the MK5. I believe that this is due to the smaller piston head. This smaller area results in less total force on the knife edge, and as such, any friction on the piston shaft due to o-ring extrusion (or just increased pressure at high tank pressures) have a corresponding higher impact on the movement of the piston.
What I find is that my MK10s typically do not lock up as reliably as my MK5s for as long a time. I was rebuilding MK10s maybe twice as often as MK5s; still a few years of solid use before I noticed any creep. That's the biggest difference, longevity between rebuilds. Both regs have delivered plenty of air for my use and I have never noticed any difference in in-water performance. I do like the fact that it's easier to find MK10s (and pistons for them) in pristine condition, and the fact that the modern DIN retainers work in them.
The MK15 may be my current favorite, and I'm sad to see them not supported any more. I have a handful of seats for mine and will continue to use them until I don't have any more. The MK15 got a bad rap as being hard to rebuild (it's not) and there was a real problem with the initial seat material and external IP adjustment mechanism. I think this is what sunk the MK15 more than anything else. SP loved the aerodynamics of the rounded piston head they went to in the MK20, even though until the composite piston came around, lock up on those was not as solid.
