What’s the difference between a MK5 and a MK10?

[halocline]

Mine looks most like the Black one ... but the 2 split rings are the same thickness ..I am the first to take this apart ..Imagine my surprise when I took the top cap off to inspect and maybe "flip" the H.P. seat ..It's not in the cap any longer ...weird

If yours looks like the black one, it is the composite piston, meaning the piston head is plastic. There are no plastic split rings used on that one, just the two o-rings. That is the same piston in the MK25. If your piston head is metal (SS) then you have an early MK20 piston, which works fine. But it uses one 022 o-ring and a plastic split ring. The split ring holds the o-ring in place. The picture I attached shows each generation of piston with the correct arrangement of o-rings and split rings.

Thanks for the reply ..Yup .I have a piston "bullet" ...thing is when you go to use it the 3 piece bushing/o-ring deal pushes out the top of the reg .. I read about a trick putting the spring and seat in to hold the bushing/o-ring assy while you put the piston in ..I did that ..had it installed ..These "newer" pistons seem less sharp than the old mk5 & 10 .. anyhow I got that together but couldnt get the cap on ..It went past the first wiper ring and the o-ring but I think the o-ring caused the second wiper to extrude rather than allow the piston to go in ..Looks like the mk25 uses a different Piston without the wiper rings ...

You need the bushing tool to hold the bushings and HP o-ring in place while you push the piston/bullet into place. I would not try to rebuild one of these without that tool.

Scubapro Mk20/Mk25 Assembly Tool

Scubapro Mk20/Mk25 Assembly Tool The Mark 20 Assembly tool is used to install the piston stem bushing system and retain the bushing system while the piston is inserted into the main body.The top illustration to the right shows the relationship of the tool and bushing system before the parts are...

scubatools.com

 

[Pressurehead]

Ah, another fanboy for sealed regs!

Scubapro Mk19, perhaps?

Watching your YouTube videos did this to me.
The Mk10,Mk10+ and the Mk25 now have little use .
This is only the first stages, how I now tune the 2nds is another story [G260, G250s, S600 etc]
I am borderline OCD now.

Oh, and loving it.
And sorry, off topic.

 

[MR12111]

If yours looks like the black one, it is the composite piston, meaning the piston head is plastic. There are no plastic split rings used on that one, just the two o-rings. That is the same piston in the MK25. If your piston head is metal (SS) then you have an early MK20 piston, which works fine. But it uses one 022 o-ring and a plastic split ring. The split ring holds the o-ring in place. The picture I attached shows each generation of piston with the correct arrangement of o-rings and split rings.


You need the bushing tool to hold the bushings and HP o-ring in place while you push the piston/bullet into place. I would not try to rebuild one of these without that tool.

Scubapro Mk20/Mk25 Assembly Tool

Scubapro Mk20/Mk25 Assembly Tool The Mark 20 Assembly tool is used to install the piston stem bushing system and retain the bushing system while the piston is inserted into the main body.The top illustration to the right shows the relationship of the tool and bushing system before the parts are...

scubatools.com

Thank You for all your help .. Here's a pic of the 1 I have with the parts removed ..I said it looked most like the black one because it just looked closest .. At this point I'm about to chuck this thing in the trash and go with some old one in good shape ..mk 5 or 10 ..

 

[halocline]

That's the early MK20 piston, and that's much better than having the middle generation brass tipped one. The one you have gets on of those white plastic split rings, then a 022 o-ring. The white plastic ring goes on the ambient pressure side of the o-ring. It's basically a spacer/backing bushing to keep the o-ring positioned in the groove. If you took two of those plastic split rings off, somebody made a boo-boo last time around.

I have been using MK5s and 10s for decades. I have never owned (for any length of time) a MK20 or 25, until very recently when I put together a pair of MK25s to eventually replace my MK15s, because I will run out of MK15 seats and they seem to be unavailable. Anyhow, I like the MK20/25 just fine. The IP recovery is pretty impressive. They are actually easier to rebuild once you are familiar with them because you do not have to remove and install the HP piston o-ring in that groove deep in the regulator body. (There's a great tool for that, BTW, made by herman on this forum.) They are also better for HP tanks IMO; the bushing system is designed to prevent o-ring extrusion at the piston shaft. The only problem with the MK20/25 is the cost of rebuild kits. So don't give up on it yet, just get the scubatools bushing installation tool, and you should be ok.

