Scubapro MK 20 creeping badly

[J_Rod]

Hi Gurus,

I have a MK20 that cycles from 100# to 135# IP. It gets back to 135 in about 3 seconds....BUT, then it will creep to as high as 160# over 30 minutes, then it stabilizes,but over 150#....I have read about the piston evolution of the MK20 series, and know that I have the brass tip piston which is known to creep, but not this badly. Prior to taking the 1st stage apart, it was 135# cycle, then creep to 150#, which is why I decided to take it apart in the first place (other than I was waiting for a chance to start working on my newer 1st's. I have serviced a MK10 and it works perfectly). I thought it might be that I disturbed the piston/seat marriage, but after cycling about 100 times, it still creeps badly. I have ordered a service kit off of the e...but I think a composite piston may be the actual issue (it only has 1 big o ring at the base of the piston, btw. Schematics show 2 bigs ......any advice on where I can procure a comp. piston? PM me if necessary. I am currently developing a relationship with a new LDS owner, so parts are more of a challenge than in the past...Thanks in advance!

 

[guruboy]

Did you replace the seat after you took it apart?

 

[J_Rod]

Did you replace the seat after you took it apart?

I did not. I have not gotten a service kit yet. Didn't figure I had much to lose, since it creeped to 150# anyway. If I can find a comp piston, I will service it. That would basically upgrade it to a Mk25 as I understand. Using it to practice on.

 

[fmerkel]

2 piston o-rings? No....1 piston o-ring (90 hydrometer) sandwiched between 2 plastic guides.
If it leaks you get leaks out of t2 ambient chamber, but that doesn't cause creep.
If you use a soft 70 duro or don't get the guides right the high pressure side chews up the o-ring over time.

 

[J_Rod]

2 piston o-rings? No....1 piston o-ring (90 hydrometer) sandwiched between 2 plastic guides.
If it leaks you get leaks out of t2 ambient chamber, but that doesn't cause creep.
If you use a soft 70 duro or don't get the guides right the high pressure side chews up the o-ring over time.

Yeah, that's what I thought, but it looks like a schematic I have shows another. Wanted to verify. Thank you for the o ring material heads up too.

I found a piston. $55 shipped. I'll update after I get it done. Any tech tricks appreciated! I've read if no Nitrox, silicone grease is OK instead of Christo lube?

 

[snowdog61]

My MK20 has 2 of the blue striped o-rings on the piston (it had been upgraded to the composite in my case, on the right in the pic ). One is on the piston itself, with the other sandwiched between the piston and the black spring alignment perch (don't have the schematic nomenclature for this part). My advise - inspect the orifice tip very carefully for any defects and check o-ring contact surfaces (LP bore especially) to make sure they are entirely clean and smooth. You are fine with regular silicone but I prefer cristolube as you can use it on all o-rings including silicone - one lube to rule them all (well mostly).

IP creeps are due to loss of seal between the orifice and the seat. If the seat and tip are fine, it should lock up with only a few psi creep until it stops again. My thinking (and I'm sure there will be comments on this) - Poor surface finish on the LP barrel (drag) where the piston o-rings move can cause the slow 'post-lockup' creep while the parts settle in and finally achieve a full seal at the seat. Another possibility is drag on the piston stem (thinking this is why you see lower durometer o-rings getting chewed up - the high delta P along with a low duro part). The seat and orifice mostly seat (you see the needle quickly settle then move) but the parts are still just slightly moving as the oring relaxes. You need that little extra oomf (the few psi creep you see) to get it to equilibrium. I have a harder time believing the piston oring drag since your delta P is only IP to ambient. it is a possibility but the cross section of the oring(s) is really small - may be a contributor but.... More likely is the HP stem seal - it just makes more sense to me. As others have said, a higher durometer oring should help. I'm still fighting a 7 psi creep-to-lock and don't want to pull hers apart again. It maintains a lock once it gets there (137 psi). If I wait, maybe I'll find the stem oring chewed which would make the post mortem easy.

 

[redacted]

What we have here is a failure to communicate.

One HP duro 90 (85) 2-010 on the pist6on shaft sandwiched between two bushing on all Mk20 piston versions. Works best when the bushings are installed correctly. There is a top ans a bottom bushing and each bushing has one surface designed to be in contact with the o-ring. If you get it wrong, it bwill probably work but not as reliabloy as it should.

One piston head o-ring, duro 70, on the brass piston in a large single groove with a plastic backup ring.

Two piston head o-rings, duro 70, on the piston head, one in each groove, with the composite piston.

 

[fmerkel]

What awap said.
He pointed out I was using the plastic guide without the ridge improperly. At a glance it looks symmetrical-it is not. One side is completely flat, the other side is slightly rounded and has some small marks (casting?) near the edge.
The flat side toward the O-ring to support it. Same with the other guide with the ridge, large flat side to the O-ring. This combo creates the support for the O-ring that is missing when SP change from the MK10 which had the O-ring internal to the bore. Good support, hard to change. Now it's easy to change, with compromised support IMO.

 

[halocline]

I have had good luck polishing the brass tip piston with micromesh, basically you 'lap sand' the edge by gluing some micromesh (1800-2400 grit) on a hard flat surface, and running the piston edge over it. Then you use some sort of cone (I use a pencil or chopstick with micromesh wrapped around it) to get the inside of the edge, then very fine micromesh (like 3200 or higher) around the outside just with your fingers. That should really clean up the soft brass tip and cut down on the creep.

I've also used micromesh to polish and clean the edge on the composite piston. You don't actually want to change the profile of the edge, just remove any really tiny scratches and foreign matter. It's amazing how much difference in lock up it makes. Again, 3200 or higher.

Another handy use for micromesh is to remove any glaze on the piston shaft where the HP o-ring sits. You certainly don't want to scratch the piston there, just light polish. It seems to hold the lube a little better.

 

[Rusty Shackleford]

What we have here is a failure to communicate.

One HP duro 90 (85) 2-010 on the pist6on shaft sandwiched between two bushing on all Mk20 piston versions. Works best when the bushings are installed correctly. There is a top ans a bottom bushing and each bushing has one surface designed to be in contact with the o-ring. If you get it wrong, it bwill probably work but not as reliabloy as it should.

One piston head o-ring, duro 70, on the brass piston in a large single groove with a plastic backup ring.

Two piston head o-rings, duro 70, on the piston head, one in each groove, with the composite piston.

If I remember right, the mold injection marks should be AWAY from the O-Ring. correct?