Scubapro MK15 servicing question

[bada3003]

I'm a novice at servicing 1st stage regs and would appreciate advice on a SP MK15 that I recently got used from ebay. The seller had no info on recent service nor its working condition. However, it was cheap and hoped it might serve as a practice tool to hone my skills before I would work on my MK25 (new) and MK17 (used but recently serviced), eventually. I read Wolfinger's book (1st stage parts), got some SP tools from scubatools.com, and jumped right in.

It wasn't just for curiosity's sake: the reg had a problem. When I connected it to an AL80 tank, there was a noticeable leak with air exiting from the ambient ports. IP was showing 90-100 PSI so one possibility was a failed/missing o-ring that sealed off the wet ambient chamber from the two IP chambers. Disassembly using SP tools was straightforward. I wasn't able to find a MK15 repair manual online but was able to compare the parts during disassembly with the diagram in

10-600-000_mk15_rev_b.pdf

To my newbie eye, there were no missing pieces and the various o-rings (and parts) seemed to be in reasonable shape. Except, the backup ring of the piston which sits next to an o-ring in an o-ring groove below the head of the piston. I've attached a picture where a crack in the white backup ring (seems to be plastic) can be seen:



The amber o-ring forms a good seal with the head of the piston except at the crack where there is a gap as gleaned with an o-ring pick. The piston stern o-ring looks ok but can't be sure until the reg is re-assembled with the piston backup ring (and adjacent o-ring for safe measure) replaced and subject to testing. I ordered MK15 repair/maintenance kit 10-600-041 through ebay so will know (assuming the damaged backup ring is part of the kit) soon. Sorry for the long-winded set-up but I wanted to be clear.

My questions are two-fold:

1. Should I apply Christo-lube to all o-rings and parts during assembly? That seems to be implicitly implied by Wolfinger's book but I wanted to be sure.

2. Should I be applying separate grease/gel onto the spring inside the ambient chamber? If so, is there a recommended type/make? Should I be very liberal with it? As you can see in the picture, there wasn't much grease surrounding the spring.

Thanks.

 

[redacted]

The split in the back-up ring is normal and necessary for installation. Most likely at least one of the piston o-rings is the culprit. If it is the HP piston o-ring, you will usually see quite a difference in the amount of gas leaking as tank pressure drops while the piston head will usually produce a fairly constant leak. In any case, you really should renew both o-rings while you are working on it.

I hit all o-rings and exposed threads with a light coat of christolube. Dynamic o-rings like those on the piston get a heavy application. No need to do anything with the spring or ambient chamber other than the threads. An exception is if you are going to pack the ambient chamber for cold water diving but that takes a bunch of christolube.

Those amber o-rings are probably around 15 years old so they have earned a retirement.

EDIT: BTW, when I say 'heavy' I mean I really lay it on. I lube the o-ring before installation and then I apply more christolube trying to fill the gland with lube. This is really important on the HP piston o-ring. If you ever open up a new Scubapro reg, you will see they really lather it on those dynamic o-rings at the factory.

 

[herman]

As AWAP said, the split in the backup ring is normal, they can't be installed if it's not slit since it is a hard plastic rather than a strechy rubber. I would add that you need to take a close look at the back up when you install it. Some have a concave side and a flat side, the concave side goes next to the oring. If you are not aware of that and not looking it is easy to miss.

 

[halocline]

No need to grease the spring, it's already coated with some protectant/anti-ice coating, that's why it's red. You need a set of skinny-nose snap pliers, which I assume you already have or you would not have been able to remove the circlip that holds the outer bushing in place against the HP o-ring.

Another tip, there is an inner plastic bushing on the ambient chamber side of the HP o-ring that looks like it should be removed. Don't do it! It's pressed in place and would be destroyed if you tried to dig it out. This isn't clear in the schematic that I've seen.

I'm not sure what you mean by piston "stern" (maybe stem?) o-ring or the "adjacent o-ring for safe measure". Just to be clear, there's one HP (piston stem) o-ring, which is sandwiched between the two plastic bushings, then the circlip to hold that arrangement together, then (if I remember) a chrome sleeve, then the washer/seat/retainer that goes in after the piston has been installed with a bullet.

I only have on MK15 and it get rebuilt about once every 5 years so I'm a little fuzzy on the details.

