Scubapro piston: is there a life after death?

[Zung]

Thank you gentlemen for your encouragements.

On to the real deal: I cured my MK20 #2 the "professional way" by replacing the piston, so I have the old one on hand, and I have this MK25 #3 that has an instant swing to 8 bar / 116 PSI, then a 1 min. creep to 9 bar / 131 PSI. OK, but not nice.

The "dead" piston is a current generation composite with a stainless steel shaft. Looking at the knife edge with a 15x and a 20x loupe, I can't see anything wrong (MK25#3-Before-01.jpg). But the side is a different story: there are quite a lot of very fine scratches that are not visible with the nake eyes, but very obvious with the loupe, given the proper lighting (MK25#3-Before-02.jpg).

Working the mattmagic on stainless steel is a lot harder than brass. I have to start with the 1500 grit to remove the scratches, and work my way up to 8000 grit, checking often with the loupe. After nearly an hour, I got fed up and call it a day. The thing looks good (...-After-01 & -02.jpg), even when compared to a new one (...-New.jpg), but my confidence is very low.

Using the alternative assembly procedure, I simply removed the original piston of the #3 from above, and plug the "revived" one straight in, turn on the air, and, to my surprise, IT WORKS! After less than a dozen purges, it lockups at 9.4 bar / 136 PSI and stays there. Recovery takes a second or less, and an hour later, it's still there.

Regarding herman's question, here's what I have at low air situation: tank pressure vs IP (bar/PSI)

150/2175: 9.4/136
125/1813: 9.4/136
100/1450: 9.4/136
85/1233: 9.4/136
70/1015: 9.4/136
60/870: 9.4/136
50/725; 9.3/135
40/580: 9.2/133
25/363: 9.1/131
15/218: 6.4/93

At pressure below 60 bar / 870 PSI, when the IP starts to drop a little, the lockup is less snappy and takes maybe a couple of seconds.

I'll leave the reg on for a couple of days and report back if there's any change. For now, it looks like there's a life after death. And you know what? I've very happy!

 

[Texfrazer]

Excellent Zung!

You and Matt will have to set up an Int'l School of Piston Polishing! You can give out official ISPP certifications for folks who've fixed their ISP issues.

 

[Texfrazer]

Hey Guys,

Since Zung has been experimenting with the pistons of his Mk10+ and Mk20's, I thought I might pass along a little info bulletin I ran across stating what we already know - that abrasions or other deformities on the piston can cause creep (or drift as they call it). Also of interest is that the MK10+ and Mk20 are only spec'd for Christo-Lube, and the use of other lubricants (especially standard silicone) will (their emphasis) affect the intermediate pressure.

Enjoy!

View attachment 248_MK_10.pdf

 

[Zung]

Here's another blown-up "Before" shot of the stem. There're some small scratches or roughness at the very edge I circled in red. I think this is what causes the small leak that leads to the creep, since this is the mating part.

There're also some small scratches along the body I circled in green; I don't think these can do any harm, they're not in contact with anything.

Why do these happen I don't know. I don't think it's some human error. Could it be some salt deposit that came in with the water? Could salt be hard enough to eat into stainless steel over time? Could it be some mud that came in the same way?

 

[redacted]

Here's another blown-up "Before" shot of the stem. There're some small scratches or roughness at the very edge I circled in red. I think this is what causes the small leak that leads to the creep, since this is the mating part.

There're also some small scratches along the body I circled in green; I don't think these can do any harm, they're not in contact with anything.

Why do these happen I don't know. I don't think it's some human error. Could it be some salt deposit that came in with the water? Could salt be hard enough to eat into stainless steel over time? Could it be some mud that came in the same way?

Great pics. My first suspect would be a careless tech.

 

[Couv]

Agree with Awap....rough handling or perhaps dropped. Good example of why a mat (or Mattboy) should be used on the workbench.

Edit with another thought: When placing parts into an ultra sonic cleaner or soaking tub, make sure critical edges like that are not in contact with anything.

c

 

[Texfrazer]

Edit with another thought: When placing parts into an ultra sonic cleaner or soaking tub, make sure critical edges like that are not in contact with anything.

That's a real good thought! Makes an argument for using a sub, plastic container to place inside the ultrasonic tub for pistons.

Also on this bulletin, it that states the HP seat in MK10+ and MK20's showed almost no wear after 1,000,000 (one million) cycles. If you assume an unrealistically high 60 breaths per minute (thus 60 cycles per minute), that's 16,667 minutes or roughly 278 hours of dive time and the seat would still not need changing. For those using the newer designs, I would question whether the seat would ever need to be changed. O-rings, yes. But the seat......

 

[Zung]

You mean the EB 248? I can't find any reference to 1 million cycles. I assume some marketing guy wrote it: that over 500 dives! I have one such seat that's clearly marked, but I got it from eBay, so its lineage is unknown.

Edit:
Here's the seat: the lighter "ring" inside the upper part of the cavity in due to wear. And I'm pretty sure this one doesn't have half the advertized mileage.

Thanks for the EB 242, I was looking for it: it defines how bad is not too bad for SP.

couv: "When placing parts into an ultra sonic cleaner or soaking tub, make sure critical edges like that are not in contact with anything."

You're on to something my brother: sludge, on another board, said he noticed he had less problems with creep with the brass piston after he stopped ultrasonic'ing them. Me, I always use the plastic tray and I separate the "soft" parts and the "hard" parts.

 

[Texfrazer]

Whoops! This is bulletin 242. Sorry, I forgot to attach it! Thanks Zung.

View attachment 242_MK20-MK10_INFO.pdf

 

[halocline]

Certainly I've seen MK25s that have zero creep and zero evidence of wear in the seat after a few seasons of diving.