 

[lexvil]

Thank You for all your help .. Here's a pic of the 1 I have with the parts removed ..I said it looked most like the black one because it just looked closest .. At this point I'm about to chuck this thing in the trash and go with some old one in good shape ..mk 5 or 10 ..

White ring, o ring, white ring but I have a MK 5 I’ll trade for it

need to add this, since your 20 seems to be all original you should also change the yoke retainer, there was a recall on these.

 

[halocline]

White ring, o ring, white ring but I have a MK 5 I’ll trade for it

need to add this, since your 20 seems to be all original you should also change the yoke retainer, there was a recall on these.

I just looked through every year of MK20 schematics that I have (pretty much every year from 1995-2000 when they changed to the composite piston) and there's no mention in any of them of two plastic backing rings surrounding the o-ring. Was there a service bulletin or something? It wouldn't make sense to me, all the pressure is pushing the o-ring towards the ambient side.

Good point on the yoke retainer, I forgot about that.

 

[MR12111]

I just looked through every year of MK20 schematics that I have (pretty much every year from 1995-2000 when they changed to the composite piston) and there's no mention in any of them of two plastic backing rings surrounding the o-ring. Was there a service bulletin or something? It wouldn't make sense to me, all the pressure is pushing the o-ring towards the ambient side.

Good point on the yoke retainer, I forgot about that.

Thank You for helping me .. I am the original owner of this Reg .. Got it while divemastering for this scubapro shop ..It had the D400 on it .. went to 170 ft once in the Bahamas with it ..Traded regs with another Instructor I was diving with ..He had a Sherwood Magnum ..The Scubapro really showed the difference there ..from natural breathing to trying to suck thru a straw with the Magnum ..Ha ..
Still I just didn't like this 2nd stage touchy weird ..free flowed easy ..Traded the 2nd stage for a G250 and I still have that ..didn't like it to much either ..
What I'm getting at is I was the first one to take this apart and I'm sure it had 2 "backing rings" and 1 o-ring on the piston .. Piston was a little stuck in the bore with a little green corrosion around it ...guess sitting 15 years could do that ..
The only real problem is I cannot get it back together ... The Piston ...1 backing ring goes in then the o-ring goes but that 2nd backing ring just won't go in ..seems "maybe" the o-ring is pushing on it as I'm trying to get it in and causing it to just extrude ... It had to be put in somehow the first time .. Maybe some kind of Ring Compresser like they use on car pistons ? Ha .. Just frustrating ... I am sure I have serviced/repaired 1000's of Piston Regs. Change is fine ..Just wish I knew how to assemble this thing ..It would be cool to know why they made these changes too ..The mk 5 and 10 had some problems but working on them wasn't 1 of them ...

 

[MR12111]

I have the Yoke retainer bulletin ..will address that if/when I get this thing to work ..It was a long time ago ..Betting the shops know nothing about it ..

 

[MR12111]

White ring, o ring, white ring but I have a MK 5 I’ll trade for it

need to add this, since your 20 seems to be all original you should also change the yoke retainer, there was a recall on these.

Wonder why they made these changes ..This had sat for a long time and I was just going to pop off the top cap and look at the H.P. seat and maybe "flip" it ..Imagine my surprise when I saw the seat wasn't in the cap ... Then the changes with the Piston Stem o-ring and the actual piston o-ring setup .. I just wonder what the reason for the changes was .. The Piston also seems less "sharp" than the ones I dealt with (good thing) ...the rest I could do without "Change Is Bad" Ha ....

 

[lexvil]

Wonder why they made these changes ..This had sat for a long time and I was just going to pop off the top cap and look at the H.P. seat and maybe "flip" it ..Imagine my surprise when I saw the seat wasn't in the cap ... Then the changes with the Piston Stem o-ring and the actual piston o-ring setup .. I just wonder what the reason for the changes was .. The Piston also seems less "sharp" than the ones I dealt with (good thing) ...the rest I could do without "Change Is Bad" Ha ....

The less sharp was to help prevent seat cutting, the piston head changes I don’t recall what the justification was off the top of my head but if you search “scuba pro composit piston” you’ll find some insight. The yoke retainer was a bit long and over torquing caused some body cracks, the company may still honor the recall.

for the most part you can find very nice functioning first stages for less than the cost of upgrading and servicing this mk20. On a side note have you tried changing the position of the spacer rings on the piston head, meaning putting the one on the bottom on the other side of the o ring?