Scubapro experts needed...

[50Fathom]

DA Aquamaster and others:

I don't have my parts book in front of me right now, but I have a question. I just came into a couple of Mk15s in good shape, (aside from needing a full service and overhaul) but one is missing the piston spring. Is the piston the same as the Mk25, (IE, I could just order a Mk25 spring), or if different, is the Mk15 spring still available?

Other questions to follow once I get my digital camera out.

Thanks.

ETA: I am to understand that the Mk15s are notorious for being "screamers" when the piston moves. I've heard the solution is to discard the spacer ring on the piston head and use two o-rings Mk20/25 style. Truth? What's the best way to quiet them if they scream? (aside from flipping or rotating the spring.)

 

[DA Aquamaster]

I'll check part numbers in a day or two, but I think the springs are different. In any event I have a couple update kits that I'venever used and will probably never use as any MK 15 that needed it already has it and they include a new spring. Beyond that I am unsure whether the part is still available but I can check the parts list and see if it is still available.

I have never heard of the MK 15 being a screamer any more than the MK 10 or MK 20/25. I have also never heard of replacing the backing o-ring with a regular o-ring. Off hand I suspect it would create problems with one o-ring trying to roll the other one.

Proper piston stem o-ring lube is the best defense against resonance due to piston vibration. Adding or removing a shim also changes the working range and resonance of the spring/piston.

 

[50Fathom]

Sounds reasonable. I'll stay tuned, and if I need an update kit, (or two), I'll drop you a line, so please set aside a couple for me!

As far as the piston head o-rings rolling one another, why isn't that a problem on the Mk20/25? I've wondered that ever since I saw the Mk15 used the spacer or "backing o-ring" I guess it is correctly called. If that design was correct, why change? And if either is "safe" from a design standpoint, is one better than another?

 

[Zung]

The current MK20/25 composite piston has 2 groves/journals, one for each o-ring, so they operate independently.

The previous stainless steel piston had 1 single large grove to accomodate an o-ring and a wiper/backing ring. While the o-ring is soft, the wiper is hard so they don't intefere with each other.

 

[axxel57]

The piston spring of the MK15 is part number 01.020.600, the one of MK20/25 is 01.020.225
The piston part number is 10.600.103, the one from MK20 is 10.600.155
If I remember right the first MK20 were using the same pistons like the MK15. I'd rather get me a new piston model (MK25) if you're concerned about noises than trying to use two O-rings in the MK15 piston. And if I'd do it I would check it on my Flow Bench first before doing demanding dives.

 

[DA Aquamaster]

I thought I better check before I responded to ensure there was no misinformation. As noted above the part number for the Mk 20/25 and MK 15 are different - and the original Mk 15 spring is no longer available - but SP is specifying 01-020-225 to be used for the MK 15 spring.

In contrast, SP does not specify the MK 20/25 piston for the MK 15 and while the MK 15 piston and the early MK 20 spring look the same, they are dimensionally different and not interchangeable. If they were SP would be recommending techs use the MK 20/25 piston rather than replace first stages with bad pistons that are still under warranty.

 

[DA Aquamaster]

The second o-ring and the plastic back-up ring both perform the same "wiper" function to keep the sealing o-ring clean and I don't think there is much difference between the two approaches but using two o-rings reduces the number of unique parts in the reg.

 

[halocline]

I have a spare MK15 spring somewhere in an update kit I believe as well, and my MK15 is already updated. It would be interesting to compare the MK15 and MK25 spring. For some reason I feel like the MK25 piston is a bit shorter, but I'm just guessing. My MK15 has never made any noise, unlike a few of my other piston 1sts. I feel that the MK15 is unfairly maligned because it was a bit of a challenge to service; there's a snap ring that holds the HP o-ring bushings in place that needs a long skinny set of snap ring pliers to remove and install, and one of those plastic bushings, the one on the ambient side of the o-ring, is permanently pressed into the case. I'm sure more than one uniformed tech has ruined a MK15 body trying to dig that bushing out. Mine has been super stable for years and I would far prefer the MK15 to one of the middle-generation MK20s with the creepola brass piston.

Where in central TX are you located?

 

[DA Aquamaster]

I agree - there is basically no performance difference between the Mk 15 and MK 20/25, the M15 just caused more whining among techs.

 

[50Fathom]

I have a spare MK15 spring somewhere in an update kit I believe as well, and my MK15 is already updated. It would be interesting to compare the MK15 and MK25 spring. For some reason I feel like the MK25 piston is a bit shorter, but I'm just guessing. My MK15 has never made any noise, unlike a few of my other piston 1sts. I feel that the MK15 is unfairly maligned because it was a bit of a challenge to service; there's a snap ring that holds the HP o-ring bushings in place that needs a long skinny set of snap ring pliers to remove and install, and one of those plastic bushings, the one on the ambient side of the o-ring, is permanently pressed into the case. I'm sure more than one uniformed tech has ruined a MK15 body trying to dig that bushing out. Mine has been super stable for years and I would far prefer the MK15 to one of the middle-generation MK20s with the creepola brass piston.

Where in central TX are you located?

Thanks for the help all! I've only serviced one Mk15 and it was ages ago.

I'm in San Marcos, right up the road from SA.

I looked at the one that is missing the spring, (since it's screwed together loose), and what do you know, some duffer buggered up the body right at that pressed-in ring. Oh well. I'll tear into the other one today and see if it's saveable.

I've got a Mk20 that is pretty much straight, but with the creepola piston. I've got a new piston on order, so that will get straightened up right quick.