ScubaPro MK5: The elusive o-ring #8

[JohnN]

I'm rebuilding a SP MK5 to put into service on an Argon bottle.

I have it disassembled, but cannot see o-ring #8 in the drawing below.

How does one remove (and more importantly) replace this little bugger?

View attachment SP MK5.bmp

 

[redacted]

Carefully, very carefully. It is in a groove inside the body. You need to use a pick to remove it and be very careful not to scratch a sealing surface. I use a double hook pick which has a little bend in the hook. I slide the point down into the groove along one side of the o-ring and then rotate it hooking the o-ring. Some may use the straight pointed pick and stab it. This must be done from the ambient pressure side so any scratches that result will not be on a sealing surface. Insertion is even more fun. Lube the hell out of it; twist it into a figure 8; and insert it into the hole. Then use a dowel (chopsticks work great) to move it into the area of the groove and get a piece of it started in the groove. Then keep working it with the chopstick to get the rest into the groove. Scubatool makes a tool to aide in the installation but that is only for folks looking for the easy way out (or who have little children listening).

BTW, why use a Mk5 on an argon bottle rather than a Mk2/3/200?

 

[elan]

Mark 5 on an argon bottle!!! What a waste it can do better than that.

 

[halocline]

Make sure you use a duro 85 or 90 o-ring in that position, the size is 010. If you have a 2 LP port MK5, I understand the argon use, but if you have a later 4 or 5 port model, it does seem a shame to relegate it to inflating your drysuit.

 

[JohnN]

BTW, why use a Mk5 on an argon bottle rather than a Mk2/3/200?

Mark 5 on an argon bottle!!! What a waste it can do better than that.

I got it along with a AL13 from a local person getting out of diving. He (and I at the time) didn't know what it was. So it was inexpensive and at-hand.

Except for this hidden o-ring, it appears pretty simply made and will be a good regulator to use as my first rebuild.

One additional question. . . the rebuild documents say adding additional washer #9 can be used to adjust the IP. I'm a bit confused with the physics of this, since the spring rate shouldn't change with compression. Am I missing something here?

Thanks for the assistance

 

[NotSure]

One additional question. . . the rebuild documents say adding additional washer #9 can be used to adjust the IP. I'm a bit confused with the physics of this, since the spring rate shouldn't change with compression. Am I missing something here?

You are correct that the spring force is a constant for small displacements, but the spring is slightly compressed when installed so adding a shim increases the preload spring force. The preload spring force and ambient pressure work against intermediate pressure so adding a shim results in a higher IP.

 

[redacted]

Since you are not breathing from it where the 2nd stage works best at a specified IP, you could probably just remove all the washers and end up with an IP in the 115 to 125 range which should be fine for argon.

 

[ams511]

I did one MK-5 so far so I am far from an expert. I bent a pin from a dress shirt and stabbed the o-ring to remove it. It was fairly easy to do and hard to miss. To insert the new o-ring I lubed it and used a brass pick to pull it close to position and used the piston bullet to push it into place. It went right in without any trouble. I was surprised because I am not all that mechanically inclined. I thought, is that it? Next time I will try with a plastic toothpick or one of those flossers with the sharp point because Awap is right that the pin could scratch the reg.

 

[halocline]

I think Couv has a nifty set up with a cork and a chopstick to push the o-ring in place. I use the double hook pick to remove it, just like awap (he told me about that tool) and then I just squeeze the new o-ring with a pair of tweezers, push it down in the journal, and then work it in with two chopsticks, one from each side. The best chopsticks are the small round ones with a dull tip.

I suspect that the bushing system on the MK 15 and later was as much about making it easier and faster to install that o-ring as it was about improving tolerances. I imagine that many, many MK5s and 10s were destroyed by careless techs removing the old HP o-ring.

 

[Couv]

I went a long time without having any problems inserting that o-ring. Then, I ran into a few problem children that frustrated me enough to buy the Scuba Tools tool. However, as Halo mentioned, I found a very inexpensive (depending on the quality of wine) way to make one.

http://www.scubaboard.com/forums/do...nced-piston-stem-o-ring-tool.html#post6521931