MK25 bubbles from turret swivel

[guruboy]

OK, I dismantled the MK25, the ambient chamber, swivel turret, and turret bolt with plastic washer look OK, the threads on the turret and bolt mate well and it all looks straight, o-ring looks OK, not much lube in there and a little bit of gunk and a few scuffs and scratches on the metal mating surfaces, the turret does have a bit more wobble than a new MK25, I don't see anything that looks obviously caused by an impact, any checks I should do, any thoughts?

Check it with a straight edge. Doesn't take much to cause a leaky seal.

 

[fmerkel]

I believe there should have been a small white/plastic/nylon thrust washer in there, at least there is in the MK20. That still there and not damaged?

 

[tstormdiver]

Except the MK25 doesn't use a knife edge piston.

Uhmmmm.. of the hundreds I have serviced, Yes, each & every one of them does have a knife edge. I've cut several stem O-rings in my early days. Sorry... The older MK20 did not have the knife edge (blunt),... the MK25's definitely do. In fact when I went through Scubapro's Regulator training a few years ago, We were told to use the bullet & to also use a Q-tip swab to check for burrs on the knife edge.

 

[redacted]

Uhmmmm.. of the hundreds I have serviced, Yes, each & every one of them does have a knife edge. I've cut several stem O-rings in my early days. Sorry... The older MK20 did not have the knife edge (blunt),... the MK25's definitely do. In fact when I went through Scubapro's Regulator training a few years ago, We were told to use the bullet & to also use a Q-tip swab to check for burrs on the knife edge.

The Mk20/25 composite piston sealing edge is like a butter knife. It does not cut into the HP seat material and is unlikely to cut an O-ring. A Mk5/10 piston sealing surface is like a steak knife. It has to cut into the HP seat and may cut an o-ring. The bullet tool keeps that from happening with a Mk5/10. With the Mk20/25, it simply keeps the piston edge from dislodging the o-ring.

 

[tstormdiver]

The Mk20/25 composite piston sealing edge is like a butter knife. It does not cut into the HP seat material and is unlikely to cut an O-ring. A Mk5/10 piston sealing surface is like a steak knife. It has to cut into the HP seat and may cut an o-ring. The bullet tool keeps that from happening with a Mk5/10. With the Mk20/25, it simply keeps the piston edge from dislodging the o-ring.

You might want to take another look. It is sharp enough to cut/ knick the O-rings.

 

[halocline]

You might want to take another look. It is sharp enough to cut/ knick the O-rings.

Definitely not a sharp edge like the Mk5/10/15. I don't doubt that SP is now telling techs to use a bullet on the MK25, but for several years they did not, and the composite piston has not (to my knowledge) changed during the last several years. Maybe I'm wrong about that.

We've had this discussion before, I really don't understand why you, a SP technician, can't seem to differentiate between the sharp knife edge pistons of the MK5/10/15 that are designed to cut a groove into the seat, and the rounded edge pistons of the MK10+/20/25 that are designed to seal against the side wall of the cupped MK20/25 seat. I believe that a sharp piston would quickly destroy one of those seats.

I understand that the composite piston edge is not as squared off as the brass tipped piston from the middle generation MK20, but as a group these pistons are fundamentally different than the earlier knife edge pistons. The rounded piston edge is for aerodynamics and (supposedly) longer seat life.

 

[tstormdiver]

Halocline, there is nothing to "differentiate", I call it as it is. It can & will cut/ nick the O-ring. Yes, maybe you'll get lucky & insert the piston without cutting the o-ring, perhaps thousands of times, ... but eventually it will happen, giving you an irritating whistle, or screech,.... or perhaps an IP creep. It only takes a microscopic blemish for it to happen. Don't believe or like it, feel free to contact Scubapro's lead engineer.

 

[Tbar]

Halocline, there is nothing to "differentiate", I call it as it is. It can & will cut/ nick the O-ring. Yes, maybe you'll get lucky & insert the piston without cutting the o-ring, perhaps thousands of times, ... but eventually it will happen, giving you an irritating whistle, or screech,.... or perhaps an IP creep. It only takes a microscopic blemish for it to happen. Don't believe or like it, feel free to contact Scubapro's lead engineer.

I agree that you can nick the Piston o-ring, but I have only done this when I have taken the ambient chamber off not when I have done a full re-build.

As far as this nicked o-ring impacting on IP creep, I am not sure I understand how that will occur, the gas comes past the o-ring and out the ambient chamber when it fails. Or are you suggesting this can increase friction on the piston?

 

[halocline]

Halocline, there is nothing to "differentiate", I call it as it is. It can & will cut/ nick the O-ring. Yes, maybe you'll get lucky & insert the piston without cutting the o-ring, perhaps thousands of times, ... but eventually it will happen, giving you an irritating whistle, or screech,.... or perhaps an IP creep. It only takes a microscopic blemish for it to happen. Don't believe or like it, feel free to contact Scubapro's lead engineer.

In a balanced piston 1st stage, IP creep can only be caused by a faulty seal between the piston and seat. Damage to the HP o-ring would not cause IP creep. You are confusing issues.

Also, to say that there is nothing to differentiate between the MK5/10/15 style of piston and the MK20/25 is simply false; they seal differently and there is a fundamental difference in the edge design. You should know that. I realize that they are telling you to use a piston bullet on the MK25, and I'm not disagreeing that it's a good idea, provided that you have a bushing insertion tool that allows for it. But it's nothing like the old sharp pistons.

You mentioned that SP is recommending that you use a Q tip to check for burrs on the piston edge. That's an interesting idea and thanks for mentioning it. Can I ask what they recommend if you find a burr? New piston?

 

[axxel57]

I think one probably gets away with no damage of the piston stem o-ring by not using the bullet, and of course is the piston edge of the MK25 composition piston not nearly as sharp as the one of the MK5/10/15, but it is also definitely not as blunt as the ones of the early MK20s.

The 'Leading Engineer' of SP who conducted the Technician Course recently was using a bullet.

As a Pro I have a lot of specialized tools and of course also a couple of bullets.

I don't take chances in my work, so I usually use the bullet.

I'm pretty sure for DIYer it's not really necessary to use that tool if they carefully install the piston, but otoh it's not really an expensive tool...........