Tore My Rix SA-6B Apart

[rob.mwpropane]

The "guide pins" as you call them are the plastic 1/4'Diameter that fit into the outer bearing retaining ring
and stop the rod end bearings clattering onto the swash plate housing six (6) in total 2 per rod end bearing. This is not the part your want stop calling it a guide pin you will confuse the natives.

The "tiny" stainless pins your describing are part number 17-757 Pin, Valve

Granted its known to the rest of the world and most engineers as a dowel pin
but Pin, Valve describes the application better as far as the product is concerned.

Dowel Pin. Heat treated stainless steel hardened 1/32" diameter x 1/4" long
per ANSI ASME B18.82 standard series base diameter if you want the full description.

Granted they are small I wouldn't say tiny its not a watch part just don't drop them on a rolling deck in the dark I guess LOL Iain

Part # 17-758? I ordered a few of those today, I did not realize I labeled them wrong. I don't think that I broke it, I didn't drop / force anything. When it came out it was shorter than the rest. I'm not sure.

There was a guy that posted a while back that he would make some videos about the heads... not sure whatever happened there but I bet if he did that I wouldn't have gotten so much wrong!

Heck? That what a steel razor blade or Stanley knife blade is for. If the gasket is suck on the stainless valve plate scrap the old gasket off with the razor blade getting it under the hardened gasket and only if required lap the valve plate with a fine 600 -1200 grit wet and dry or if in doubt use Brasso a brass polish on a flat glass plate (just clean the paste off well after) Don't scratch the valve plate or dig chunks out of it.
I use an old Buck rosewood handled knife for the purpose but that's classed as a luxury tool now a days

If the gasket is stuck inside the aluminium head again I use an old Parker brass flat blade end of a O Ring pick tool (sharpened) Its also used for lifting rings and removing piston o rings

Pardon Our Interruption

I did use a razor blade for the last little bit, but mostly I used a hard plastic scraper.

To line up the valve onto the dowel pin discharge side I use a pair of Waldes Truarc straight circlip pliers (old school) if I was buying new today I would use a pair of Knipex Straight circlip pliers 3-10mm
The essential part you need to be aware is only poke them into the holes that are on the recessed half of the valve that way you don't "form" the holes on the ground flat side half and cause the reed valve to leak over your created raised edge.

This also ensures you don't fit the valve upside down or you will blow yourself up.

Use this principle for both the 3rd and 2nd stage you should also note that the hole you're grouping for is in line with the valve head body mounting holes.

Snapping the dowel pin by janking the valve around "looking" for the hole to fit the valve body into the dowel pin will make it shorter yes just try not doing that in future. Iain

I have those and could have used them but it seemed just as easy to line the holes up.

How imperative is it to have lube on the o ring? Mine came with none. Maybe just a little to make things slide together easier?

The pressure breaker rings are a matching pair and should be in a small plastic bag. Don't swap the halves over with another matched pair or you end up with two mis matched pairs.

You also should have asked this question before ordering these rings, We stopped using them some time back at the Skunk works and use for our current builds just two additional pairs of compression rings instead.
Talk about the right hand nor knowing what the left is doing.
All you need to remember is Rule 1. Engineers rule.
On the bright side after your 3 month wait you saved yourself $2 I guess. Iain

Well.. I did post a list of parts in this thread that I thought I needed for the rebuild. I went off of the manual and I even called Rix to see if they thought I needed anything else.

I would have preferred to do it right and not have to have waited the 3 months.

If the "new" way is to just add another 2 compression rings then why are we all still passing around an outdated manual? Heck, that's even the same manual that Rix sent me in ~ Oct. when I placed the order?

You can see in this pic that there's no way I can fit that in there. Rix told me tolerances for that part are .070-.073. The slots on my piston measure .072 and .069 wide. How come they're out of spec (or maybe they're not and things have changed?)? And to top it off, one of the halves measures .075, so it's out of spec.



I'll echo @-JD- , do they get o rings underneath if using regular compression rings?

It did have an o-ring but years ago I will pull up the part number for the old rider ring 0-ring when back at the works It was a carry over from the SA-6A

The reason the groove is still machined is because changing the cad drawing and dfx file for the CNC is much too complicated for highly paid engineers.................Nah just kidding

The other reason is keeping the piston groove is consideration to the "weights" in balance on the swash plate running at 1500 RPM on a 32mm stroke. Consideration to the mass weight acceleration and deceleration at TDC and the need to match the weight transfer over the other two pistons over the piston stroke over the travel length.

If by way of example you were to change the gas molecular weight or say you want to change to nitrox
engineering considerations would require changing from a fully aluminium piston to say a bronze lip design and that additional weight flying about would have effect. Now for ease you could consider say more grooves or deeper grooves in the piston or a scooped out back to lighten the weight to match the original weight etc etc hence why if you have no change in gas or pressure or rod loading and no real need for the o-ring itself it was removed but the groove remained for the purpose of weight reduction.
Clear as mud I know but one day I may learn to give better explanations. Iain

So no o ring, but used to be one, but shouldn't get one because it will throw the weight off.. got it... clear as mud

I do appreciate your posts!

 

[iain/hsm]

So no o ring, but used to be one, but shouldn't get one because it will throw the weight off.. got it... clear as mud

On the first stage SA-6 piston. No not quite.
There was an 0-ring on the SA-6A first stage piston but was removed on the SA-6B as not really required.
The machined groove on the metal piston remained in order to retain the reduced weight the piston o-ring groove gave. For reasons of balance and for consideration to moments of inertia.

The design philosophy for a compressor requires the weight or mass of the piston to determine the piston force needed for piston acceleration at its compressive stroke.

