Stuck MK10 Piston cap (Scubapro)

[Zung]

The MK5 turret bolt takes a 11/16" socket, preferably with the chamfer ground off for better grip, but I've successfully used a regular cheap socket without any problem. If your has 4-5 LP ports, remove all the plugs and poke a screw drive through the holes for grip.

Regarding the piston, I would clean it up with some metal polish compound, put the whole thing back together with a new HP o-ring (2-010 Duro 90), pressurize and check for leak. If it leaks, you gain yet another paper weight, because these pistons are getting really hard to find.

 

[knowone]

A C spanner acts only on one side at two points with a pulling and pushing motion forcing thread against thread

That's no good?

Use a hammer with replaceable rubber and plastic heads and bash as much as possible "the crap out of the dry seized reg" whilst holding it as much as your hand can take.

That's good?

Or a lump of pine

Like the grinding you do unscrewing or screwing if you don't relieve spring pressure with your palm

 

[Zung]

... Use a hammer with replaceable rubber and plastic heads and bash as much as possible "the crap out of the dry seized reg" whilst holding it as much as your hand can take...

That works wonderful with stuck jam pot cover. I'll try it next time I come across a stubborn reg.

 

[redacted]

Parts for the Mk5 are usually pretty easy to find but you have to go to the junk yard. That is, look for less desirably configured Mk 5 (like the one you have) and harvest the parts. I usually plan to pay about $30 for a desirable Mk5 and under $10 for the clunkers. If you look at some of Halocline's posts you will find he talks about using very fine sandpaper (micro mesh) to clean up such scratches, so the piston may be savable. But your Mk5 sounds like it has all the undesirable features and then some. You will probably be better off looking for desirable models like this: eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

 

[Couv]

Manow,

Congratulations on your progress so far.

What you have is not a beat up MK 5, but a MK 5 with character. Now that you have it disassembled you are almost done. Unless the piston stem has very deep gouges in it or corrosion that is really deep, you should be able to re-surface that area. I know others believe in polishing or braking the glaze of that area, but like an engine's cylinder wall or the master cylinder of your brake system, the surface should be abraded. How abraded? 400 grit crocus cloth or sand paper. The reason being, you want that piston stem to hold lubricant. Try not to make that surface just scratched in a circular pattern. You want what machinist call a cross hatched pattern. If you really want to get anal about it (I'm probably the only Usual Suspect who does this-please don't tell anyone) also, slightly abrade the inside of the o-ring where it make contact with the piston stem.

Keep us posted.

Couv

 

[halocline]

I don't think I'd be comfortable putting a visible cross hatch pattern on the piston shaft. Remember we're not talking about a surface that has to capture liquid lubricant, and we are talking about a surface that has to seal airtight against a 3000 PSI pressure gradient. I use micromesh 2400-3600 for the piston shaft. I'm sure that leaves very small scratches but nothing that would allow either air to pass or any significant edges for the o-ring to get abraded.

 

[DA Aquamaster]

Mk 5 pistons are no longer made. The early style were straight while the later style have a step near the head that prevents the piston from "cookie cuttering" the seat. It's a rare occurence but will result in a total stoppage of gas.

A rough piston stem will cause excessive wear to the O-ring. Excessive clearance will also cause the o-ring to extrude and become pinched. That snips off little cesent shaped pieces of the o-ring and a leak results. If the stem is bad, get a new or serviceable used piston - or in this case, by a non beat up Mk 5.

E-bay always has them and most are rebuildable. I have rebuilt Mk 5's if you want one guarenteed to work. I have one 5 port Mk 5 currently but it's the older single 3/8" HP version. I'll throw in a 3/8 to 7/16 adapter.

 

[Zung]

... I know others believe in polishing or braking the glaze of that area, but like an engine's cylinder wall or the master cylinder of your brake system, the surface should be abraded. How abraded? 400 grit crocus cloth or sand paper. The reason being, you want that piston stem to hold lubricant...

I don't think I'd be comfortable putting a visible cross hatch pattern on the piston shaft. Remember we're not talking about a surface that has to capture liquid lubricant, and we are talking about a surface that has to seal airtight against a 3000 PSI pressure gradient...

I'm a bit confused. According to the Parker Handbook, Section V, Dynamic O-Ring Sealing, Paragraph 5.3, Surface Finished: "The most desirable surface roughness value is from 10 to 20 micro-inches." That's roughly .25 to .50 micro-meters, or microns.

Now the highest graded Micro-Mesh is 8000, equivalent to 3 "Micron" according to the conversion chart here. That gives "scratches" that are nearly invisible to the naked eyes, and way finer than a 400 grits sand paper.

Something's wrong, there's a discrepancy that's 2 orders of magnitude, please educate me.

 

[redacted]

I'm a bit confused. According to the Parker Handbook, Section V, Dynamic O-Ring Sealing, Paragraph 5.3, Surface Finished: "The most desirable surface roughness value is from 10 to 20 micro-inches." That's roughly .25 to .50 micro-meters, or microns.

Now the highest graded Micro-Mesh is 8000, equivalent to 3 "Micron" according to the conversion chart here. That gives "scratches" that are nearly invisible to the naked eyes, and way finer than a 400 grits sand paper.

Something's wrong, there's a discrepancy that's 2 orders of magnitude, please educate me.

In layman's terms, does this mean that 500 to 1000 grit is what should be used for the final cleanup of a piston shaft?

 

[Zung]

In layman's terms, does this mean that 500 to 1000 grit is what should be used for the final cleanup of a piston shaft?

That's what I ask:

• couv saith: use 400 grit - That's about 35 microns
• Parker saith: surface roughness should be 10 to 20 micro-inches - That's .25 to .5 microns, or 60,000 Grit

Something doesn't add up.