No love for the MK15, which in my mind has just about the best design of all of them. Here is my rundown with some of the main pluses/minuses:
1. MK5
If I had to choose only one of the SP balanced piston models that own to keep, throwing away all the others, I would probably choose the MK5. Why? Because it has extremely stable IP, seats are readily available and will be long after my diving career is over, and the long term reliability is off the charts. I have several MK5s that sit for 5,6,7 years, and then lock up perfectly. This is outstanding. It costs very little to service them as well.
MK5 minuses: It is difficult to recommend buying them to less experienced divers, because there are a variety of models, some with poor yokes for current tanks. Some of the older ones used brass turret retainers, which are weakened by poor servicing, and in general parts are harder to come by. The MK5 and MK10 do not use the bushing system to minimize extrusion on the HP o-ring, and as such are not ideal for high pressure tanks. One last minus is that the MK5 uses a proprietary DIN retainer which is a pain to find. I finally found a pair of them so this is no longer a problem for me. Awap, ever the innovator, used some SS washers to fit the slightly shorter SP universal retainer to his MK5s.
2. MK10
I have several MK10s and I use them and rely on them frequently. They are the best of the SP balanced piston regs for very cold water IF you have the late model which accepts the SPEC boot and the boot itself. They are easier to recommend for purchase than the MK5 because they all have good yokes, SS turret retainers, and they can use the widely available universal DIN retainer. They're everywhere so parts and donor regs are a breeze to find.
MK10 minuses: They do not have the IP stability of either the MK5 or MK15. I am certain that this is due to the smaller diameter of the piston head. There are two ways this lack of stability manifests, 1) They start to creep after a few years of steady use, instead of the MK5 or 15, which I literally have seen last a decade without creeping. 2) They have more of an issue with IP rise at higher tank pressures. I've had MK10s measure something like 140 PSI at 3400 PSI and 125 300 PSI. I have been able to mitigate this a bit by using 90 duro polyurethane o-rings and lots of PTFE grease, but still, the friction co-efficient on that HP o-ring seems to have a significantly more pronounced effect than on either the MK5 or any of the later regs.
3. MK15
I currently use MK15s as my main cave regs, mostly because 1) They are outstanding performers with seemingly lifetime rock solid IP stability, and 2) because I have a stash of seats and bushings for them, and since they are supposedly no longer supported, I want to eventually use up my seats and bushings before the world ends in a fiery blast. The big plus with the MK15 is that it combines the great IP stability of the knife edge with the bushing system that minimizes HP o-ring extrusion. These things are as bulletproof as it gets.
MK15 minuses:
The big issue is parts availability. The earliest MK15s had a tragically bad design in the seat retainer in a dumb attempt to add external IP adjustment. This early seat retainer was quickly removed from service after causing a few pistons to cut right through the seat....oops! The redesigned retainer (which was installed as a recall) works perfectly. Another minus is a design quirk in which one of the bushings is not user replaceable, it's pressed into the body. While this bushing does not bear the brunt of the pressure gradient and rarely wears out, some over-eager-but-not-too-bright techs would hack away at them and in the process ruin the whole reg. There are also a few specialized tools required to service MK15s; the bushing installation tool is unique, there is a special set of snap ring pliers needed to install the snap ring that holds the bushings in place (that snap ring was replaced by a spring in later models) and the seats are unique. Forget about any aftermarket seats; there just were not that many of these regs sold.
4. MK20
I have only owned a few MK20s and have sold each of them after not using them very much. I don't have anything against them, but I don't want to spend a fortune on seats (have you seen the cost of a MKI20/25 rebuild kit?) and I am good enough on air so that I don't need the 300SCFM flow capability of the newer piston/seat. (That's a bit of a joke. 300SCFM is the equivalent of emptying a AL80 in 17 seconds, and if you need that kind of air flow, you might want to work on your air consumption)
MK20 minuses: When you buy one of these, you have no way of knowing what piston is in it. There are three possibilities, and one of them is a disaster and should be immediately replaced. That adds at least $75 to the cost, and that's if you can find a friendly dealer to sell you a new piston. You also have to determine if the yoke/DIN retainer recall has been performed, and the only reliable way to do that is to remove it. I know of a SP dealer that was so idiotic that he really thought that the recall simply involved replacing the saddle, not the retainer itself. If you buy one and it still has the old, sloped retainer, you can still use it, and supposedly any SP dealer will do the upgrade. But then you are forced to bring your reg to a dealer and let them work on it. This is unacceptable to me, but I do have a bit of a bad attitude about dive shop techs as a whole. The ones on this forum are all, I'm sure, excellent. But you guys are not the majority out there.
5. MK25
I have never personally owned a MK25, I'm just way too cheap for that (hehe) but I have worked on friends' MK25s. They're fine......they do address the issues of the MK20 with the pistons and the retainer, and the MK25 piston locks up pretty well and stays stable for a decent amount of time. There is no doubt that they are among the most successful and highest performing 1st stages ever sold. The IP drop on these things is teeny-tiny under even the most aggressive purge, and they've been around for long enough so that they pop up used all the time at a decent price.
MK25 minuses: Complexity is one; I don't really like the external IP adjustment because it is a potential leak, an extra little set screw and tiny o-ring to fiddle with, and I never had any problems with the decades-long proven design of the shims for IP adjustment. Sure, I can see why someone who works in a dive shop is going to prefer being able to adjust customers' regs IP without taking them apart, but since that doesn't apply to me, it's not a benefit. The composite piston, which locks up so beautifully, has a design flaw, which is that it has an internal o-ring that is not officially replaceable. That means when that o-ring wears out, it's new piston time. I have MK5/10/15 pistons that are at least 30 years old.
The rounded piston that is shared by the MK20/25 was developed for the primary reason of increasing airflow. This is really a sales thing IMO. The earlier regs flowed WAY more air than any two divers would ever need at any depth and IP recovery was pretty damn fast. I think SP just wanted to be able to market a regulator that flows 5 times (or more!) the amount of air that could ever squeeze through a scuba valve anyways.
Alright, that's the longest post I think I've ever made on any internet forum. So I'll stop there, even though there could be more to talk about!
