MCH-6 low fill rate and O2%

[NAM001]

Are you referring to the 3rd stage rings? The 4th stage piston does not have rings.

Yes the final stage is usually a floating piston and in my case has non metel rings on it ,,,,, more of an O ring type which if scarred will leak into the crank case. Again an air flow gage on the crank case will tell how much is leaking by. I suspect you will find a 80% leak if not more. If you buy another piston get the asssembly in a cylendar. then you just tap it in to place because the rings are already compressed.

When you finish the repair then check the crankcase vent and measure teh air flow. If you have say a 6 cuft compressor and the blow by is .8 then yo uare pumping 5.2 cuft a min. check the blos by at 2k 3k and 4k psi. thta should give you a good perspective on how the compuressor shoudl preform at various back pressures. Mine was 6 cuft at 3k and 5 > 5.5 cuft at about 3800 psi.. of course then check it in dymanic performance by filling a tank from a fully charged filter system. a BPR on the fill whip will make that work for you.

 

[SurfLung]

Trouble Shooting Slow Flow

- I've had slow fills due to a head valve leaking... It was the 3rd stage leaking back into the 2nd stage. But the symptom was pretty obvious: The 2nd stage over pressure valve was releasing. Disassembled and cleaned the 3rd stage head, reed valves, and replaced O-rings. That solved it.
- I've had slow fills after replacing all the piston rings. Worked great at first but got slower and slower. I went through each piston one by one from 3 to 2 to 1. The leak was in the big old #1 piston that has only one ring. The ring itself was a little too long and overlapped itself one time too many when compressed.This caused the ring to stick in the compressed state in the ring groove. I un-stuck the ring, trimmed it short enough not to overlap in the groove, made sure it compressed and expanded freely before re-installing. AMAZING how the fill speed returned when the 1st piston could compress air again.
- I have had slow fills due to the wrong sized sheave (pulley) diameter. Have you confirmed the RPMs instrumentally? I was not aware that my used RIX SA3 had been modified to run slower with a different diameter sheave (pulley). My friend AntiqueDiver calculated the correct pulley diameter and it was much bigger than what was on the motor. We changed the pully and the fill rate corresponded more closely with the proper RPMs. Later, I bought a laser RPM meter and was able to confirm the actual RPMS of the pump. I then bought an adjustable sheave and was able to tune the RPMs to almost exactly 2300 rpm (factory spec).

 

[zider]

@SurfLung It's sounding more and more to me like an issue with the piston rings on the 3rd stage. The 3 rings on the 1st stage and the 4 rings on the 2nd stage all looked pristine when I had the unit apart. These also reinstalled with no issues at all. The 3 rings on the 3rd stage were a different story. They appeared to be made of a different material and were soft, so maybe graphite? The 3rd stage rings looked ok at disassembly but I really had to manhandle them to install the piston into the cylinder for reassembly. As I noted in my original post, I needed a second set of hands to push the ring from the back side so I could compress it by the spring. We also had to use some plastic punches to push on the rings as they were nearly impossible to compress with your fingers.

@SurfLung I have not actually measured the RPMs with a gauge. The motor pulley is 3 inches and runs at 3600 per the motor manufacturer' s specs. Using the standard 100mm pulley on the compressor that should work our to just around 2750 RPM, which is darn close to the pump spec of 2800.

@KWS My MCH-6 is an old one so it doesn't have the air bleed port found on the more modern units. The only access to the crank case with the unit assembled is the oil filler tube (see picture). Do you think I could hook some tubing to this port to test the rate of air seeping back into the crankcase?

 

[SurfLung]

@zider
- I just thought of something. Does MaxAir or Coltri have an insertion tool for that piston? My RIX SA3 3rd stage might have been just as difficult as your's except I had an insertion tool... Basically a tapered tube. I was able to gently install the 3rd stage piston into the tube, gently squeezing each ring set under the lip until the entire piston was in the tube. THEN, installing the piston with rings into the cylinder was a simple matter of positioning the tube in front of the cylinder and pushing the piston into it.

 

[zider]

@SurfLung Man I wish they did but as far as I know this doesn't exist. The youtube video below shows Coltri's recommended installation steps at around the 3:20 mark. It looks super easy here but that was not my experience.

 

[NAM001]

@zider
- I just thought of something. Does MaxAir or Coltri have an insertion tool for that piston? My RIX SA3 3rd stage might have been just as difficult as your's except I had an insertion tool... Basically a tapered tube. I was able to gently install the 3rd stage piston into the tube, gently squeezing each ring set under the lip until the entire piston was in the tube. THEN, installing the piston with rings into the cylinder was a simple matter of positioning the tube in front of the cylinder and pushing the piston into it.

Dont you have the option to replace a drop in cylendar/piston assy when you replace the 3rd stage. Im not sure but I thought the cylendar came out like a piston would come out of a cylendar. pull the old one out and replace with a new cylendar and piston and rings unit. No damage to the picston rings or problems with a damaged wall that could destroy the next set of rings. I seem to recall something about a tapered funnel like thing to insert a piston when changed only.

 

[zider]

Yeah, if I replace the piston and cylinder then I would just need to pop it on The connecting rod with the retaining pin. The thing is that the piston and cylinder will be over $200, where replacement rings are only $20. I might try replacing with new rings first before jumping all in for another $200. The piston and cylinder themselves were in nice shape when I had the unit broke down.

 

[zider]

@KWS i attempted to measure blow-by into the crankcase by seeing collecting the flow out of this oil vapor recovery port on the side of the oil filling port (see pic). After reaching about 3000psi, I timed how long it took to fill a gallon ziplock bag. It took ~1.5min so it appears the blowback first the crankcase is less than 0.1cfm. It might not be the most accurate way to test but it seems to me that that a couple CFM aren’t flowing back into the crankcase. Do you still think there could be issues with the piston rings?

 

[SurfLung]

Yeah, if I replace the piston and cylinder then I would just need to pop it on The connecting rod with the retaining pin. The thing is that the piston and cylinder will be over $200, where replacement rings are only $20. I might try replacing with new rings first before jumping all in for another $200. The piston and cylinder themselves were in nice shape when I had the unit broke down.

On my RIX SA3, the easy 3rd stage piston kit includes the assembled piston and insertion tool and costs over $700. Since I had actually broken a piston, I had to buy a new one. But I discovered if I bought the piston and rings separately and assembled them myself, it cost half as much. When repairs get into the $700 range, and a brand new Coltri is $2995, you begin wondering if you should scrap the old and buy a new one. It was a close call but for approx. $350 in parts, I got the RIX back up to running as good as new... So it was worth it. The thing is, a lot of my repairs have been a guess as to whether the thing I was repairing is what was causing whatever problem I was chasing. I enjoy figuring stuff out. But, I have to say I'm not so sure I have saved much money buying used and fixing up! (A brand new Coltri MCH-6 or MAxAir 35 could be paid off in a year at $250 a month).

 

[rjack321]

@KWS i attempted to measure blow-by into the crankcase by seeing collecting the flow out of this oil vapor recovery port on the side of the oil filling port (see pic). After reaching about 3000psi, I timed how long it took to fill a gallon ziplock bag. It took ~1.5min so it appears the blowback first the crankcase is less than 0.1cfm. It might not be the most accurate way to test but it seems to me that that a couple CFM aren’t flowing back into the crankcase. Do you still think there could be issues with the piston rings?

don't do this too often, the oil mist in there is a lubricant to the first stage.