Stuck MK10 Piston cap (Scubapro)

[redacted]

It won't hurt to get the reg wet inside - as long as you actually get it open in a timely manner to clean it and dry it. The problem occurs if you are not successful and walk off for a week or two...

I have even left them soaking for weeks with all ports and inlet open. But that is soaking in clean fresh water with a change of water every few days, usually along with another attempt to disassemble often including another cycle of hot/cold.

 

[redacted]

The MK5 is pretty much out of play because I can't get the drive extension through the tiny yoke hole, and the thin yoke really isn't something I want to deal with right now anyway. Nonetheless, I'll keep it around because I will eventually figure out a way to remove the yoke. I pulled it apart, less the yoke. It looks okay inside, and I noted the piston is much larger than the MK10's. The thin yoke is a concern, and until I have better understanding of the risks, I'll set it aside.

The Mk5 yoke retainer has threads that are protected from water incursion by the retainer o-ring unlike most newer scubapro models. That means that retainer should be a bit easier to manage than your Mk10 or the Mk20/25. I built a 1" yoke nut socket that takes a 1/4 in drive extension just for such cases but for a one time removal, the nut in the vise or a very large crescent wrench will do the job. For 3000 psi use, I do replace the lightest yoke but do use the intermediate yoke marked "3000" in spite of the Scubapro warning not to. No problems yet. It is my understanding that if they fail, the failure mode is the yoke stretches out and will no longer seal properly. So, little leak rather than large lose of gas. There are a lot of Mk5 models floating around, mostly at pretty good prices ($30 or less). I look for heavy yoke, two 7/16ths HP ports, and 5 LP port swivel.

 

[Six2Life]

For 3000 psi use, I do replace the lightest yoke but do use the intermediate yoke marked "3000" in spite of the Scubapro warning not to. No problems yet. It is my understanding that if they fail, the failure mode is the yoke stretches out and will no longer seal properly. So, little leak rather than large lose of gas.

I have an Oceanic MK5 copy from the 1990's with a yoke that appears to have the exact same dimensions as a SP intermediate 3000 yoke. The only difference is that it has a larger yoke screw and is marked "4000psi". If the failure mode of the older SP yokes is stretching then it doesn't make sense that the Oceanic would be rated for 1525psi higher with a larger screw being the only change. Unless there was also some change made to the metalurgy of the yoke material that increased it's strength without changing it's dimensions. So either the failure mode on the SP would have to have something to do with the smaller screw, or the Oceanic "4000" marking is very optimistic, or the SP intermediate yoke is fine for pressures higher than 2475psi.

 

[Zung]

I have even left them soaking for weeks with all ports and inlet open...

Same here, one time, one stubborn MK10 with a stuck HP plug that had never been opened in its entire life (I guess). Soap and water for a couple of week, no side effect whatsoever.

... Unless there was also some change made to the metalurgy of the yoke material...

Metallurgy is the magic word, and also my worst nightmare in school: I guess it's a common occurence, so everybody is using large safety margins. Even if somebody did a proper tensile strength test, a safety margin is still required.

 

[DA Aquamaster]

Chrome plated brass is very durable stuff and soaking in water results in relatively little free oxygen available to promote corrosion. A few drops of water inside a regulator however, can get interesting as there is now moisture and lots of O2. It can get really interesting if there is also a bit of salt in the mix to facilitate the exhange of ions. Add an older reg missing the chrome to the scenario and it can result in a picture of the reg's badly corroded insides on the "wall of shame".

 

[redacted]

Chrome plated brass is very durable stuff and soaking in water results in relatively little free oxygen available to promote corrosion. A few drops of water inside a regulator however, can get interesting as there is now moisture and lots of O2. It can get really interesting if there is also a bit of salt in the mix to facilitate the exhange of ions. Add an older reg missing the chrome to the scenario and it can result in a picture of the reg's badly corroded insides on the "wall of shame".

It seems to me that the only place it really matters on a piston 1st is on the dynamic sealing surfaces which are exposed in the ambient chamber anyway. I assume you are talking about the kind of corrosion that darkens the brass rather than the verdigris that seems to require both salt and air to form.

 

[Zung]

Chrome plated brass is very durable stuff and soaking in water results in relatively little free oxygen available to promote corrosion. A few drops of water inside a regulator however, can get interesting as there is now moisture and lots of O2. It can get really interesting if there is also a bit of salt in the mix to facilitate the exhange of ions. Add an older reg missing the chrome to the scenario and it can result in a picture of the reg's badly corroded insides on the "wall of shame".

I've got one where the chrome inside the cap, where the piston rides, is completely gone; got it used, so I have no idea what happened. I've got another one with a couple of places worn to the brass. After a bit of smoothing with Brasso, they work fine, but I don't know for how long.

Does anybody know much is a cap for the MK20/25? I've seen $20 in a pricelist from 2002; must have gone up.

 

[axxel57]

Part 10.713.102 Cap is 13.81 US$ 2011 US Catalog and 7.50 Euros plus 19%VAT German Catalog 2011.
Don't know what the SP Dealers charge......

 

[Zung]

Thanks axxel57. The price seems to be a bit low. Can you check the part number please? My 2004 MK25 AF schematic says the cap is 10600102.

 

[Manow]

Thank you to everyone interested, and helping with feedback and suggestions! Lots of progress! Okay, the MK10 has been stripped, cleaned in soap & water, then in Vinegar-water. I put some cheap silicone (trident) lube on the o-rings, and reassembled it. No issues. When I get a proper re-build kit, I’ll build it right with some cristo. I have a beginner understanding of the MK10. I managed to get the yoke off the MK5. Zung and AWAP were right on the money. I put the nut in a vise, and the vise makes contact with the nut, as the nut extends out past the small yoke by fractions of an inch; enough that I could get a good purchase on it. Unfortunately, I wasn’t able to get the swivel cap nut off. I tried a few different sockets, and I’m still not clear on the size of the nut holding the swivel? Also and here’s a BIG QUESTION: on the MK5 Piston, I noted what felt like stamped letters on the piston shaft. On closer examination, I see rough spots on the piston shaft, small scratches basically. Right about where the piston shaft passes through the little o-ring. Maybe these can be polished out, or maybe I need a new piston? Overall, this MK5 looks like a train wreck on the outside. It looks like someone pushed a screw-driver through the ambient ports, and used this as a technique to open the reg. When I first received this reg., it had a bolt screwed into one of the HP Ports, and a toilet anchor screwed in the other HP Port. Also, the HP Ports are the same size as the LP Ports, so an adapter will be necessary, or I guess I could get an older 3/8” HP hose. The thing is scratched up pretty good, with a few light gouges. (Read: ugly Reg.) Also, just for fun, I installed the MK10 Yoke on the MK5, and it appears to fit okay. Any comments or feedback are appreciated.