Did Scubapro ever make a MK20/25 composite piston with a rounded knife edge?

[Six2Life]

I have a later production MK20 that I got from a friend. It is stamped "T.I.S." and has a composite piston but the knife edge seems quite rounded and I don't see or feel the slight flare at the end of the piston. I was just wondering if any of you have run across different versions of the composite piston or if maybe the one I have was monkeyed with in the past.

 

[Zung]

I have 7 composite pistons, plus a steel, some of them old, some of them quite receng, and all of them look like this. I guess you can call them rounded, and the flare is very hard to see:

 

[Six2Life]

That looks like the one I have. I have also had a chance to inspect what I believe was a newer production composite piston in a dive shop that definitely seemed to have a more noticeable flare right at the tip. From what I've read on SB the newest piston has a reputation for excellent lockup while the rounded end brass pistion is known to be a creeper. Mine is producing some moderate creep (about 5-7psi) so I was wondering if that was due to having a more rounded profile as opposed to the somewhat sharper profile that I think I observed on a brand new piston.

 

[redacted]

That looks like the one I have. I have also had a chance to inspect what I believe was a newer production composite piston in a dive shop that definitely seemed to have a more noticeable flare right at the tip. From what I've read on SB the newest piston has a reputation for excellent lockup while the rounded end brass pistion is known to be a creeper. Mine is producing some moderate creep (about 5-7psi) so I was wondering if that was due to having a more rounded profile as opposed to the somewhat sharper profile that I think I observed on a brand new piston.

The problem with the brass piston was deposits that formed on the large sealing surface that resulted in creep. I you clean and lube that surface every 3 to 6 months, it works OK.

 

[ams511]

I disassembled two mk20s last night and pulled the pistons. Both pistons appear to be brass-tipped. However one has a black piston head while the other is green and looks like a TIS. What gives? I thought there were only three piston types. What do you guys recommend to inspect the knife edge? Also what to you use to remove the ambient chamber from the body? I have the scubatools multi-tool and although it works well on a mk5 it sucks on a mk20.

 

[elan]

I use the multitool . I always put an old LP diaphragm between yhe tool and the bodyand it always works well

 

[halocline]

I disassembled two mk20s last night and pulled the pistons. Both pistons appear to be brass-tipped. However one has a black piston head while the other is green and looks like a TIS. What gives? I thought there were only three piston types. What do you guys recommend to inspect the knife edge? Also what to you use to remove the ambient chamber from the body? I have the scubatools multi-tool and although it works well on a mk5 it sucks on a mk20.

There should be no knife edge on any MK20 or MK25 piston; they should all be much rounder-tipped than the earlier piston edges (except the MK10+). Here are photos that DA Aquamaster posted a few years back of the three generations of MK20/25 pistons. I don't think there are any others, except maybe there's a newer one by now that looks a little different. The composite has a plastic head when viewed from the top of the head. I use a jeweler's loupe to inspect the edges. I have found that polishing the brass edge piston with micromesh will slow down the creep but the best solution by far is to replace the piston. Then a MK20 really works like a MK25 except even better IMO due to the lack of the external IP adjuster.

 

[Zung]

I use a hook spanner, 10 bucks from an automotive tools shop, works wonderful. Mine says 34-36, guess it's the diameter in mm; you can also take your reg along to try for size.

The piston is more tricky: a shop once quoted over 80 bucks for a MK10 piston. Of course I declined.

 

[ams511]

I use the multitool . I always put an old LP diaphragm between yhe tool and the bodyand it always works well

It could be my poor technique. I do not use a vice. As Harlow suggested, I put the first stage on the tank and loosen the chamber from the body. What is funny is that it works well for the MK5s but the MK20s are much more difficult. I used a piece of those orange German towels but still got some scratches. I was thinking of using an inner tube patch next but maybe I will try the diaphragm. Thank you for the advice.

---------- Post added April 3rd, 2013 at 03:55 PM ----------

There should be no knife edge on any MK20 or MK25 piston; they should all be much rounder-tipped than the earlier piston edges (except the MK10+). Here are photos that DA Aquamaster posted a few years back of the three generations of MK20/25 pistons. I don't think there are any others, except maybe there's a newer one by now that looks a little different. The composite has a plastic head when viewed from the top of the head. I use a jeweler's loupe to inspect the edges. I have found that polishing the brass edge piston with micromesh will slow down the creep but the best solution by far is to replace the piston. Then a MK20 really works like a MK25 except even better IMO due to the lack of the external IP adjuster.

Yes, the edges are rounded not sharp. Thank you for posting the photos, however I am still confused. Looking at the second photo, the pistons are the ones on the middle and right. Looking at the first picture both bottoms are like the middle. Why don't you like the external ip adjuster?

---------- Post added April 3rd, 2013 at 03:57 PM ----------

I use a hook spanner, 10 bucks from an automotive tools shop, works wonderful. Mine says 34-36, guess it's the diameter in mm; you can also take your reg along to try for size. The piston is more tricky: a shop once quoted over 80 bucks for a MK10 piston. Of course I declined.

Thanks for the tip. Of course the question is where to purchase a piston at a reasonable price?

 

[elan]

If it is not urgent just monitor ebay, they do show up from time to time. I have seen them a couple of times during past half a year, they were in a range of $30 - $40.