This 2650 series Thomas compressor has run near constant for almost 4 years now, and the diminishing air output is beginning to reflect all those hours of use. So before ice cover sets in, I thought I would do a complete rebuild of the top end. This is what came in the kit: new cylinders, cups, reed valves, and o-ring gaskets.



First step was the removal of 8 screws that hold the head on. A T25 Torx driver was needed for this.



I was surprised to find that the cylinders(?) were essentially free floating, captured between the head and the main body. They came off with the head.



To get to the piston cups, which would be comparable to a piston ring, I had to disassemble the piston by removing one screw.



The piston is made in two pieces, with the cup in between.


The difference between the old cup on the left, and the new replacement one on the right, is pretty noticeable, as the new one has a larger outside diameter. The old cup tended to stay compressed, while the new one was soft and flexible.



Lots of wear on the cylinders also. A definite ridge could be felt with my finger.


Replacing the cups was the trickiest part of the entire procedure. I finally settled on placing the cup in the cylinder, placing that assembly over the lower piston half, then placing the upper piston half on top of it, and tightening the screw while hoping everything remained centered. Surprisingly, it worked.


On to the valves. There are 4 total, two for the intake, and two for exhaust...which in this case is the pressure outlet. They are simple, flapper style valves, similar to the old reed valves.




These are the four parts that comprise the valve assemblies. A metal flapper, a rigid stiffener (plastic?), a square metal retainer, and the hold down screw. Note that the two intake valves do not utilize the plastic stiffeners.


From here, it's just a case of replacing the formed O-rings and reassembling. Very straightforward. Once I had everything back together, I hooked up the pressure gauge and flipped the switch. What a difference! I can actually hear the stronger compressor output.

Next step, designing a better air filter.

Thanks Tony. Good stuff!

re: better air filter. Think B&S cone shape. It might entail some welding and fabrication, but nothing you can't handle.

Great info instruction of how to rebuild a rocking piston. Some brands are slightly different but still quite similar for rebuilding. Thanks for taking time to show how to do this!!!

Sparkie, a couple simple filter designs I'v used that are tremendously better than "factory" was! Fist change was to use a 5 micron, paper water filter on the original filter base/cap by replacing the stud a 12" piece of all-thread.

I eventually used a 9" OD, cheaper Diesel truck air filter cartridge with a piece of all-thread through a couple 3/16" thick aluminum disc's I cut from scrap with the 1/4" inlet drilled/tapped for a 3/8" hose barb x 1/4" MNPT....It ran 3+ years without being cleaned with zero flow restriction noticed in my vacuum gauge I put in.

I also used a Luber Finer 750 metal cartridge oil filter by plugging one end with a rubber freeze plug and then used the rubber from a plug, drilled it our some and screwed in the 1/4" MNPT x 3/8" hose barb into the other end to expand/seal the center hole of the filter on the other end. You get far less restriction, MUCH longer service life, and will filter out much finer particles than factory filters.

FWIW, The Lubri-Finer is really easy to use as a filter, plus the metal casing makes it critter proof...(a mouse chewed holes into the air between the metal screen mesh)

Thanks all. One thing I should probably mention, is the need to reinstall the valve bodies (plates) in the same orientation as they were removed.

Here is one of the two valve bodies, with the reed assembly in pieces:


And here is a shot showing the compressor with both valve bodies in place, awaiting the installation of the head...note that the reed valves in both bodies are aligned in the same manner, as in the reeds are 'up' in the photo, closest to the white pvc. It is possible to install these bodies 180 degrees rotated, which would place the valves "down". I believe the compressor would still function, but the inlet/outlet ports would be reversed.


If only one valve body were rotated 180 degrees however, the compressor would effectively be working against itself, with one cylinder providing pressure while the other cylinder was trying to intake. Probably cancel each other out.

Finally got around to fabricating the air filter. My design considerations were low weight, compact size, inexpensive, and easy to access with no tools required.

The original filter was a simple, rectangular box, with the air cleaner being an integral part...in other words, it wasn't possible to change just the element itself. I didn't like that, as it's bound to translate into higher costs. And in fact, the o.e.m. filter retails for around $22-$26. To be fair, the original unit is probably a hepa filter....not needed for an aeration application, so no reason to pay for that.

Original unit on left, my design on right. Both utilize 3/8" female pipe threads.


Some of you undoubtedly recognize my filter choice as being used on a pushmower engine. I calculated the displacement of the compressor at around 92cc's, and the filter is intended to flow adequate for a 190 cc engine....sounds good to me.

All of the components. When installed, a simple 60 degree rotation of the filter housing will unlatch it, and it will pull straight off. Reverse for installation.


Height of original element.


New element.


All of the components. I have about $20 in a one-time shot for everything, with replacement filters running $5-$6.

Great thread Tony. So, my uncle used to rebuild everything he owned, and he used Marvel Mystery Oil to lubricate all the top end parts as they were installed. Do people still do that, or is it better to rebuild stuff dry (non-lubricated)?

Very nice detailed rebuild instructions. These pumps are really pretty simple to rebuild after you have done a couple. I also did not like the idea of paying over 20 dollars for a single use/disposable air filter for these pumps. I found a very compact air compressor filter with a metal housing and replaceable paper filter element on E bay. They cost $8.85 w/shipping and have the 3/8" male threads. Replacement filter elements are $5.75 w/shipping. I have been using one of these for about six months and it has worked great so far. If anyone would like, I can post a link (if appropriate on this site) for them or you can PM me.

Al, if there is direct metal-to-metal contact I would definitely add assembly lube. The way these pumps are made, (with non metallic piston "ring") I don't think it is necessary.

Troy, your air cleaner sounds like a pretty good score! I agree, these pumps are very simple. And with the way they are designed, they can be rebuilt almost indefinitely.