Why you don't make bushings out of plastic...

[Genesis]

Ok, this is a rant. You've been warned.

I have a bunch of SP Mk10s, which, I might add, I believe are some of the nicest, simplest, and best-breathing 1sts for warm-water (50F+) ever made. I can tear these things down in my sleep and every O-ring in there can be replaced without a hint of trouble or hassle.

I also have ONE - repeat - ONE - MK25.

Anyway, the last couple of times I used it I noticed that I could hear a bit of a hiss behind my head. It couldn't get it to leak in the rinse tank, but I assumed it just need an overhaul - no big deal - IP was stable, etc.

Ok, so I tear it down.

On close inspection I find that one of the piston bushings - the one on the HP side of the HP O-ring - is cracked.

Its made out of ORDINARY PLASTIC.

Now I caught this before there was a catastrophic failure, but let's face it folks - this is a 2 cent part and its in a critical place.

It also doesn't HAVE to be plastic. There could be a real, honest-to-god grove in the case like on the Mk10. This "bushing thing" is hokey to start with.

Its been about a year since I bought it - its due for an annual, yes. But - and this is the rub - I have absolutely no idea how long this thing has been cracked like this. It only leaked SOMETIMES, when things aligned under pressure just wrong.

But if a SECOND crack had developed as a consequence of unequal loading (quite possible), resulting in a separation of the bushing into two pieces, the result might have been a total failure of the HP O-ring due to extrusion through the resulting gap, which would have been very, very interesting - especially at 110' or so.

I'm NOT impressed with this.

Yeah, I have overhaul kits, so the 2 cent part will get replaced this evening, and that will be that.

But I am most unimpressed that SP has this design in their "high priced, high performance, high-zooting" regulator.

I'll calm down now.....

 

[ElectricZombie]

Weird...I've never seen a cracked bushing on a Mk25 before. ScubaPro has been know to send out parts that have some sort of manufacturing flaw in them.

I agree with you that the Mk10 is a much simpler design and easier to work on...I wish they would start making them again.

 

[Zaphod]

Are you sure it's really cracked or does it look like it's been cut? There was a period of time where SP instructed their techs to cut that bushing. They later started including a cut bushing in the kit. They've since changed their mind and the bushing is no longer being cut. The little white slip of paper inside the overhaul kits has a little note on it to that effect.

If I remember correctly, the the cut was there because they said the bushing would shrink in cold water and bind against the piston.

 

[Genesis]

This is NOT the "soft" bushing - that one I know about, and know about the cut that they made in it intentionally for a while.

This is the inside (HP side) of the "triple stack" of the inside (HP side) bushing, HP O-ring, and outside "shouldered" bushing that go in the body.

The INSIDE one has a crack down its length.

The "face" of that is the surface against which the HP O-ring seals (it, and the one on the other side of the O-ring, are what locate the O-ring in the bore.)

 

[oxyhacker]

Rant on , you will get no argument from me. But why limit it to warm water? I've had my Mk5s and 10s under the ice many times, and in 40 degree quarry tunnels without ever a hint of trouble. The old grease pack method was simple and very effective, it's only disadvantage today is that the high cost of O2-safe lube makes it an expensive option for regs that will see high FO2s.

It's hard not to wonder if the plastic bushes in the newer SP BPs were really necessary to improve the performance, or just a way of deliberately complicating the design to both make it harder to copy, and harder for non-SP dealers to service. Both the Oceanic BPs (which started out as pretty much exact SP MK5 copies, but have evolved) and the Atomic have been able to get performance comparable to the Mk20/26 with considerably simpler designs.

Genesis once bubbled...
Ok, this is a rant. You've been warned.

I have a bunch of SP Mk10s, which, I might add, I believe are some of the nicest, simplest, and best-breathing 1sts for warm-water (50F+) ever made.

 

[Genesis]

Look at the IP drop "under load" when you inhale on the Mk20/25/10.

Almost identical. In fact, I challenge you to be able to tell me which is which if you can't see the reg at the time

My Mk25 is back to normal, but I'll be damned if I'll buy another one. Mk10s, on the other hand, I'm always on the prowl for at a nice price

 

[DA Aquamaster]

oxyhacker once bubbled...
It's hard not to wonder if the plastic bushes in the newer SP BPs were really necessary to improve the performance, or just a way of deliberately complicating the design to both make it harder to copy, and harder for non-SP dealers to service.

I'm paraphrasing an SP tech from another post but SP went to the bushings as some of their less able techs had trouble getting the HP O-ring in and out of the groove in the Mk 10 first stage body. Performance improvement had nothing to do with the move to bushings.

I agree the MK10 has more than adequate airflow. The MK 20/25 may have more, but then it is additional flow capacity no one uses anyway. And this additional flow capacity is an aggravating factor in the cold water freeze up problem.

You can also pick up some of the performance difference betwen the Mk 10 and Mk 20/25 by uorading it to a MK 10 Plus. The upgrade kit includes a new piston with the rounded sealing edge and the conical seat. But in my experience the MK 10 Plus seat was less reliable and I had problems getting a whole season out of them. Consequently mine all got converted back to Mk 10 status. It's hard to improve on perfection.

If the MK 10 patent had not expired I doubt Scubapro would have had much motivation to develop something new. Unfortuantely they have uin many ways went down hill since. If they were smart, they would reintroduce the Mk 10 with perhaps a few changes (300 bar service pressure, etc). It would be simpler than trying to resolve the MK 20/25 freeze problem.

O2 compatibility is not a problem with the SPEC kit on the MK 10 as long as the 02 is 40% or less.

 

[Genesis]

It bites.

I've had MAJOR problems with the seats in these.... including NEW seats that don't seal well!

Oh, they'll seal, but they have a nasty little creep in them that makes you less than happy....

For some reason the Mk20/25 don't have THAT problem, even though (surprise!) the seats are interchangable (yeah, I know they say they're different, but they're not....) Probably a difference in the piston.....

 

[Zaphod]

Genesis,

If you still have the bushing and would like to, as one of their techs, I can send it back to SP and try to get an explanation of what happened. I'm sure it won't change your overall perception of the reg, but it might be interesting to see how they handle it.

 

[Genesis]

nothing to lose, right?

PM me an address and I'll toss it in an envelope. I still have it