Air leak test won't work

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Air leak test won't work

Postby brentporter11 » Sun Oct 24, 2004 9:39 am

I have tried everything. The gasket and brass o-ring are snugged
down, but not over tightened. I have put a bit of lubricant on these
pieces to ensure no air gets by, and I have sealed the crankcase-
cylinder-cap joints with silicone TWICE!! I have been irritated to
no end for two days trying to get this thing to work and I just can't
do it.
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Response to Air leak test won't work

Postby stan.hornbaker » Sun Oct 24, 2004 10:49 am

I assume you have an MM-1 Coffee Cup Engine Kit.

1. The two joints around the Displacer cylinder (short plexigas ring) must be sealed air tight with silicone as a part of the assembly.

2. The Power Piston (slicone rubber) diaphragm must be air tight.

3. The brass gland assembly for the Displacer Rod must be air tight EXCEPT that slight amount of leakage is permitted.

4.When making the air test, rotate the propeller till the diaphragm is in the bottom position and let it rest there until there is no tension visible. Pull up gently on the piston rod and let the diaphragm snap back indinating the the seals have been properly made.

On-line instructions for assembly are at URL:


and can aslo be reached from the Stirling Store MM-5 Kits under the price as "On-Line Instructions."
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Response to Air leak test won't work

Postby bnjwood » Sat Mar 01, 2008 7:54 am

Hello Brent,
I see your post is from October of 2004; I’m not sure if you finally got your engine running or if you set it aside out of frustration over the leakage. I too experienced the same problem just yesterday evening (2/29/08) as I was joyfully putting the finishing touches on my brand new (and very first Stirling) MM-5. Having taken my time over 2 days to carefully assemble my engine, I was very much looking forward to having it up and running before bedtime....what a glorious way to finish my day. When I tested for air leaks, I found that I had one. By the time I got to this test it was late, I was tired and as you can imagine....disappointed. So I set the engine aside and decided to get some rest before trying to find and fix the leak.

After getting that rest and with a cup of fresh coffee in my hand, I re-sealed the cylinder cap and cylinder ring by applying silicone sealant carefully on the outside joints of these two pieces and disassembled and reassembled the Piston assy. (about 3 times)....and it still failed the test. I then decided to make a leak testing tool to help me locate the culprit. To do that I first removed the piston diaphragm, then made a flat plug about the same size in diameter as the brass washer; I made it from some thin gauge sheet plastic but I think a plastic lid from a coffee can would work nicely. Using the brass washer I marked the four stud hole locations on the plastic and then punched out holes just large enough so that the plug would fit properly in place of the diaphragm. I then punched a hole in the center of the plug and inserted a 2 foot long piece of small dia. (4 mm) clear flexible tubing in the hole; the hole must be a little smaller than the tubing as a good seal needs to be made where the tubing goes through the plug. I then put the plug on the 4 studs (the black rubber washer is still in place), put the brass washer on top and snugged the 4 nuts down. I was then able to blow on the tubing creating pressure inside the displacer or suck on the tubing creating a vacuum. While holding the engine up to my ear and listening to different areas as I was blowing or sucking, I was able to hear the hissing of the leakage area. Unable to find the exact location of the trouble, I went to the kitchen and found a container (Rubbermaid I think) that was shallow but wide enough for the engine to set inside it, and filled it with water. I then carefully submerged the displacer cylinder in the water while blowing on the tubing and looked for air bubbles; I was surprised to find none, the leak was not in that area. CAUTION...sucking at this point could draw water inside the displacer cylinder if you had a leak; this would create additional problems...if you try this make sure you only blow.
I then continued to submerge the engine up to the point where the piston diaphragm and base of the displacer tube were underwater; still no bubbles. Now I'm really scratching my head and wondering where this leak is. After carefully drying off the engine I listened again for the leak. As I was doing this, I felt a slight puff of air on my hand that was holding the handle. Further investigation revealed that the air was coming from where the displacer rod passes through the Brass tube on the crankcase. After my discovery, I sealed the leakage area (I took the rubber tubing provided for the connecting rods and placed that over the displacer rod and then plugged the end of the rubber tubing with the small hex wrench supplied with the engine kit; this nicely sealed the leakage area) and then used the test tool; no more hissing could be heard....I had found the leak. I then reassembled the piston and tried the leak test again; this time it worked...the piston snapped back as it should indicating a vacuum inside the engine. I then removed the temporary sealing device (hex wrench and rubber tubing), continued with the assembly of the engine and once finished took it for it's test run on a cup of boiling water. And it ran. It ran just as I was expecting it to, and ran for about 10 minutes till I removed it from the heat source to do some experimenting with ice...it ran on ice too.

If you were never able to find that leak, you might want to try making the test tool. I'm sure my engine will run even better if I can seal the leak at the displacer rod; I'm not sure how I'm going to do that, possibly with unwaxed thread wrapped gently around the area (don't want any fibers to get inside the brass tube).

I hope you have had, or will have, success with your engine as I have with mine.

Bud Wood
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Response to Air leak test won't work

Postby bnjwood » Mon Mar 03, 2008 7:49 am

Also, if your sealant is not cured completely only pressurize the chamber; you don't want to chance having any of the vapors enter your mouth. You could use a vacuum pump just to be safe.
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