These rules however do not appear to apply to Coltri or Nardi or other Chinese junk due to the magic forces of ignorance. Just saying. Iain

 

[iain/hsm]

How imperative is it to have lube on the o ring? Mine came with none. Maybe just a little to make things slide together easier?

You need to define what 0-ring your talking about

1. The piston ring 0-rings that sit behind the compression rings No. Install dry use gloves and don't get your greasy fingers all over them

2. The three (3) 0-rings that sit in the heads of stages two (2) and three (3) Yes these need lubrication
use Krytox or Christo-lube MCG111

3. Put a small amount of grease into a small plastic zip lock bag or similar drop the dry 0-rings in and
squish them about until evenly lubricated fork them out with your brass 0-ring pick and pop them in place with your gloved hands.

4. Also consider with old pumps in for service lubricating the internal walls of the heads this will help in locating the valve into position during any slight rotation required of the valve plate onto the dowel pin
dischage side. Iain

 

[rob.mwpropane]

You need to define what 0-ring your talking about

1. The piston ring 0-rings that sit behind the compression rings No. Install dry use gloves and don't get your greasy fingers all over them

2. The three (3) 0-rings that sit in the heads of stages two (2) and three (3) Yes these need lubrication
use Krytox or Christo-lube MCG111

3. Put a small amount of grease into a small plastic zip lock bag or similar drop the dry 0-rings in and
squish them about until evenly lubricated fork them out with your brass 0-ring pick and pop them in place with your gloved hands.

4. Also consider with old pumps in for service lubricating the internal walls of the heads this will help in locating the valve into position during any slight rotation required of the valve plate onto the dowel pin
dischage side. Iain

Thank you. What about the addition of o rings under the new compression rings?

Quoting @-JD-;

For those of us doing future service. Should those 2 additional compression rings (in lieu of pressure breakers) be backed by o-rings like the original 4?

 

[iain/hsm]

If the "new" way is to just add another 2 compression rings then why are we all still passing around an outdated manual? Heck, that's even the same manual that Rix sent me in ~ Oct. when I placed the order?

Not quite a "new" way don't forget the background dynamic here.
The Rix SA-6 as a recreational scuba compressor was discontinued 1st July 2017 to you lot.
And declared on the forum on August 20th 2017. Further that spares were going to be available through to 2022. That was a five year spares support program from Rix from date of retirement of the model.

Now the manual you have is correct and current to your build and states two pressure breaker rings on the 3rd stage. Further some 7 years later and 2 years after the spares support retirement concluded Rix are still showing support for recreational spares albeit with a 3 month delivery to re stock an already discontinued part. Maybe some slack is required.

Conversely on our side of the planet the (no longer called SA-6) is happily still being sold for pure air and liquid air applications for the military, pure air authentic atmospheric air sampling applications measuring pollutants in the atmosphere to the parts per billion and quad trillion range for laboratory purposes.
These purest of pure air compressors make scuba breathing air with its allowable 5mg of oil per m3 look questionable at the very least. A healthy life is a rare asset to those who don't care. Iain

 

[iain/hsm]

You can see in this pic that there's no way I can fit that in there. Rix told me tolerances for that part are .070-.073. The slots on my piston measure .072 and .069 wide. How come they're out of spec (or maybe they're not and things have changed?)? And to top it off, one of the halves measures .075, so it's out of spec.

View attachment 830695

When I'm back at the works Monday I will look into this

The piston 3rd stage has six (6) grooves for your build 4 for the compression rings and 2 for the pressure breaker rings. The groove width you have quoted 0.70"- 0.73" is simply wrong much too tight
The one groove you quote at 0.75" is nearing the mark but still too tight

Now I'm not going to declare or disclose an engineering dimension on an open forum
but to suggest that the groove has "lipped" and peened in over at its edges. Iain

 

[iain/hsm]

Thank you. What about the addition of o rings under the new compression rings?

Yes all the 3rd stage compression rings need the O-ring under.
We use original Aircraft specification Dupont Viton with a known cure date and compound
I will not disclose the specific compound for reasons given earlier.

I suggest not using some 10+ year old hydraulic O-ring with an out of spec dimension used by others
compressor suppliers with more carbon black filler in them than smokers lung. Iain

 

[rob.mwpropane]

When I'm back at the works Monday I will look into this

The piston 3rd stage has six (6) grooves for your build 4 for the compression rings and 2 for the pressure breaker rings. The groove width you have quoted 0.70"- 0.73" is simply wrong much too tight
The one groove you quote at 0.75" is nearing the mark but still too tight

Now I'm not going to declare or disclose an engineering dimension on an open forum
but to suggest that the groove has "lipped" and peened in over at its edges. Iain

Let's see if I can get this right, because that's not the groove width I quoted.

My piston grooves themselves measure .069 and .072 as shown in the picture. Rix said these grooves should measure .076-.078

The pressure breaker rings pulled from my machine all measured .069 when pulled off.

The new pressure breaker rings all measured .0715-.072 and then one measured .075. Rix told me that the spec for the PB rings were between .070-.073

I can understand you thinking that it may have "lipped or peened" but as you can see in the pictures there's no deformity at all (at least that I can see or measure).

If I can just order 2 new compression rings (18-C1791-4.5P) and 2 expanders (123-012-5) then knowing why this piston or newer pressure breaker rings are not matching up might be a moot point. I'd still like to know why they're off.

 

[Gone for diving]

You could always widen the grooves to spec.. in a lathe of course.

seems like your piston is an older version... or something

 

[1bubbleoff]

I recognize something in that picture....* cough cough*....