Sherwood Magnum (SRB 5300) 1st stage rebuild...need opinions

[konish]

Howdy!
I'm new to the board and pretty new to SCUBA as well, but have been pretty mechanically inclined all my life. Anyway, my brother sent me 2 full rigs with Sherwood Magnum 1st and 2nd stages and Sherwood Minnimus octos.

Both sets haven't been used in something like 10 years but were stored in reg bags inside the house and were serviced a year or so prior to being stored. I did some pool stuff with them and they worked fine, but I figured I'd freshen them up with a parts kit from NESS before I dove them in open water.

I downloaded the manual and did a ton of reading and research here before I attempted to take them apart. Not to belittle anyone's trade at servicing these but there just isn't that much to 'em, and I have been familiarizing myself with the ports (especially the dry-bleed system) and parts.

Here is where my lack of experience shows. Both piston bores had scratches and a little corrosion. I cleaned them up with a really, really soft polishing tip on a low-speed dremel and managed to lightly polish them up.

However, one of the bores has a ring the runs the entire circumference of the ID, and a noticeable scratch running at a slight angle to the bore centerline (see pic)

This was after polishing and the pics make them look a lot worse than they are. The edges are smoothed and feathered, but you can still just barely detect them with a toothpick (but *just* barely).

The other bore had some corrosion that cleaned up quickly and overall the bore is in great shape but I noticed this little "dimple" in the bore.

Again, the camera make it look worse than it is. The edges aren't sharp and I can only detect it if I put the tip of a toothpick into the dimple, otherwise, it does not catch on anything that is flat to the bore surface.

Question is whether or not these are going to present a problem or should I try and source some 1st stage bodies? Hone and use a thicker OD o-ring? I'm guessing that they'll be okay as they don't seem to affect the o-rings (no tears, scuffs, flat spots or dull spots).

I've rebuilt pneumatic and hydraulic components with similar defecst in sealing bores etc and never had an issue with a ruptured/torn o-ring, but none of that was life support equipment either.

Thoughts? Thanks in advance

R/
Dustin

 

[axxel57]

Probably you don't have to worry about the condition of the bore, have seen much worse still working fine. This part is specially polished by the manufacturer to insure a smooth movement of the small piston o-ring.
Normally it would be easy to find out if there is a problem, because most of those unbalanced 1st stages have water chamber, so under pressure with such a high pressure differential air would enter the main spring chamber and if you hold it in water it would show you bubbles coming from the water chamber.
These Sherwood models have a "sealed" balancing chamber, so little air is passing through the flow restriction screw into the main spring chamber, balancing the LP as usually the water is doing. It's exiting through the one way bleed valve in the water, so if there is a problem with the bore surface or o-ring, you couldn't identify.
Use a IP gauge (without it you shouldn't start to rebuild any reg), check the IP and look if it drops a bit after some seconds. This could point to that particular problem, but of course the IP could drop also for other reasons.
Don't use the Dremel aggressively, if the chrome come off the surface of the brass, you will have fast a lot of corrosion on those places.

 

[konish]

Probably you don't have to worry about the condition of the bore, have seen much worse still working fine. This part is specially polished by the manufacturer to insure a smooth movement of the small piston o-ring.
Normally it would be easy to find out if there is a problem, because most of those unbalanced 1st stages have water chamber, so under pressure with such a high pressure differential air would enter the main spring chamber and if you hold it in water it would show you bubbles coming from the water chamber.
These Sherwood models have a "sealed" balancing chamber, so little air is passing through the flow restriction screw into the main spring chamber, balancing the LP as usually the water is doing. It's exiting through the one way bleed valve in the water, so if there is a problem with the bore surface or o-ring, you couldn't identify.
Use a IP gauge (without it you shouldn't start to rebuild any reg), check the IP and look if it drops a bit after some seconds. This could point to that particular problem, but of course the IP could drop also for other reasons.
Don't use the Dremel aggressively, if the chrome come off the surface of the brass, you will have fast a lot of corrosion on those places.

Axxel,
Thanks. I have an IP gauge on the way and your reply confirms what I have read and observed about the dry-bleed system works. Just to be sure I understand this correctly, it would seem that the IP on these regs will never "lock-up" in the traditional sense due to the dry-bleed system.

Essentially, I'm guessing the dry-bleed is balanced by allowing the IP chamber to bleed off pressure through the one-way valve and dry-bleed fitting even when the piston closes against the valve seat. From what I've read and researched, other piston regs (that are working properly), the IP pressure would remain static indefinitely until a breath (or purge) is used to depressurize the chamber thus causing an pressure imbalance.

With the Sherwood system, the dry-bleed system allows pressure to decrease from the chamber on top of the piston face (through the hole on the piston stem) which then forces the HP seal away from the valve seat via the mainspring. Once the HP circuit is opened, pressurized air fills the IP chamber again, flows through the piston stem, which re-pressurizes the piston chamber. This increase in pressure on the piston face overcomes spring pressure, causing the piston to rise in the bore effectively closing the HP circuit(?) Obviously, this cycle continues to "modulate" the seal away from the HP valve seat as it pressurizes and depressurizes the IP chamber...is that about right?

I can't see how it would work any other way as the design intentionally leaks air to keep the piston chamber dry and balanced...but of course if it doesn't work this way, I'd really like to get a better understanding of it.

I was *very* careful using the dremel...I've been ham-fisted with the thing in many other applications in the past and it'll do damage faster than most realize if not monitored but I appreciate the words.

Anyway, I guess it'll have ot come down to throwing the parts kit in and seeing what the result is, but I do feel better that the bores didn't raise any safety alarms. In my anal-retentiveness, I'll probably end up opening the 1st stage periodically just to monitor the o-rings and bores until I feel satisfied that all is working well and without cause for concern.

R/
D

 

[axxel57]

Hi, you got a quite good grip on how the 1st stage is working if I understand you right (my English is not perfect).
But the IP is locking up, if not it would affect the overall performance of the reg and adjustment would be very difficult. The amount of air leaving the 1st stage is too small to be seen on the gauge while testing.
The hole of the flow restrictor screw is laser drilled, extremely small and leaves only tiny amounts of air through. This is why it can be clogged easily if your tank (and the filter) is dirty or water enters through the one way bleed valve backwards into the 1st stage.
That happens much faster than you might expect, this is why those Sherwood regs better have to be rinsed after the dive while still on the tank and under pressure.
And this is also why you would probably feel a slight increase in the inhalation (cracking) effort if you should start a straight fast descent into the depth. The 1st stage would need some time to catch up (balancing) with the demand in that particular depth.