How to restore a Knife Edge Piston

[buddhasummer]

Deleted, off tangent rant. This is the DIY forum isn't it?

 

[Angelo Farina]

Would you allow your young son/daughter who has just received OW certification to use this regulator...and all the other old junk that comes along with it...SPG/primary second/octo/inflator hose...etc...

This is a relevant point, and merits to be discussed further.
Disclaimer regarding my own position and view: I am an engineer, with a PhD in Applied
Physics, and I started servicing my own SP regs only after being trained on their maintenance directly at the factory where they are manufactured, here in Italy (Casarza Ligure, near Genoa). This means that not necessarily my view can be extended to general users. Keep this perspective when you read the following.

Yes, a reg is a life-supporting device, hence it deserves proper care and maintenance. I would never leave it to someone not possessing at least my same level of qualification and expertise. Particularly when this equipment has to be used by my sons or my wife. I and my wife did train our sons to scuba diving since very small age (they were about 4 years old when we started diving with them in the sea).
For children so young it is better to keep the equipment to minimum: a small 5-liters cylinder, no BCD, no second reg or octopus, no SPG, a thin wet suit (possibly a shorty), mask and fins.
We did choose to use old MK5+109 for these dives, for their reliability. Only problem is that they are a bit heavy in the mouth.
Of course serviced and properly tuned by me. I could not ever think as one can bring such young children underwater using rented equipment, not carefully serviced personally.
In conclusion, I never think that having a reg serviced in a shop can be safer than servicing it yourself, and this holds particularly true when the reg has to be used by your sons. And vintage equipment is usually much more reliable than new items.

But now back on topic.
The modification described in this thread is interesting, but I had never taken this route.
Shortening the piston means that its sealing dynamic O-ring will start working dangerously close to the edge of the machined surface in the regulator's turret. The risk that the movement of the piston causes the O-ring to exit the machined surface and starts leaking, or, worst, get trapped on the edge locking the piston in a position which closes the flow, is significant and not worth the minimal cost of replacing the whole first stage. I know that Couv has a number of MK10 in mint conditions, if I get a damage as the one shown here on the piston of one of my glorious MK5, I could not think anything else than purchasing a "new" MK10 for replacing it.
I would never buy a new reg as the one you suggested, indeed: in my opinion a reg becomes safe to use only after at least 5 years of being used, and serviced at least twice (the last time by me, of course). I have seen new regs fail at their first usage... QC in the factory is never good enough, 5 years of usage, instead, give proof that everything is OK.

 

[halocline]

T
Shortening the piston means that its sealing dynamic O-ring will start working dangerously close to the edge of the machined surface in the regulator's turret.

What do you think, you chop off the end of the piston shaft with a hack saw? Jeesh....we're talking about a few thousandths of an inch AT MOST and you seem to think that will put the piston out position?

The MK10 service kits come with 3 different seat heights, and using one of the shorter seats would drop the piston head position in the turret hundreds of times more than polishing the edge. Is that dangerous too.....

 

[Open Ocean Diver]

This is a relevant point, and merits to be discussed further.
Disclaimer regarding my own position and view: I am an engineer, with a PhD in Applied
Physics, and I started servicing my own SP regs only after being trained on their maintenance directly at the factory where they are manufactured, here in Italy (Casarza Ligure, near Genoa). This means that not necessarily my view can be extended to general users. Keep this perspective when you read the following.

Yes, a reg is a life-supporting device, hence it deserves proper care and maintenance. I would never leave it to someone not possessing at least my same level of qualification and expertise. Particularly when this equipment has to be used by my sons or my wife. I and my wife did train our sons to scuba diving since very small age (they were about 4 years old when we started diving with them in the sea).
For children so young it is better to keep the equipment to minimum: a small 5-liters cylinder, no BCD, no second reg or octopus, no SPG, a thin wet suit (possibly a shorty), mask and fins.
We did choose to use old MK5+109 for these dives, for their reliability. Only problem is that they are a bit heavy in the mouth.
Of course serviced and properly tuned by me. I could not ever think as one can bring such young children underwater using rented equipment, not carefully serviced personally.
In conclusion, I never think that having a reg serviced in a shop can be safer than servicing it yourself, and this holds particularly true when the reg has to be used by your sons. And vintage equipment is usually much more reliable than new items.

But now back on topic.
The modification described in this thread is interesting, but I had never taken this route.
Shortening the piston means that its sealing dynamic O-ring will start working dangerously close to the edge of the machined surface in the regulator's turret. The risk that the movement of the piston causes the O-ring to exit the machined surface and starts leaking, or, worst, get trapped on the edge locking the piston in a position which closes the flow, is significant and not worth the minimal cost of replacing the whole first stage. I know that Couv has a number of MK10 in mint conditions, if I get a damage as the one shown here on the piston of one of my glorious MK5, I could not think anything else than purchasing a "new" MK10 for replacing it.
I would never buy a new reg as the one you suggested, indeed: in my opinion a reg becomes safe to use only after at least 5 years of being used, and serviced at least twice (the last time by me, of course). I have seen new regs fail at their first usage... QC in the factory is never good enough, 5 years of usage, instead, give proof that everything is OK.

 

[Perryed]

What do you think, you chop off the end of the piston shaft with a hack saw? Jeesh....we're talking about a few thousandths of an inch AT MOST and you seem to think that will put the piston out position?

The MK10 service kits come with 3 different seat heights, and using one of the shorter seats would drop the piston head position in the turret hundreds of times more than polishing the edge. Is that dangerous too.....

This was my thought exactly. Also the fact that the knife edge embeds itself deeper into the seat deeper than anything trimmed off . Now if it had a chip in the 1mm range, yea, replace. But even .1mm or so? Go for the rebuild and enjoy the process.

 

[Angelo Farina]

It must be said that using an higher seat, the head of the piston comes back to be exactly where it was planned to be, so there is no risk for the O-ring.
Instead I do not like the idea of removing the plastic washers for having the piston staying lower...
So one of the proposed mods is acceptable to me (using MK10 seats), the other is not (scraping the original seat, making it further lower, and then removing the shims).

 

[kelemvor]

Got a line on a source for mk10 pistons? My honker isn't all that creepy but I think during the next service I might replace it.

 

[Couv]

Got a line on a source for mk10 pistons?

I might have a MK10 piston-PM me if you need one..........BUT.............

My honker isn't all that creepy but I think during the next service I might replace it.

If you mean you have a MK7 (Honker) you'll need a MK5 piston.

 

[SCUBA BAH]

Thank you.

 

[Bloody Knuckles]

I'm a long time reader and a first time poster. I first read this thread a little over a year ago and decided to review the procedure and get a refresher on the technique.
I've come to realize a few things...
1) RSingler, your the balls. Thanks for all you do
2) Couv...Although we only shared a few emails together. I truly miss you and your guidance on this forum. I hope the "Angelfish" are treating you well.