 

[bada3003]

Thank you for your very helpful comments.

awap: Thanks for pointing out the split backup ring which I misdiagnosed as being a culprit. Looking at the MK15 diagram again, the component is drawn with a gap which I hadn't noticed earlier. Will follow your recommendation to exchange both o-rings and apply christolube heavily. The service kit should arrive later this week. If there's still a leak, will check for leak variation as tank pressure is varied.

herman: Don't have the backup ring with me but will check if there's a concave side when I get back home.

halocline: Yes, "stern" should be "stem". I didn't remove the copper retaining ring and piston stem o-ring, one, thinking that I had identified the problem as the split backup ring, two, because I didn't have a sufficiently skinny snap ring plier, and three, the piston stem assembly looked to be the most tricky to put back together.

Based on the comments, replacing both dynamic o-rings seemed necessary so just ordered a snap ring plier and Scubapro Mk15 Bushing Tool from scubatools.com. Not sure if the latter was necessary but hopefully will help with reassembly. Will watch out for the inner plastic bushing. The schematic shows two o-ring washers, one on each side of the ambient chamber. I'm assuming one of them is the inner plastic bushing that should not be disturbed.

 

[halocline]

You also need a piston bullet to install the piston without destroying the HP o-ring. You might consider getting Vance Harlow's book on regulator repair, it has lots of information I suspect you will find helpful.

The bushings are not on either side of the ambient chamber, they are on either side of the HP o-ring. This o-ring resides in the channel between the ambient chamber and the HP seat. I'm not sure if this is obvious to you or not, if so, sorry.

 

[bada3003]

Thank you. I ordered the piston bullet too. I have Harlow's book as well but haven't had a chance to read it. Perusing it briefly, it seems to be more DIY oriented (as the book title indicates) with many helpful pictures. Will have to schedule some time this weekend to read it.

Thanks for the clarification on the inner plastic bushing "on the ambient chamber side of the HP o-ring". My confusion stems from the schematic indicating (I could be reading it wrong) that the amber HP o-ring is wedged between a o-ring washer (component #9) and a retaining washer (component #7). That is, not between two #9 plastic bushings. The other o-ring washer (same component #9) is shown located on the ambient chamber side. Your description makes more sense so I may be misinterpreting the MK15 schematic which I found at frogkick. They also call the components "washer" when "bushing" may be more accurate. I will pay attention during final disassembly when the snap ring plier arrives.

Lastly, followed herman's suggestion and checked the two sides of the backup ring. Both sides appear to be flat.

 

[halocline]

Thanks for the clarification on the inner plastic bushing "on the ambient chamber side of the HP o-ring". My confusion stems from the schematic indicating (I could be reading it wrong) that the amber HP o-ring is wedged between a o-ring washer (component #9) and a retaining washer (component #7). That is, not between two #9 plastic bushings. The other o-ring washer (same component #9) is shown located on the ambient chamber side.

Yep, my schematic says the same thing; essentially, you would install a nylon bushing (9), then the o-ring (8), then the brown plastic retaining bushing (7), then the snap ring. The bushing on the ambient chamber side is also nylon and appears identical to part (9). As I said, it's been a while since I've had mine apart. But since the air pressure on the o-ring is forcing it towards the ambient chamber, it's the bushing (9) directly on the downstream side of the o-ring that takes all the force. So I assume they felt that an upstream nylon bushing was not necessary.

When SP went to the MK20, they replaced this arrangement with a bushing/o-ring/bushing/spring arrangement that was considered easier to install.

 

[bada3003]

The follow-up tools from scubatools.com arrived a couple of days ago, disassembled and the parts are as noted in the schematic. The retaining washer, bushings and small piston shaft o-ring took some effort to dislodge. As awap remarked, parts may not have been replaced for a while (possibly never). The kit finally arrived today but a key piece, the large amber piston o-ring, was missing. Oh well. The seller's gonna send a replacement Monday. On a side note, I did get a chance to read the first two chapters of Harlow's book. Seems to be an excellent book. The lengthy quote from Nevil Norway's book is a nice touch.

 

[herman]

I may be off base but don't assume you don't have one just because there is not an amber one in the pack. Is there a black one of the correct size? awap and couv do a lot more with SP than I do but I know at one time SP kits were a rainbow of colors but have since gone back to basic black for the most part.