SP MK-3 - First Stage Piston Seat

[Bryan]

The HP seat is an interference fit and when I install them in a Scubapro or USD pistons it usually requires me to center the seat on the piston and then turn it upside down on a block of wood and push down on the piston base pushing the seat in place. They do not slip in easily like the seats on a MK5/10 do. I suspect that is why a removal tool for this type of seat is made.
Picture below shows the removal tool and a piston/seat from an Aquarius (I didn't have a Scubapro handy) but the process and results are the same. The caliper is on a new replacement seat for a Scubapro piston regulator.

[rhwestfall]

Thanks Bryan!

I wish you had pulled a MK-2/3 seat to measure. I did find my old micrometer. Seat from the package from VDH is 0.193" [from the scale: 0.1 + 0.075 + 0.018]. I'll be in the digital age by Monday......

Can anyone confirm this is the correct diameter for the MK-3 seat? As I said, this one is too big for my piston.

It is holding nicely just over 130 psi with 1000 psi on it, and didn't budge over 12 hours. Really don't want to pull it apart to solve this riddle....

EDIT - I miss-read what you wrote - okay, that is a SP seat you measured...... well, the one for this piston appears to be smaller..... plot thickens...

[rhwestfall]

okay, I pulled it apart...

0.100 + .075 + .003 = 0.178

should I be expecting these to "be the same" part and interchangeable (w/ some force)?

additional info: the OD of the piston stem is 0.225"

[antique diver]

I have some that are pretty close at about 0.181 diameter, and 0.091 thick.
Might work, as these are supposed to be a tight fit and should press in as Bryan says.

I can drop one in the mail if you would like to try it.

[rhwestfall]

I don't think it is workable. Height on this seat is [0.100 + 0.050 + 0.012] = 0.162

the VDH supplied seat is [0.100 + 0.025 + 0.007] = 0.132

it just keeps getting stranger....

reassembled, IP of 140, cycled it, and stayed. Bled it off, and re-pressurized, and 130 again...

[Bryan]

Hard and soft seating by Pete Wolfinger.....Shared with his permission.

https://www.dropbox.com/s/fwxneiqler4ws ... g.pdf?dl=0

[luis]

Everyone, thanks for the input.

As this is a really early model (2 lp, weak yoke, hp thread is same as lp, integral yoke stem), I'm wondering if Greg Barlow might have some insight as to the possibility of a piston change in the early model to a next generation that is now standard....

I'll update as I find out more.

I would not call that description a really early model. All the Mk-2 and Mk-3 from the 60's , 70's, and into at least the early 80's followed that description. The new yoke retainer didn't come out until much later. I am not sure, but I think it was the early 90's. I am guessing the change came about because of the introduction into the US of the DIN connection.

I started servicing these regulators in 1971 working at Divers Service Center. The regulators that I am most familiar are from that era and some of the earlier ones from the 60’s.

If you post a picture I of the outside I can tell which generation Mk-3 you have. Is the piston cap of your regulator rounded or flat on the bottom? Is the piston head the size of a quarter or a nickle?

The Mk-2 predates the MK-3 by a bit over a year. The MK-2 came out around 1963.

The first generation Mk-3 had a flat bottom and was around from about 1964 to 1971. The second generation came out around 1972 and was around until the early 80's. You probably have a second generation because they are more common. Both had the same yoke and main body configuration (same ports). The main body part number was the 102-7 for both the Mk-2 and the Mk-3.

Below are a couple of diagrams. Notice that the these diagrams have the old part numbers with fewer digits.

If you notice the parts numbering system, the first three digits referred to the regulator where this part was first used. In this case the 102 was because the part was originated in the Mk-2.

Look at the diagrams below and you will see that the seat was the same 102-4.

The Mk-3 diagram below (second generation) only introduced 5 new parts. The new Mk-3 parts all start with the 103 prefix.

I did service some of the first generation Mk-3, but I do not have a diagram for them. As far as I remember and the documentation I remember the only difference between the first and second generation Mk-3 was the piston cap, but not the piston. Note that I am going by memory (but that is also what the Scubapro regulator timeline says). The piston numbering (103-4) doesn't seem to indicate that there was a previous revision (but this is not a sure sign). The first generation Mk-3 were (and are) relatively rare.

Here is a copy of the vintage Mk-2 diagram.




Here is a copy of the Mk-3

[rhwestfall]

its the exact one on the ScubaPro page timeline, listed as the 1964 first generation... they label the image as a 1964.

this one, but I've swapped in a MK-5 heavy yoke on it right now (I have the skinny yoke). Exact same bottom style.

[luis]

That would be a first generation. Those are not as common.

I don’t remember having a diagram for that Mk-3, but I will look… Next year.

Below is what the Scubapro timeline says, but there are so many incorrect pictures in that timeline that I honestly trust my personal memory more that what they published. The picture they are showing as the second generation Mk-3 is incorrect. The picture looks more like the first generation.

I wrote comments on the Scubapro zone of ScubaBoard specifically identifying errors on their timeline, but they didn’t seem to care. The errors that I pointed out were only the ones that can be easily verified by their own catalogs, but it didn’t seem to matter.

1972 — 1981
MK3 2nd gen
The MK3 2nd gen was introduced in 1972 and is the same as the first with the only change being the bottom of the first stage which is now rounded.