SP MK 10 PLUS: please help ID

[halocline]

If it creeps, try polishing the brass tip with a product called micro mesh. Start with 1500 grit, lap sand (to flatten the seating surface) then polish the outside of the edge. Repeat this process with a couple higher grits, and see if that helps. I got one of the brass-tipped MK20 pistons to stop creeping that way, but eventually replaced it with the MK25 piston and the difference was impressive. The MK25 piston is super stable. I've had friends go years of regular diving without any creep at all.

 

[Zung]

Thanks mattboy, I'll look for it.

Now, since I was attracted to the dark only last summer and I can sorely feel my lack of experience, please tell me if I've got it right: creeping means HP air leaks into the IP "chamber" until 1 of the 2 following situations occurs:

1. The IP increases, adds more pressure to the piston and "balances" the HP and the creeping stops
2. The "balance" can't be achieved, the IP keeps going up until the 2nd (or something) freeflows

The 1st case only puts somewhat more strain on the seals, and the second case can be more "embarrassing".

The reason why I'm asking is: my primary is a Mk20/G250 from the 90's (don't remember exactly when, must among the early production), and my wife's Mk10/M5 has been converted to a Mk10 Plus in 1996. So I assume both pistons are not the latest & greatest. However, we've never experienced any embarrassing situation so far.

Any thoughts?

 

[redacted]

Pretty hard to tell what is going on without putting an IP gauge on it. For the most part, creep is a symptom and not usually a problem until it has progressed quite a bit. While you are breathing the reg, the effects of creep will usually go unnoticed as you will cycle before the HP leak will build up pressure enough to become a problem. But creep does not get better or go away by itself. Without an IP gauge, it might take a year or more to notice a problem.

 

[Zung]

I have an IP gauge, but I'm a warm water sissy and don't own a tank:depressed:
But I'll get organized, rent, borrow or steal, cross my heart.

The only "real" experience I have so far is from last October in Dahab, Egyptian Red Sea. I finished servicing son #2's G250 there; his Mk10 was one of my spares, serviced in 2000 by a LDS and put in a closet since. The Mk10 started at 123 PSI, quickly crept to 130, then slowly to 145 over about 2-3 minutes. The G250 was tuned accordingly: freeflow + 30°. 10 dives later he didn't complain, but 145 is still within specs.

 

[halocline]

Some MK10s don't seem to lock up as immediately as, say MK5s, but creeping from 123 to 145 is really excessive. If one of my MK10s did that I'd take it apart immediately. Basically, when you take a breath or tap the purge, you want to see the IP gauge dip, then come back up to where it was immediately when you stop, and then stay there.

You're right, IP creep is a slow leak at the seat, which causes the piston to need more force to completely seal out HP air from the IP chamber. Higher IP itself is not hard on the seat, because the IP is working against the spring force. For an example, if you add a shim, you increase the spring force, so you need an increase in IP to counteract that. What is hard on the seat would be an increase in the seating force, IOW how much pressure against the seat does the piston need to seal completely. This is primarily a function of how perfectly (or imperfectly) the piston stem edge matches up with the hard plastic seat.

I think the early MK20 pistons were okay; it was the middle generation that had that brass tip. It's obvious when you look at the piston which one you have.

The G250 2nd stage will be fairly tolerant of IP changes, as it's balanced and the rise in IP will result in a corresponding increase in balancing force in the balance chamber. So, you could tune it to a low IP, say 125, and then it might not freeflow at 145.

I have rebuilt MK10s, then not dove them, let them sit for a few months, put an IP gauge on them, and they've creeped a bit. I've found that they need to break in a little. I have never experienced this on a MK5. Awap thinks this must be due to the smaller piston head on the MK10 not providing as much leverage on the piston edge, and as usual, he's probably right. Anyhow, the point is, you could hook the errant MK10 up to a tank, cycle it a few hundred times (light tap on the purge) let it sit pressurized for several hours, cycle some more, then see how it looks.

If you're going to get into reg repair you're of course going to need a tank. Any cheapie will do the job.

 

[DA Aquamaster]

A Mk 10 should lock up fairly solid and not creep more than 5 psi. After a break in period the crrrp should be essentially zero.

A damaged seat will cause creep as well as a damaged knife edge on the piston. If changing the seat does not resolve it, changing the piston will rsolve it about 99% of the time.

A situation where the IP creeps up to some level then stops suggests an imperfection in the knife edge. With enough pressure it will fially seal, but only for a while as that imperfection will over time damage the seat and a leak will result.

 

[halocline]

A damaged seat will cause creep as well as a damaged knife edge on the piston. If changing the seat does not resolve it, changing the piston will rsolve it about 99% of the time.

I was able to find a NOS MK10 piston a while back, and it locked up close to perfectly. Further, there was less IP rise at very high tank pressures; I think this was because the piston surface in the area of the HP o-ring was in pristine shape, where my used ones have some wear there, increasing friction. So, I took some micromesh and carefully polished out the glaze on the stem, then wrapped the micromesh around a pencil point and used it to clean up the inside of the edge; I was pretty careful. It actually worked out pretty well, and I still have it as a back up piston. It's tough to find MK10 pistons new, isn't it? Does SP still have any in stock?

As always, I appreciate your responses on these threads and always learn something.

 

[DA Aquamaster]

I am pretty sure new pistons are still available from SP.

 

[Zung]

and I need this board's collective wisdom:

I can't find the washer that goes between the swivel retainer and the swivel cap, part #01-060-500, position 16 in the revision C 12/97 schematic, or pos. 25 in the rev. M 02/01.

From the attached cutaway, I guess this part (painted red) doesn't serve any critical purpose other than soaking up the slack in between and maybe providing some friction.

I find that a 2-013 o-ring kinda fits OK in there. Question to the board: if I did this, will there be any dire consequences?

Thanks

 

[halocline]

One very important thing that that washer does is allow the turret to move freely without tightening or loosening the turret bolt. I'd find a teflon or hard plastic washer that fits instead. Actually, I've found that washer at a few SP dealers, it shouldn't be that tough to find.