"A bit old"....these regs are newer than almost every reg that I use. A couple of issues with the MK20 are: 1) Find out if the recall on the yoke retainer has been done, if not you should get that done at some point. 2) If the piston is the middle generation MK20 brass-tipped piston, be prepared to get a MK25 piston as a replacement, usually about $60, and the reg will perform MUCH better over the long haul. For O2 use, the MK 20 would be fine I guess; SP makes a 'nitrox' kit for that reg that I imagine has a more O2-friendly seat and viton o-rings. That leaves only the bushings in the air path and I'm not sure what problem they would present. The D400 is a different story because you will never find the balance spool o-ring in viton, it's 4.5mm by 1 mm, and you will never find the seat in viton. I have no idea what, if any, impact that would have on using it for high percentage O2. IMO (which is hardly an authority on high percentage O2 use) a 109 might be a better choice for O2 use; there would be basically 2 o-rings and the seat in contact with the air, all of which could be found (or stamped) out of some fluoroelastomer. But I'm really guessing.
As has been suggested, it's likely if you get the MK20/D400 in good tune you'll want to use it as your primary reg, nothing breathes better IMO outside of a pilot, and you'd have be a little nuts to use one of those as your primary.
---------- Post Merged at 06:45 PM ---------- Previous Post was at 06:39 PM ----------
A D400V would certainly be a departure from the "usual."
Unfortunately SP seems to be heading in the other direction and phasing out support for the D series other than rebuild kits. The pilot, air1 and D series do represent some very elegant engineering, and it's engineering that really does have an impact on performance for a change, not just something to impress breathing machines or include 'features' that sound good on the dive shop sales floor.
I'm afraid that because conventional 2nd stages are already very good, far better than is necessary to enjoy diving, and because the vast majority of buyers will not be able to appreciate the subtlety of the D series design, there's little reason for anyone in the industry to return to it. SP did sort-of-pay-hommage to the D400 with the short lived X600, but it contained none of the real geometry and design advantages, it just sort of looked even uglier than did the D series.
