I have a MK20 Ultralite that needs a new piston. Someone at one time degraded the edges of the two o-ring retainers which I think has compromised it somewhat. Instead of being a perfect circle it looks like someone sanded down the edge. Maybe a fifth of the width of the depth of the groove. The regulator creeps just a little then settles at about 147 psi. I`ve used it on 30-40 dives with no problems but would like to get a new piston. I don`t know if anyone out there has one available or what the cost is. Please let me know what you all have available and the cost. If it`s too high I guess I can always upgrade to the MK25. Thanks in advance guys!
Mk 20 piston
- Thread starter Perryed
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Lets have a picture. The flattened edge may be normal as I've seen what you describe on NOS MK25 pistons.
Re the creep, see @rsingler 's post on how to dress a piston knife edge. How to restore a Piston Regulator that has IP creep
Re the creep, see @rsingler 's post on how to dress a piston knife edge. How to restore a Piston Regulator that has IP creep
seen brand new ones like Couv's/yours. darned if I know why...
It just looked unnatural to me. Guess I`ll look elsewhere for the reason it`s doing what it does. It does final settle at about 145 and I can live with that. I had it upgraded due to their tendency to explode on occasion and went ahead and had it supposedly serviced at the same time. After the trip I opened it up because it didn`t look to have had anything done except for the update for the saddle. This is why I do my own work now. Thanks for the help in eliminating one possible aspect. I really love the help you can get here!
fmerkel
Contributor
Yup, have some with that same 'nick'. Came that way.
I have polishing materials and equipment to work the piston end. Great tutorial from rsingler. We`ll see what kind of luck I have doing that. May just need a new seat also as I doubt it was replaced on the "rebuild".
A couple of thoughts here.
As long as that flat does not have extruded metal at the edges that cutout will not hurt a thing. It is on the pressure side of the oring gland so any extrusion will happen on the opposite side of the piston. Run your finger over the ends of the groove and if does not catch, leave it alone. Since that groove will not hurt anything and more than one has had them, it may be some form of indexing or retaining notch for one of the machining processes....or it was stamped out of the edge of the material but still met QC requirements...no clue but in any case it will not impact the performance of the piston, on the other side would be a totally different matter. .
As for the knife end, Couv correct me if I am out of line here but a few psi of creep after lock up that stops is not a big deal, messing with the knife edge is asking to make it worse, that is more of a last ditch effort to restore an edge with an obvious defect that continues to leak past causing IP to keep going up. 145 is not an excessive IP, out of spec some maybe but still better than destroying a working piston over. Would it not make sense to adjust HP seat height to lower the IP, assuming there are no shims that have been missed rather than take the chance of damaging what appears to be working knife edge?
As long as that flat does not have extruded metal at the edges that cutout will not hurt a thing. It is on the pressure side of the oring gland so any extrusion will happen on the opposite side of the piston. Run your finger over the ends of the groove and if does not catch, leave it alone. Since that groove will not hurt anything and more than one has had them, it may be some form of indexing or retaining notch for one of the machining processes....or it was stamped out of the edge of the material but still met QC requirements...no clue but in any case it will not impact the performance of the piston, on the other side would be a totally different matter. .
As for the knife end, Couv correct me if I am out of line here but a few psi of creep after lock up that stops is not a big deal, messing with the knife edge is asking to make it worse, that is more of a last ditch effort to restore an edge with an obvious defect that continues to leak past causing IP to keep going up. 145 is not an excessive IP, out of spec some maybe but still better than destroying a working piston over. Would it not make sense to adjust HP seat height to lower the IP, assuming there are no shims that have been missed rather than take the chance of damaging what appears to be working knife edge?
Thanks for the information herman. I think a good overhaul will get this back into shape just fine. The LDS I used does not have the best track record when it come to servicing and like I said the only reason it went there was because they were an authorized Scubapro dealer and could do the case cracking and blowing up underwater up grade.Got the kits, will do it myself and report back. Also it is my understanding that the MK 20`s have a slightly blunted edge. Maybe it will just take a while for it to "find" it`s seat. We`ll see.
Right, a few psi creep is fine unless it is a long slow crawl-then I would dress it. Rob's method is pretty fool proof. Of course, make sure the the soft seat is absolutely spotless-in fact if you (the OP) have a spare, replace it.
I didn't consider that; it could be a feature that assist in the manufacturing process. I assumed it was there for weak-fingered technicians so they could more easily install the o-ring. i.e. install the o-ring 180 deg from the notch, then pull the tight bit over the notch.
BTW SP likes the 2 o-ring configuration in some of their applications. The idea is that one o-ring serves as a wiper and the other o-ring actually does the sealing. Addtionally, SP recommends using the 2 seal configuration as a reservoir for grease. So be liberal with your application of lubricant in that space.
I could be having a senior moment here, but I don't think the MK20 has the adjustable seat, so adding/removing shims is the only way to adjust IP through normal means.
Please know the saddle is not the update. It is supposed to indicate that the update has been performed. The real update is to the yoke/din retainer/fitting to help prevent cracking of the first stage body when Magilla torques the fitting.
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