Scubapro MK20/G250: Ten Years After

[Zung]

Another data point: 20 years?

I just scored a near-NOS G250 off the 'Bay, paid a bit too much, but that's off topic. It has the 4th generation decal on the purge button, all metal adjustment knob, and medium-short VIVA adjustment. According to the SP Museum, that's 1991. Can any guru confirm the dating?



Tested as-is, it worked fine, the cracking pressure is 24mm/.94" of water. But because it's at the very least over 10 years old, I opted for a service.

Inside, everything is shiny as it should; I don't think this thing has seen much water, if any. It has the old poppet with the metal stem, and the seat is deeply marked.



I replaced all the soft parts and fitted a current S-wing poppet, tuned it to the standard 1 hour pass sealing point. Now the cracking pressure is 22mm/.87" of water: nothing gained but peace of mind.

So: possibly 20 years of engraving doesn't seem to affect the performance of this reg. Would you dare diving it? I wouldn't, that why I serviced it, but there's a pretty good chance it's still good as-is.

 

[halocline]

The only things that could make it dangerous to dive would be a massive leak or lever failure, and that's assuming you have no access to an alternate air source AND you don't have immediate access to the surface. You don't dive like that, do you? So sure, I'd "dare" to dive it.

OTOH, I usually-not always- rebuild a reg when I get it, even if it passes an initial inspection, mostly because I'm curious to see it inside, and once it's apart it makes sense to replace seats and o-rings, neither of which cost me very much. Plus, we do this for fun, so rebuilding is hardly a chore. You did the right thing, and now you have a reg that you know inside and out.

 

[Couv]

Nice find.

I'm impressed that the cracking pressure was so low considering the depth of engraving on the soft seat. Did you receive that G250 with the adjustment knob cranked in all the way?

I cannot make out from the pix. Is that a flat, replaceable, balanced soft seat? I don't think I've come across those before. That would be the best of both worlds-an Awap seat that can be used in a balanced poppet.

Lastly, I would not have any issues diving a reg that passes inspection either.

c

 

[Zung]

Bro, the knob was nearly all the way out when I got it, I think.

The metal ring in the middle of the seat appears to be some sort of rivet. I too had some hope it could be an "awapable balanced seat". but the rivet resisted my (light) attempts to remove it. Maybe I'll try harder next time, it's dead anyway.

 

[Six2Life]

I spent about 3hrs yesterday diving a G250 that sounds like it is very similar to the one you just got Zung.

A couple months ago I bought a MK20/G250/R190 set off craigslist that looked practically new. Looking at the scubapro reg museum indicates it was bought new somewhere in the 1996-1999 timeframe. The lady I bought it from said it was used for her OW certification and that's it, never serviced. When I threw it on a tank it had a pretty healthy ambient chamber leak. Awap tore down the MK20 and fixed the leak but the G250 didn't really get serviced. He pulled the internals out to take a look at them but no adjustments were made to the orifice or anything. It has the older poppet like Zung's. I didn't see the seat but maybe awap can remember if it looked similar to the photo above. I'll try to post a photo of it later on.

Should 10-15 years of seat engraving raise or lower the cracking pressure? I don't know what mine is but I do know it doesn't take much to get it to freeflow. Yesterday while diving I turned the venturi to min, and the spring tension all the way in and it would still freeflow if I pulled it out of my mouth underwater.

 

[Couv]

Seat engraving in itself does not necessarily increase cracking pressure. However, a heavily engraved seat is sometimes symptomatic that there has been a bit of freeflow that was corrected by adjusting the orifice in, which does increase cracking pressure.

On an adjustable regulator, a seat can also suffer deeper engraving if the adjustment knob is run in while in storage. As Brother Zung received that reg with the knob out, the seat was engraved, but still had low cracking pressure has me scratching my head.

 

[Six2Life]

Here is a photo of the seat out of my G250. It looks like it is engraved pretty deeply as well. Also it looks like the top hat poppets that I've been pulling out of some 109's. How many different poppets did SP use before the S-wing?

 

[Zung]

Here's what mine looks like from the side.

 

[Six2Life]

That hole near the seat sure looks similar to the one on the Duro poppet that I assumed was there to aid in pushing the seat out for replacement. If the hole in your G250 poppet is there for the same reason I'd think that one you have would be pretty desirable for a DIYer. If it is indeed awapable...

It looks like the seat slides under the metal center rivet. I wonder if you could push the outside edge of the seat up through the hole on the side, and once the outside edge is clear grab it and try to work the center hole out over the rivet. Although maybe the rivet is staking the seat in there pretty good.

I'd post a picture of a side view of my poppet but I just dropped that set off at the local SP shop to have the yoke retainer on the MK20 replaced. Just picture a top-hat poppet from a 109 but blue and with a metal tail sticking out the back.

 

[Zung]

Tried that, there're actually 2 holes, 1 on each side, but the center rivet is holding the seat down very firmly. Oh well, the Trident seats are not that hard to get...

Edit

But, wait! Look what a properly sized watchmaker's screwdriver can do!



External diameter 6.95mm, width 1.95mm, and the diameter of the pin is 2.00mm. It's flippable and awapable! Just let me figure out a way to punch the center hole...