MK-5 vs. MK-10

[ams511]

Which is the better first stage, the Mk-5 or the Mk-10? and why is it better?

 

[redacted]

I was diving Mk10s for a while but have stopped using my Mk10s in favor of Mk5s (and a couple mk7s). I was having trouble with IP stability over the range of tank pressures with my Mk10s. In some cases I was experiencing a 10 to 15 psi drop from 3000 psi to 500 psi. I did not see even half that much drop with my Mk5s. I attributed the problem to friction between the -010 HP piston o-ring and the piston shaft. I was using duro 90 o-rings so they should have been hard enough. I tried viton and nitrile and finally urethane which is supposed to have better dynamic characteristics including extrusion resistance than the other materials. It did show some improvement but I was not as good as my Mk5s. I decided the problem tracked back to the designed piston head diameter. With the larger diameter piston head the IP produces more force on the piston head and requires a stronger spring to counter balance that force. The force of friction between the o-ring and the piston stem is probably the same in both design. So, with the smaller diameter piston head and lower associated forces, the force of friction with that o-ring is proportionally larger with a dirfference of about 2:1 which is about the difference I saw in IP swing between the two 1st stages.

I think there is a reason Scubapro went back to the -022 o-ring size piston head in its newer BP designs (Mk15/20/25).

 

[aquaregia]

I'm also seeing a 15psi drop on my Mk10, and paired with the 109 you really can feel the pressure drop as you get low.

My Mk5s are 2LP versions whereas my Mk10 is a 5LP version. I use the Mk5s for my double 72s and the Mk10 for singles. DIN conversions are expensive for the Mk10 but largely unavailable for the Mk5.

 

[redacted]

DIN conversions are expensive for the Mk10 but largely unavailable for the Mk5.

I did manage to come up with one Mk5 din. I rigged a 2nd one using the universal din retainer and plastic saddle from the Mk20 recall upgrade. That retainer is about 1mm too short to bottom out in the body when properly torqued with the side o-ring sealing without being crushed. I added a couple brass washers at the bottom as spacers. The I cut away the inner band on the plastic saddle and it fits OK with no binding. I have been using it for a couple years now with no problem. Without the added spacer, the O-ring is crushed and the saddle will bind on the din wheel. The washer should be of the correct OD and ID to distribute the torque as an original retainer would.

 

[halocline]

I'll take a 5 port, excellent condition MK5 over a MK10 any day, but for most divers MK10s are a little more practical, for 3 reasons. 1) some MK5 parts are harder to find; while there is a great NOS selection of MK10 parts out there. 2) MK10s all had SS turret retainers, while some MK5s had brass ones. The SS retainers are better; the older brass ones could easily be damaged by severe over torquing. A small group of us led by Zung went in on a reproduction run of SS turret retainers for the MK5, so we all have enough for our lifetimes, but if you buy a MK5 with a brass retainer that's been manhandled, you'll want to look for a replacement. 3) MK10s are much easier to convert to DIN if that's important. In fact, I use MK5s on my single tanks and my MK10s are set up in DIN for my doubles. 3A) in terms of buying, there simply are more great condition MK10s available; I've never bought one that was in bad shape, but I've seen a few fairly knackered-up MK5s on ebay.

DA Aquamaster wrote an excellent post about why he thinks SP went to the MK10 design, which includes one piece of milled brass for the ambient chamber and HP piston channel, as well as the smaller piston. He believes that SP wanted better tolerances on the piston channel, undoubtedly to address the extrusion problem of the HP piston o-ring and improve alignment with the ambient chamber/piston path, and that was more cost effective with the smaller diameter (or something like that). Later, when SP went back to the larger size, there was better machining technology to ensure the alignment with a 2 piece system. Or at least the post was something like that.

I like the MK5s because the lock up is SO stable and lasts forever. My MK10s will start to creep a few PSI after a season or two. I keep waiting for my tribolube-packed MK5 to show some signs of wear, but it's been a few years and the damn thing won't budge from 125 PSI. Like awap, I've dealt with the IP rise at higher tank pressures by experimenting with o-rings, but I've also tried new pistons and really polishing the piston shaft and cleaning the edge, and both of those things have helped significantly. I use them with balanced 2nds so I never notice anything in terms of increased breathing resistance.

The MK10 does have a higher flow rate spec, but I couldn't begin to tell a difference in breathing performance.

 

[Couv]

Most of the important stuff has been covered, but here are a few more points to consider:

MK5 can usually be bought cheaper than a MK10
MK5 is (subjectively) better looking.
MK5 is a little heaver-could be a plus or minus.

Some of the older MK10s do have brass turret retainers. One of my first eBay purchases had one and it was sheared so I got it really cheap. I also notice less IP swing with a 5, but only with a gauge. I have not been able to tell the difference while diving. Having said that, all my 2nd stages are balanced.

Re: the quicker IP recovery of the MK5. It's my guess that due to the larger piston head, more overall volume (larger accumulator at the same psi) exist in the cap of the 5 vs the 10; which makes me question the idea that a MK10 has a higher flow rate. Perhaps there is some volumetric efficiency design feature not apparent to us mortals.....where is LuisH when we need him?

One plus I find in the 10 over the 5 is the hex broach in the turret retainer. It's easier to remove/install and torque than the retainer in the 5. You don't need any Awap-ground-down-special socket. You can also put the final torque on the retainer from the outside by removing the end and inserting an allen bit through the turret. If you do it this way, remember you are torqueing counter clockwise.

 

[ams511]

DA Aquamaster wrote an excellent post about why he thinks SP went to the MK10 design, which includes one piece of milled brass for the ambient chamber and HP piston channel, as well as the smaller piston. He believes that SP wanted better tolerances on the piston channel, undoubtedly to address the extrusion problem of the HP piston o-ring and improve alignment with the ambient chamber/piston path, and that was more cost effective with the smaller diameter (or something like that). Later, when SP went back to the larger size, there was better machining technology to ensure the alignment with a 2 piece system. Or at least the post was something like that.

Actually that was the post (see number 6) that made me ask the question. I purchased some regs on Ebay and was going through them trying to decide what to keep and what to rebay. I prefered the MK-10 mostly for cosmetic reasons but I figured I would ask first to see what others think.

 

[DA Aquamaster]

Larger piston head on the MK 5 equals better response time and less IP drop with falling tank pressure as the Mk 5 and Mk 10 are. It perfectly balanced (drop in ip is about 4-6 psi.

 

[ams511]

as the Mk 5 and Mk 10 are. It perfectly balanced (drop in ip is about 4-6 psi.

I am unsure of what you are trying to say.

 

[Zung]

Interestingly, SP published better numbers for the MK10 in terms of flow rates at low tank pressures in its 1988 catalog:

• MK5: 87 SCFM @ 2000 PSIG, 48 SCFM @ 300 PSIG
• MK10: 80 SCFM @ 2000 PSIG, 73 SCFM @ 300 PSIG

And bragged about "... Tight orifices or bottlenecks were enlarged to allow the air to flow faster at lower pressures. The piston diameter was reduced and the stem shortened allowing for the use of a smaller, more lively, stainless steel spring."

I have 3 each (I think) MK5 and MK10, but we dive mostly with the MK20/25, so I don't really have any meaningful day to day experience. But I did have a lot more problems restoring the MK10 back to specs, mostly because 2 of my MK10 are actually MK10Plus with known issues with the brass piston stem and the sub-standard white HP seats. The MK5 just needed a cleanup and are ready to go.