Scubapro MK20 - shredded piston O-ring

[fmerkel]

I want to thank awap, halocline, and couv for their particular input. I think it could be really useful. I must admit I have trouble visualizing the O-ring getting extruded/pinched in those confines. OTOH, I also can't believe all the air you need to breathe goes through that tiny hole in the piston.

Wahoo! Scubatools piston insertion guide and bullet just came.
Put in a new Scubapro O-ring and guides per instructions > system IP locked right up at 130 with a smidgen of creep to 135. Got it in a bucket of water for long term creep and leak test now. Maybe this build will 'go the distance'.

RANT - 'official' Scubapro manual makes no mention or differentiation of the orientation of the smaller 'bottom' piston guide. Since I have a 50/50 possibility of putting it in right, some of my issue might have simply been the luck of the draw.
In fact, at least in the copies I have, it's almost entirely slanted toward the MK25 with only oblique references to the MK20. I've looked for a MK20 specific manual and have not found one. I've lurked on a huge number of the Scubapro threads by you guys so I know the usual sources.

couv (or anyone) - so where does one get aviation grade EPDM and FKM in small quantities suitable for a home use?

 

[rhwestfall]

couv (or anyone) - so where does one get aviation grade EPDM and FKM in small quantities suitable for a home use?

from someone who has had to purchase more than "small quantities suitable for home use"....

 

[redacted]

Mk20 and Mk25 are the same as far as the piston is concerned. I had to go through a couple manuals to verify the orientation of the inner bushing. But if you think about the physics of what those bushings do, the correct answer is rather clear.

As far as extrusion goes, although an o-ring is a solid, it does have some fluid properties in that it does change shape a bit to fit its container. Under high pressure, it may change shape quite a bit; enough to flow into the cornersw and grooves of its environment. When that happens, there is an increase in the contact surfaces. Friction between those surfaces will create forces that will effect IP. If there is not enough lubrication, it may even heat and damage the surface; hence the nibbling.

It has been quite a while since I ordered o-ring, but I think this was my source: O-Rings, Inc. Since 1958 - Buy Online

 

[fmerkel]

It makes sense....once you explained it.
I didn't consider the o-ring support surface. Extrusion was not part of my consideration. I was concentrating on the inner structure of the lower guide thinking how that was oriented helped stabilize the end of the piston. Live and learn. I usually got a year out of the o-ring before it started leaking.

I have an O-ring store the other end of town that will sell to retail customers off the street. Next time I'm in that area I'll go in and have a more educated conversation of what they have that I could use. For now I still have a couple more MK20 kits.I bookmarked your link. Thank you.

You guys ever re-use the piston O-rings? By eye and feel they seem fine after a year.

 

[redacted]

I am used to getting 5 years or more from a full service. When I do that service I replace seats and dynamic o-rings. If I happen to open a 1st just to inspect and lube, I usually do not replace any o-rings.

 

[fmerkel]

I bring a small rinse bucket for camera gear to the dive. After the cameras the regs go in (warm water), then the lights. All that gets another soak-rinse when we get home, so I'm pretty conscientious about that. After a year there is still some obvious crud/corrosion inside the piston chamber. I assume that's pretty normal?

If this piston O-ring actually holds up....this time....I might reasonably expect to open it up, clean, lube, and simply re-assemble what's there for a couple/three years? This is based on reasonable inspection and replacement of parts as necessary.

Got to admit, one of the things I like about my MK14 (MK20 is the wife's), is just opening it up, finding it nice and clean, cleaning it anyway, lube, and generally just put the same components back in. It just works.
******
G250 HP vs other 250's

Can I shift gears and questions to 2nd stages?
I overhaul what I have, so I don't have the extensive experience some of you folks have. The MK20 is with a G250HP.
My MK14 has a much simpler/older G250. Seems the G250HP is not nearly as well thought of in the form as the G250. Just from an overhaul perspective I find the G250 is simple, easy, and works quite well, maybe breathes better than the HP based on my highly technical inhale test.

I find the HP some kind of bored technician's overkill brainstorm. It works, but it's a pain to work on and tune compared to the older model. Is this the issue with the HP, that it's kind of a mess?

I JUST pulled the guts out of my G250 and put it in my 109 (old original poppet). It seemed like it works well so I'm looking forward to trying it next dive. I appreciate the information I've gotten from this site to help with that conversion.

 

[redacted]

If you are finding deposits in the ambient chamber, you are probably not soaking it enough. Mine have no such problem but I go with an 8 to 12 hour soak after SW trips.

I don't have a G250HP but I do have its older cousins, the G500 and S600. There is really no reason any of those balanced barrel poppet 2nds should not breath the same other than condition and tuning. The added complexity gets you a seat saver feature but I am happy just using a purge depressor for storage.

 

[fmerkel]

Oh, that's a long soak. I thought an immediate rinse, then a later/longer soak (5"-10") should do the job.

 

[Couv]

A good rinse between dives and days of diving is all that is needed because salt crystals do not have time to form. However, after a dive trip and before storage, a long soak is required-I like to soak everything overnight. Besides, I'm usually too tired and jet lagged to do much more than throw all my stuff into a water filled trash bin anyway.

Re acquiring seals in small quantities, RH alluded to an o-ring swap thread we had going in the DIY section. Try to locate it can bump it or start a new one.

Here is a cut and paste from a previous post I made re purchasing seals.....some of the info may be a little outdated:

I like the folks at O-Rings, Inc. Since 1958 - Buy Online
Once there click on the BUY ONLINE button
Then in the search window type in: m83248/1 (for Viton duro 75) m83248/2 (for Viton 90)

Then scroll down until you find the size you need and click the BUY NOW button next to it.

Note that buying only one will cost you a lot more per o-ring, so play with the quantity buttons and you will find that (just for instance) for a m83248/2-010 if you buy 70 of them, they are only .10 each and one will cost you .25

G250HP: See this recent thread What reg is this pt.2 & "Top 5 SP regs"

 

[fmerkel]

I definitely have the older 250 with metal adjustment knob, and the wife has the HP. The thing seems to work fine but it's far more of a hassle to overhaul and tune, which seems to be the consensus. I find it interesting that SP put out a bulletin to 'upgrade' older units to the HP. People did not seem to stampede to their LDS to do this.

I just took the internals out of the 250 and put them in a 109. Operation seemed completely normal so I think it was an OK donation. Looking forward to trying it. I used it with the stock poppet for ~6 months and it worked fine but the poppet has taken a huge set and it doesn't breathe as well anymore in that configuration.