Scubapro MK20 Maintenance Issue/Question

[redacted]

Speaking of seats. The MK10 plus/MK20/25 apparently take the same seat. I've inspected a few of them and see a number written on the "piston" side surface. I don't remember the numbers, but I do recall that they were not all the same. What is the significance of these numbers? Some older white MK 10 (not plus) seats also have numbers on them that I assumed were the precursors to the + = - (or is it X -l ll.....Ben Casey might know) markings we see on the gray seat kits; but I'm not certain. If that is the case, why would SP go back to a numbering system?

♂, ♀, ✳, †, ∞

A batch marking would make sense in case a bad run ever makes it through QA. That location makes it as easy as possible to check.

 

[halocline]

I only have two of those seats, one has a 7 and one has a 4. I can't tell any difference between them.

 

[redacted]

I only have two of those seats, one has a 7 and one has a 4. I can't tell any difference between them.

Look close. It is 3.

 

[halocline]

I thought you might say that the difference is that "4" has an extra line than "7".

 

[Boiler_81]

IP gage on sale for $7.95 Intermediate Pressure Gauge with BC Quick Disconnect Fitting

Fair enough. I'd say that there were several contributing factors:
1. Have never heard of this condition before
2. Have never experienced it before (have 1200 dives on my own gear)

I will certainly buy one now and use it appropriately. How often or, after what events, should one test IP? After each service, before each trip, before each dive?

Best,
Tracy

 

[guruboy]

View attachment 389032

I think these are the three MK20 pistons. Although the brass one does look like it's more rounded, you can see that none of them have the sharp knife edge from the earlier regs. It just wouldn't work with the new seat. And I'm pretty sure this whole rounded edge design was developed to increase the air flow around the piston edge, not so much to increase IP stability. The MK15 has pretty great IP stability, just not the gaudy flow rate numbers of the MK20/25. Who knows what they were trying to accomplish...

According to DA Aquamaster the early Mk20 stainless Pistons did have a knife edge.

Scubapro mk15 mk20 and mk25

 

[halocline]

According to DA Aquamaster the early Mk20 stainless Pistons did have a knife edge.

Scubapro mk15 mk20 and mk25

Just a few days ago I rebuilt a MK20 with the early stainless piston. It is not a knife edge piston similar to the MK15. It's not as rounded as the brass piston that followed it, but it is not a sharp edge piston that cuts into the seat. It has kind of a flat edge. I don't think a sharp edged piston would work at all with the MK20/25 stye seat. Comparing the early stainless MK20 piston to the composite piston, the edges look pretty similar; under magnification the composite piston has a little more curve around the edge while the earlier piston is pretty flat on the edge.

I'm really familiar with it because I had two creeping MK20s I was working on almost at the same time, one with the early piston and one with the composite. This meant I was looking at the edges carefully with a jeweler's loupe. I used micromesh to clean up some tiny scratches on the stainless piston. Now it locks up well.

 

[rhwestfall]

question: I've got a MK-20 with the brass piston, and also have a composite piston sitting here to update it. I don't have any of the MK-20 tools, but with the flat edge (not knife edge) can I just pull it and replace the composite? No insertion or alignment tools.... (I don't have the bushing/piston tool at this time). Does the seat and retainer hold the bushing in place if it's all assembled?

 

[guruboy]

I think everything should stay in place but it might not seal well on the used HP seat.

 

[guruboy]

Just a few days ago I rebuilt a MK20 with the early stainless piston. It is not a knife edge piston similar to the MK15. It's not as rounded as the brass piston that followed it, but it is not a sharp edge piston that cuts into the seat. It has kind of a flat edge. I don't think a sharp edged piston would work at all with the MK20/25 stye seat. Comparing the early stainless MK20 piston to the composite piston, the edges look pretty similar; under magnification the composite piston has a little more curve around the edge while the earlier piston is pretty flat on the edge.

I'm really familiar with it because I had two creeping MK20s I was working on almost at the same time, one with the early piston and one with the composite. This meant I was looking at the edges carefully with a jeweler's loupe. I used micromesh to clean up some tiny scratches on the stainless piston. Now it locks up well.

Cool. Good to know. Where were the scratches? On the flat of the piston edge?