The Stovebolt Site Forums The Shop Area The Tool Chest New compressor piston/sleeve too tight!

As I earlier mentioned in a different post, my Sears 25 Gal. upright compressor wouldn't build air. I finally decided the old piston was worn and bought a replacement.

Got the new part yesterday and put it in. From all appearances an exact replica. The old one was so worn you could see daylight around the edges. The problem is the new one is quite stiff in the sleeve and the motor isn't able to kick it over for a full revolution and kicks the breaker. It's an airless compressor but is there some lubricant I could shoot on the cylinder to decrease the resistance? If I can get the motor to just start it I'll leave things open and break it in for 20 minutes.

Hello, Just off the top of my head.I would not trust it to "power through" and break in without first knowing what clearance you have between the piston and sleeve (bore). Do you have any specs for the compressor? Did you check the rings end gap? Rule of thumb is .003 for every inch of bore for end gap.You don't want them to butt up.Can you measure the old piston and the new? It could be a hone to fit with .xxx clearance.Let us know.

"oil-less" compressor? Most have a piston ring that is non metal and gap-less. That will make the ring end gap hard to figure. If you can remove the ring and fit the piston you can check the fit. It should be a free running fit and that was about 0.003 inch. Get a long feeler gauge at 0.002 and 0.005. Put the feeler in the bore and slip the piston in with it. If you can pull the 0.002 and not the 0.005 you are about right. If the piston was rough you might need to hone the cylinder just enough to smooth it out. When you re-install the piston with the ring be careful not to catch or roll the ring as you go in. Then see if you can move the piston in the bore. If you can the motor should be able as well. Air less is what you are trying to fix

It's an oil-less compressor. Well this is a real simple piston/crank/sleeve combo. Nothing to adjust. The ring comes premoulded to the piston. The ring is made of some sort of hard rubber/leather. The cylinder wall isn't much more than a pop can. My guess is that tolerances never came to mind when designing this "Craftsman" compressor. Just sayin'

What I ordered is supposed to be a direct replacement for my model. This is what it is. Now here is something I noticed that may be of interest. When in compression stroke there is much resistance. That is to say with all valves open. I would think there would not be resistance. But then I'm not sure what is going on in the head of the unit. I assume that the air goes through a check valve into the tank. Fairly simple?

WM - I digress from your compressor issues. What's the story behind your 26 Model T Truck?

Allen, I found it as you see it a couple years ago from a local guy. It was restored some 25 years ago but still looks grand. All I've done is new gas tank, axle seals, and manifold. Starts good. It takes all your concentration to drive the thing. You think our old trucks are slow, try 25 mph top speed but 17 is all you want to hit!

I just got done replacing my piston and sleeve in my 3 1/2 hp Craftsman Oil less compressor. Wish I'd seen these, I bought mine from Sears.....
This is the second time I've replaced the piston and sleeve. When first got the compressor it seemed to wear out in under a year. Replaced it and it lasted for nearly 20 years. Must have improved it some.
Was going to replace it when it needed replaced this time, but looking at new ones, I found none that rated very well on the different sites. So I decided to just rebuild this one again. I also replaced the check valve in mine.
When you replace the piston and sleeve you should run the compressor unloaded for 15 to 20 minutes for break in, allowing it to not build pressure. I usually leave the drain clear open and the regulator open too.

John

Thanks John, I too have left the drain and regulator open for the break in. No luck. I still have to take it apart and see why it is so tight yet. One reason it probably is, is because the piston does not pivot about the crank, making the ring change from round to oval as it travels up and down.

Yeah the piston actually rocks in the piston sleeve. Makes an odd wear pattern on it. This last kit I got looks like they plated the cylinder or something.

John

Just an update. The cylinder has minute machine grooves that I lightly sanded and with the help of a little WD40 got it running again. Now I just have to lower the contact settings so it doesn't blow the pressure relief valve.