SP MK-20 - IP Creep

[rhwestfall]

I have a MK-20 that when initially is pressurized, is at 130. Cycle it, and is s-l-o-w-l-y makes its way to 140, then is rock solid, even after days, and numerous cycles. Seat was changed, and still the same result (creep, as well as unknown history, was the reason for a rebuild). The other one I have locks and holds.

I'm wondering if the spring may be the culprit..... maybe "weak" on the top end of its compression.

Any other thoughts?

 

[redacted]

I would be looking hard at the piston sealing surface. Which piston is it?

 

[rhwestfall]

I would be looking hard at the piston sealing surface. Which piston is it?

IDK - Shop serviced as I was out of time. He told me of the results, and I wanted to get some outside input...

please elaborate.

 

[halocline]

There are 3 generations of MK20 pistons; the middle one has a brass tip and is a notorious creeper. If that's what you have, either live with the 10 PSI creep or get a MK25 composite piston. Those are much more stable for lock up. They seem to be aerodynamically better as well; IP drop during demand is tiny.

Some of my MK10s creep 7-10 PSI over 15 minutes or so no matter what I do. As long as they don't creep any higher it's really only an annoyance. This is one reason I like MK5s better.

I don't think it's the spring; if the spring was worn I think you'd notice IP kind of jumping around, locking up up someplace different after each breath, and you'd have more stability at low settings (like 120) than at higher settings. But no creep, because don't forget the spring force is what the supply pressure overcomes to attain IP. Less spring force, lower IP lock up.

 

[Couv]

I have a couple of MK25 NOS pistons SP p/n 10600155 Halo, is that the preferred unit?

 

[halocline]

I have a couple of MK25 NOS pistons SP p/n 10600155 Halo, is that the preferred unit?

That's the part number I have. Hmmm, maybe I'll buy a MK20 if you'll sell me a piston.

 

[DA Aquamaster]

As noted above the Mk 20 had three pistons. The early ones were stainless steel knife edged pistons. The middle production pistons used a brass rounded tip seating surface and a concave seat (intended to improve flow rate by eliminating the sharp edge), while the late production Mk 20s used a composite pistol with a stainless steel piston stem with a with a sort of knife edge seating surface and an Acytel piston head. That same piston was carried forward to the Mk 25 but it's been updated over the years with a mirror polished stem to improve cold water performance. The only difference between the last Mk 20 and the first Mk 25 was the adjustable seat retainer on the Mk 25, and that was the single minor change where Scubapro finally decided to call the Mk 20 a Mk 25.

Personally, I think the change in model numbers had a lot more to do with Scubapro wanting to give all it's balanced first stages odd numbers - Mk 11, Mk 17, Mk 25, Mk 21, etc.

Given the rounded seating edge, a mushy lockup is the norm for a Mk 20. Normally, you expect a Mk 10, 15, 20 or 25 to lock up with no more than maybe 2 psi of drift in the first 5 seconds, and no drift at all is very common, particularly once the seat breaks in. However on a Mk 20 with the rounded tip piston, 10 psi of drift in the first 30 seconds isn't abnormal.

Provided it does ultimately lock up at 140-145 psi, I would not worry about it. If it becomes a problem it's a quick and inexpensive fix to put in a Mk 25 piston, and if the reg is still under warranty the tech will do it for free when it comes in for service. A really good tech will replace it for free even if it is not under warranty as SP will swap the part out at no cost to the dealer.

The Mk 10 also has a knife edge and locks up very solidly as well. It's the Mk 10 Plus, developed at the same time as the Mk 20, that can have a mushy lockup as it used a steel piston with a slightly rounded seating surface and the same concave seat as the Mk 20. The first few years were interesting for the Mk 10 Plus as Scubapro searched for a seat material that was soft enough to seal, yet durable enough to survive a couple hundred hours of use before starting to creep. But, the seat material used now is great stuff and it shares the same seat with the Mk 20/25.

 

[akdeepdiver]

Last time i went through a SP service course, the factory rep teaching it told me the "ideal" IP for the MK20/25 was 137psi. So if it's locking at 140, in my mind, not an issue.

 

[Zung]

All has been said by the gurus, so I only add a few observations of my own, as this happens to me as well. As pointed out already, it's the less than perfect mating between the edge of the piston and the seat that creates the creep. The causes I've been able to determine are:

1. Bad seat: unlikely in your case if it's been serviced, hopefully with a new kit. More likely if the seat has been removed and put back, and the groove doesn't line up with the edge
2. Some foreign matter on the seat, maybe salt crystals or the whatyoucallit white stuff. I have this sometimes after the gear have been sitting for a while between dive trips. It'll go away after you use it for a while, or if you cycle the gear a lot, maybe 50 to 100 times.
3. The piston edge is scratched. This can be fixed with the Matt (halo) Magic I reported some times ago
4. Brass piston stem: hopeless: trash it, or keep it for training purpose with the Matt Magic above.

 

[oceancurrent]

1. The piston edge is scratched. This can be fixed with the Matt (halo) Magic I reported some times ago

I am interested in learning about the Matt Magic, but can't find the thread. Could you post the link or quote the original post (how to do it, where to find the product)?