Here's the basic:
- MK5: the ambient ports are located in the cap, the part that's right below the swivel. At that time, SP decided the crown of the piston should live in the cap.
- MK10: the ambients ports are on the body, 1 story below the cap, where the yoke is. SP changed their mind and moved the crown to the body.
Another way to tell them apart: the 5 has a metal saddle, the part between th yoke and the body, the 10 has a plastic saddle.
Inside, the 5 has a big piston with a -022 (i'm sure) crown o-ring, and the 10 has a smaller piston with a -017 (I think) o-ring.
I agree with most of the above - the stuff in regard to wether the ambient holes are in the swivel cap or in the reg body. But you will encounter Mk 5's with plastic saddles now and then so it is not definiitive. The last of the Mk 5s used solid black plastic saddles and many earlier Mk 5s have ended up with Mk 15, 20 and 25 saddles.
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The best approach I have found for replacing the piston stem o-ring on the Mk 5 or Mk 10 is to use a brass o-ring pick from parker seals (comes in the 2 brass o-ring pick set sold on scubatools). start on the ambient chamber side, poke the o-ring in the hole, nudge one edge of it into the groove with the rounded end of the pick, flip the reg over and use the V shaped end of the pick to slowly push the free end of the o-ring down (toward the ambient chamber side). The o-ring will most of the time roll right into the groove. In some cases you may have to nudge it in here and there with the rounded end. Occassionally you get a stubborn one with an overly tight groove and you may have to hold the captured end in the groove with a straight o-ring pick with one hand while you press it in place with the V shaped tool in the other.
Technique is probably important, but with the above methods you can get one in place consistently in about 15 seconds.
I have never liked the tool Peterbuilt makes for this. It's quicker and easier doing it as noted above.
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When removing the piston stem o-ring, only use a brass o-ring pick. steel picks will scratch the surfaces and if you create a scratch in the groove itself, you will end up with a leak and there is no way to repair the damage. The other o-ring pick in the parker set is perfect for this.
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Scubaporo made two major changes in the Mk 10 compared to the Mk 5.
1. Smaller diameter piston head.
2. Bearing surface for the piston head in the body rather than the swivel cap.
Number 1 was a mixed bag. the smaller head increased the less than perfect balance issues caused by the straight piston stem, and it required more IP drop to get the same work out of the smaller piston. But the msaller piston was lighter and displaced less water in the ambient chamber when it moved.
Number 2 was a problem as any wear or scoring that occuured there require a new reg body on the Mk 10 rather than just a fairly inexpensive and easy to replace swivel cap. It also made packing the ambient chamber with environmental silicone without leaving voids much harder than on a Mk 5.
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The Mk 15, 20 and 25 have all used a variation on a bushing system intended to allow the tolerances in the reg body to be both more accurate and renewed with each annual service. Too much wear in that area on a Mk 5 or Mk 10 will result in O-ring extrusion or pinching and that will eventually result in leaks, in some cases whistling noise (as the trapped piece of o-ring causes the piston to vibrate as it moves) and eventually complete o-ring failure.
The bushing system in the Mk 15 was not popular as it required long reach o-ring pliers to remove the C-clip retaining the bushing, but the Mk 20 and 25 use a spring to accomplish this and all the bushings are removeable and replaceable.
And in the Mk 15, 20 and 25, Scubapro went back to the full size piston head and they also went back to having the bearing surface for the piston head o-ring in the swivel cap. .
