superstar
Contributor
mattboy, I measured the IP, at 2000 psi its 145 and at 1000 psi its 142 so I can guesstimate if I had a full tank with 3000 psi it would be 148 and at 500 psi around 140 giving a total IP difference of 8 lbs.
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OP
redacted
Guest
The effect may not be linear. One thing that is happening is the Mk5/10 design is not perfectly balanced. The slight imbalance will account for a small and probably linear drop in IP as tank pressure drops. I understand the Mk20/25 design has a slight taper in the piston to offset this drop.
The other effect, I believe, is friction between the HP o-ring and the piston stem. If there is any extrusion, the surface area involve gets larger and that effect should be non-linear. And, with the smaller forces that come into play with the Mk10 due to the smaller piston surface area, that effect may be much more noticable on the Mk10 than the Mk5.
halocline
Contributor
I suspect it will rise the most between 2500-3000 due to increased friction and more extrusion with the softer o-ring. At least that's what happened with mine.
superstar
Contributor
If the piston stem o-ring fails, is high IP the result? If the original IP setting was 140psi and now measures 148psi, this leads me to believe the 70 duro piston stem o-ring is not holding up. Also would a mushy IP lock-up be a symptom of a soft piston stem o-ring?
OP
redacted
Guest
If the piston stem o-ring fails, the result is a leak from the ambient chamber holes. Most o-rings will change shape a bit under high pressure differentials. I'm not sure where this shape change becomes extrusion, but with more sever extrusion, the o-ring will tend to be cut and "nibbled" away until it fails. short of the nibbling and subsequent leak, I guess the o-ring is working because it is still sealing. While a duro 70 might do the sealing job effectively and reliably in some cases, I can't think of any reason why a duro 90 would not be as good or better.
Zung
Contributor
If the piston stem o-ring leaks, the HP air would go out through the ambient chamber ports, and that's it. Creeps and slow lockups are sadly more expensive to fix.
I had a couple of similar cases:
I had a couple of similar cases:
- MK10 Plus #2: IP was 145 PSI, and lockup was 50 sec.. After a full service with a new seat, IP is 133 PSI and lockup is under 1 sec. So it was the seat, and a classic case of a reg begging for attention. Cost was $22 for a service kit.
- MK20 #2: after a full rebuild including a new seat, IP was 142 PSI, and lockup was a full minute. I replaced the piston, and IP is 129 PSI, and lockup is under 1 sec. So it was the piston. Cost was $22 for the kit, and $42 for a piston.
Sorry, lockup?
Zung
Contributor
Loosely: time for the needle to swing back up to nominal after a drop due to a breath
Texfrazer
Guest
Hey guys,
I've run across two additional pages with o-ring sizing (2nd stages). It's all one file with the first stages (4 pages total - the first two are the same as superstar posted earlier in this thread, though without the bleed through when scanned), but I thought it might be of some use to everyone.
View attachment O-rings.pdf
I've run across two additional pages with o-ring sizing (2nd stages). It's all one file with the first stages (4 pages total - the first two are the same as superstar posted earlier in this thread, though without the bleed through when scanned), but I thought it might be of some use to everyone.
View attachment O-rings.pdf
In a sense, yes. If the o-ring is extruding and causing the piston to slow or hang up before mating properly with the hp seat, it can cause high IP. Typically you would see this when the IP is correct at low supply pressure, but over at high supply pressure. That is why it is prudent to use the proper hardness o-ring for that application. As the others have pointed out, a leak past the stem (or piston head) o-ring would allow bubbles to exit through the ambient holes.
Couv
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