How to properly seat piston rings?

[kwindham]

I ordered some new piston rings for the third stage of my Bauer VT12 compressor, and presumed that slipping the cylinder over the rings would be a trivial task. Boy was I wrong. By wiggling and rotating the cylinder, I can nudge the first two rings into the cylinder bore, but the third ring repeatedly cracks in half with only a trivial amount of pressure. I burned through $50 of rings trying all manner of lubricants, wiggling, rotating and every other means of coercion I could think of. It is truly startling how easily the rings can break.

Is this a typical problem? Is there a trick to assembly that I am unaware of?



Here are some theories I had about possible reasons it may not be working:

- Honing the cylinder may have scratched the cylinder bore entrance bevel and maybe it's catching on a tiny scratch?
- I noticed the new rings did not have a one degree bevel like I had expected they might. Did I get the wrong rings?
- There doesn't appear to be any way to get a ring compressor on the piston while inserting it. Is there a sleeve or special tool for this?
- The old rings seem to fit ok, but not the new ones. Are the new rings bad (ironically they both measure exactly the same, dimensionally)?

Any thoughts?

 

[rcontrera]

Carbon rings can be a real pain if you put uneven pressure on the rings. We use a simple ring compressor.

 

[Peter_C]

You either need to find a way to gently push the rings into position, or the proper way as Ray suggests you need a ring compressor.

From your topic, I thought you meant how to break in the piston rings, once the compressor was running again.

 

[windyairman]

Hi
Frist thing to do is try to fit each new ring in to cylinder.
Ther must be a small gap between the ring ends,
This can easy be checked with a feeler gauge.
Looks as if your piston is in the 20 to 22 mm range ?
I set the end gap to 0.33mm.

You can also make sure that the lead chamfer is clean & not damaged.
Rather than just pushing the cylinder on to the piston.[easy to cause damage]
Try to rock the cylinder side to side in line with the piston pin to walk the culinder past the ring.
Then next ring
Rotate cylinder 2 full turns this will help to line ring up.
same again till its in place.

Its easy to do but hard to try and write how its done.

It may also be a build up of carbon in the ring grove,So the ring cant close up.
No form of ring compressor can be used with this set up as cyliner lead in is the way ring gap closes up.
Use no force or tap with hammer to re fit asy.

You will have to convert mm to inches.

Best of luck & hope it works out

 

[Wookie]

What those guys said. I just went through this on a pair of Mako 5406 blocks. The first one was a bear, the second one was very straightforward. I ended up using a ring compressor and leaving it on overnight to get the rings to take a set. I know that carbon rings don't take a set, but don't interrupt a good story. I soaked the jug and piston down with oil and it went better the third or fourth time I tried it. I went through a lot of broken rings before I got it right.

 

[windyairman]

Wookie
Mako 5406
How did you get rings on 4th stage piston with out them breaking?.

 

[Akyla]

Glad to hear I am not the only one that has broken a 3rd stage ring putting the cylinder back on.

I have found that making sure that the ring can compress into the well cleaned groove on the piston is most important, then a good bit of compressor oil on the whole piston and bore. Try and make sure that the rings are all alinged as even as possible around the piston, not deep in the groove one side and poking out the other. Using very little to almost no real pressure when sliding the cylinder bore over the piston is important and will keep you from breaking the rings. The cylinder will just pop right on the piston if all is lined up. If it resists, do not force it and pull the piston off and start over.

The bauer fourth stage has no rings, it is replaced as complete unit and just presses into the cylinder sleeve.

 

[Wookie]

Wookie
Mako 5406
How did you get rings on 4th stage piston with out them breaking?.

Put the rings on the end of the piston square. Push down with the tip of your thumb. They will spread evenly to go around the piston. Do you have old steel rings? Steel rings are much more brittle than the new composite rings. I've broken a LOT of Mako rings.

 

[kwindham]

Hey, thanks for all of the tips, guys. I am still waiting for my replacement set of rings in the mail , but will post the results when they arrive. Sure is easy to burn up a bunch of $$ on broken ones. I'm also glad to see that I'm not the only one with ring issues.

 

[kwindham]

ok, so I got those stubborn piston rings in. Here's how I did it - the gap on the existing rings was pretty close to exactly zero. I don't know how the last owner managed to squeeze them in there, but they did. To gap the rings, I pushed an individual ring into the cylinder bore about half inch with a similarly sized socket (to ensure it is perpendicular to the bore), then measured the gap with a feeler gauge. I opened up the gap with a jeweler's file in a carefully iterative process of checking and filing in small increments to the 0.33mm that windyairman mentioned above. Then, I cut a small square of aluminum from a tin can and wrapped it around the piston. I slid the ring over the aluminum "tube" and into each groove. This kept the rings from scratching the piston, and made installation much much easier.

After all the rings were gapped and installed, I packed them with plenty of thick moly grease as assembly lube and to help center them, per the suggestion above. I'm not using the compressor for breathing air, so I imagine some sort of food safe grease would also work, in that case. At this point, I carefully inserted the piston into the cylinder by very gently pushing and pulling about 1/8" or so and rotating back and forth, feeling when each ring "clicks" into the bore and advancing slightly so as not to pop it back out. It didn't take much more than 30 seconds of manipulating for the rings to fall into place and the piston to pop into the cylinder. Worked like magic the first time with no broken rings :blinking:! I left the head off and ran the compressor for a few minutes, which heated the bore a bit and most of the "assembly lube" melted out, which I wiped up.

So, lesson learned - gap is critical to preventing broken rings and you can't use too little lube. Moly grease works wonders. The proper ring gap and the new hone job completely solved the oil leakage problem I was having. It was really bad - to the tune of an ounce of oil lost every ten minutes or so...