ScubaPro Mk10 assembly questions

[Bryan@Vintage Double Hose]

Ummmm. Okay.

I though you were saying you were going from the HP seat side and pushing on the piston where it enters the O-ring and extruding it enough to allow the piston to pass without damage. But now I see what you were actually describing. Which in my opinion is still wrong as there is no way you can be 100% sure you did no damage to the O-ring. That's the reason for the bullet tool and there is absolutely no reason NOT to use one. Then again if they are your regulators you can do as you like. Skipping the bullet tool on a customers regulator is taking an unacceptable chance.

 

[halocline]

My approach: Gently set the piston into the channel, so that it's resting on the small HP O-ring which earlier had been very lightly lubed and then slipped into its groove. Then *gently* "walk" your finger around the edge of the piston with slight pressure. You'll eventually feel the piston slip past the O-ring. It's how I was taught. It's how I've rebuilt countless Mk 5's, Mk 10's and Mk 7's. It's how I rebuild my own set of three Mk 10's. I've never damaged an O-ring yet ...

Safe Diving,

rx7diver

I'll try that on a MK10 someday for grins. But I don't see how you can reach the o-ring, unless you use a chopstick or something. You certainly couldn't "walk your finger" around the edge of the piston. It's way down in there. And, this is a really tight fit, only a few hundredths of an inch clearance between the piston shaft and the surrounding channel. That o-ring has a huge pressure gradient on it and if there were more clearance the o-ring would immediately pop out. So I'm pretty skeptical, you must be thinking of something different than I am.

 

[rx7diver]

I'll try that on a MK10 someday for grins. But I don't see how you can reach the o-ring, unless you use a chopstick or something. You certainly couldn't "walk your finger" around the edge of the piston. It's way down in there. And, this is a really tight fit, only a few hundredths of an inch clearance between the piston shaft and the surrounding channel. That o-ring has a huge pressure gradient on it and if there were more clearance the o-ring would immediately pop out. So I'm pretty skeptical, you must be thinking of something different than I am.

Walk your finger around the *wide end* of the piston.

 

[halocline]

Walk your finger around the *wide end* of the piston.

I have no idea what you mean. The "wide end" would be the piston head. Are you trying to say you grasp the piston there, and kind of tilt it around in an attempt to sneak part of the edge over the HP o-ring, then move it around to try to work the rest of the edge over the o-ring?

 

[rx7diver]

I have no idea what you mean. The "wide end" would be the piston head. Are you trying to say you grasp the piston there, and kind of tilt it around in an attempt to sneak part of the edge over the HP o-ring, then move it around to try to work the rest of the edge over the o-ring?

Yes. Sort of. Don't "grasp" it. Rather, using one finger and your gentlest touch, gently push in on the edge of the "piston head" (as you call it) as you slowly move your finger around the edge. Don't intentionally "rotate" the piston. You'll actually feel when the knife edge finally, smoothly slides through the small HP O-ring. If you feel any sensation of "binding" or "cutting," you're doing it wrong (and you've probably compromised the O-ring!). When I was learning, it took me only a couple of tries to learn the "feel"--which is surprising, since I can be awfully ham-fisted/ham-handed sometimes. Once you've learned the feel, you won't forget.

As I wrote before, this is the only approach I have ever used, what I was initially taught in the late 1980's.

 

[redacted]

I have no idea what you mean. The "wide end" would be the piston head. Are you trying to say you grasp the piston there, and kind of tilt it around in an attempt to sneak part of the edge over the HP o-ring, then move it around to try to work the rest of the edge over the o-ring?

I believe it means he is wobbling the piston head with minimal downward pressure to ease the knife edge past the o-ring without catching and causing damage. Unless the knife edge is particularly sharp or you try to overcome resistance with added pressure, it should work. The owner/tech of a local Scubapro shop didn't use a bullet tool. He also didn't own a torque wrench. Not really good practices but you can get away with it..., until you don't.

 

[halocline]

Well, one big problem with this practice is that you would effectively scrape any lube off the o-ring with the sharp piston edge. That's not a spot where you want to skimp on lube.

And that's not even considering possible minor abrasion damage to the o-ring, maybe not enough to cause an immediate leak, but considering that's a dynamic o-ring with a 3K psi pressure differential and known extrusion/friction issues, it's probably the worst place to cut corners, in a manner of speaking.

 

[NotSure]

It's probably overkill, but I always like to run the piston bullet with a small amount of lube on it through without the piston first. It seems to stretch and align to O-ring so there is almost no resistance when the Piston plus bullet are inserted.

 

[WhiteSands]

Thanks everyone for your tips and advise.

The piston stem o-ring was a really tough one to install.

Also I'd think the piston bullet is a must have. I lubed the bullet really heavily and even then it took some muscling to push it through the stem o-ring. I'd think if you tried to push it through without the bullet, it'll definitely cut the o-ring.

If you are going to do this on a regular basis, I'd definitely buy the installation tool like the one Herman made.

 

[Couv]

Yes, the o-ring insertion tool takes a lot of the frustration out of a rebuild, and the bullet takes out the guesswork. I've installed a couple of pistons without a bullet before and did not have any symptoms of nicking the o-ring (i.e. no bubbling out the ambient chamber.) However, after second guessing myself and "what if-ing" long enough, I removed and re-installed the darn things again using the proper tools.

BTW: Notsure is correct to use lube on the bullet. Why install an o-ring that should be heavily lubricated and wipe the grease away with a dry utensil? (Saying dry tool just didn't sound right )

Here is a link to a couple of reviews of the bullet and insertion tools.

http://www.scubaboard.com/forums/do...tools-mk-5-mk-10-piston-stem-o-ring-tool.html