Which Scubapro piston 1st stage is the overall best?

[Eric Sedletzky]

Just as the title suggests, which Scubapro piston 1st stage is as good as Scubapro ever got?
All the different models and changes considered.

More in-depth discussions are certainly welcome on why some of the changes were made and what they tried to cure while possibly causing another issue.

For instance, I’m not particularly thrilled with the chronic IP creep in my MK 20 while my MK 5 locks up great. There were two models in between those, so what were the changes with them and why?

 

[fmerkel]

This should be interesting if the guys that know the inside workings and design modifications weigh in.

Got a MK20 also. IP creep is a fairly constant problem. MK2, 5 and 10, simpler to work on (fewer parts) and IP creep is (generally) not an issue. Serviced properly, they just work. MK20 I often have to mess with. It's fine once I get the creep stopped, or within acceptable limits and been in use for years (wife's reg).

I know the transition from the early models to the later ones was an attempt to both increase performance, and decrease servicing time and problems. This was at the cost of simplicity and some might consider reliability > more parts, more plastic.

 

[Angelo Farina]

My preferred one is the old MK5 in its later SPEC version. I own 4 of them, plus a 5th one which is older (no SPEC and only two LP ports, this was my first regulator, bought in 1976 or perhaps 1977).
The latest SPEC models have 5 LP ports, one of them on the top of the rotating turret. My four ones were upgraded with DIN universal adapter, the conical seat of the MK10 and the stainless-steel retainer for the rotating turret, and of course Viton O-rings everywhere.
Here one of them after the latest upgrade;

As you can see, the chrome finish is showing the marks of the age of this reg, which was built in 1982 here in Italy. However this does not affect its performances, which are still top-notch: perfect lockup at 138 PSI, very minimal pressure drop during a purge, incredibly large air flow, particularly when using a SP High-Flow rubber hose installed on the top port.
The reg is packed with high-viscosity silicon grease, which means that the service interval is approximately 5 years (depending on the usage, of course).

 

[lexvil]

MK 25 because you can adjust the IP externally…

 

[rsingler]

Well, that didn't take long!
@Angelo Farina and @lexvil are both right.
Which is why the answer is..."Atomic"

The Mk5 was the best damned piston ever made. Huge head = good sealing force. Cave cone seat is reliable. Three different seats makes changing IP easier, if not as easy as the Mk25. But the key factor IMO was the ability to seal it. When Scubapro got rid of messy SPEC sealing, they screwed the pooch. Atomic has lots of haters among the tech staff, because sealing is a PITA. That doesn't make it wrong.

If only Atomic made an adjustable IP sealed piston.
If only Scubapro made an adjustable IP sealed piston.

If you want both and no mess, dive a diaphragm or a Sherwood piston (SP2 has a magic design, but has delicate internal parts).
If you want a reliable piston that'll last for 40 years, buy a sealed Atomic. And seal it.

Sorry, @Eric Sedletzky !

But @fmerkel is right. IP creep when the change was made from knife edge to round edge pistons is an issue. The knife just gets sandblasted when HP air makes the turn to go down the piston shaft. Restoration of the piston from IP creep is not hard, but shops don't do it. A true knife edge is better, but good luck keeping the knife for 40 years (nice work @Angelo Farina !). A sealed diaphragm with a replaceable orifice is just easy by comparison. But that's not what Eric asked...

 

[lexvil]

Well, that didn't take long!
@Angelo Farina and @lexvil are both right.
Which is why the answer is..."Atomic"

The Mk5 was the best damned piston ever made. Huge head = good sealing force. Cave cone seat is reliable. Three different seats makes changing IP easier, if not as easy as the Mk25. But the key factor IMO was the ability to seal it. When Scubapro got rid of messy SPEC sealing, they screwed the pooch. Atomic has lots of haters among the tech staff, because sealing is a PITA. That doesn't make it wrong.

If only Atomic made an adjustable IP sealed piston.
If only Scubapro made an adjustable IP sealed piston.

If you want both and no mess, dive a diaphragm or a Sherwood piston (SP2 has a magic design, but has delicate internal parts).
If you want a reliable piston that'll last for 40 years, buy a sealed Atomic. And seal it.

Sorry, @Eric Sedletzky !

But @fmerkel is right. IP creep when the change was made from knife edge to round edge pistons is an issue. The knife just gets sandblasted when HP air makes the turn to go down the piston shaft. Restoration of the piston from IP creep is not hard, but shops don't do it. A true knife edge is better, but good luck keeping the knife for 40 years (nice work @Angelo Farina !). A sealed diaphragm with a replaceable orifice is just easy by comparison. But that's not what Eric asked...

I was going to mention Atomic but decided not to jump the rails here. Of course there is always the more obscure like a MK 20 with a S.P.E.C. Boot and the MK25 adjuster

 

[rx7diver]

I don't know about "best", but I own three Mk 10's which I purchased new between 1987 and 1988, IIRC. They continue to perform flawlessly. (I service them, myself.) I purchased Scubapro 300 Br DIN conversions for them in 1988 when I purchased my Sherwood Genesis (3,500 psig) HP 80 cylinders. And I converted two of my Mk 10's to "Mk 10 Plus" regs shortly after I moved to the Great Lakes area in the early 1990's. (The "Plus" conversion was intended to improve Mk 10 performance/reliability/stability on HP cylinders, I believe.)

The Mk 10 is skinnier than the Mk 5 it replaced, due to a narrower piston. From my 1986 Scubapro catalog (p. 42): "Most First Stages are engineered for maximum pressure at full tank loads. The Mk 10 was designed for maximum efficiency at a lower pressure spectrum. Tight orifices or bottlenecks were enlarged to allow the air to flow faster at lower pressures. The piston diameter was reduced and the stem shortened allowing for the use of a smaller, more lively, stainless steel spring."

I can't recall why the Mk 15 was introduced to replace the Mk 10 (and Mk 10 Plus), but at that time (in the mid-1990's) I chose to purchase Poseidon Odins to use for my back tanks, and use my Mk 10's for my deco cylinders, and so I never "upgraded" to the short-lived Mk 15's.

For the past 20 years or so, I have been using my Mk 10's almost exclusively for my rec diving when I am diving single hose regulators in temperate water, whether I am using an HP 80 or a steel 72.

ETA: I just found this old thread that contains some excellent content re the Mk 15: Scubapro MK 15 History.

rx7diver

 

[rsingler]

Of course there is always the more obscure like a MK 20 with a S.P.E.C. Boot and the MK25 adjuster

And there it is: the perfect piston. In basic black, no less.

 

[lexvil]

And there it is: the perfect piston. In basic black, no less.

It’s the UL MK 20, I don’t know how it’s going so far but James79 is working on a S.P.E.C. Boot to fit a MK 25…

 

[rx7diver]

I don't know about "best", but ...

I had a bit of time, so I grabbed my stack of old Scubapro catalogs and looked through them. Couple of things:

1. The 1993 Catalog includes a timeline that shows that the Mk 10 was introduced in 1983 (p. 8).

2. The earliest catalog that shows the Mk 15 is the 1994 Catalog (p. 20). The description of the Mk 15's (then-new) "Ultra Glide Piston Bushing System" and "Thermal Insulation Bushing (T.I.B.)" seems consistent with @DA Aquamaster's suggestion (from the earlier SB thread that I linked in my previous post) that the Mk 15's design is significantly different than the design of its Mk 10 (and Mk 5) predecessors.

FWIW,

rx7diver