MM-5 leak, displacer/power piston contact

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MM-5 leak, displacer/power piston contact

Postby bnjwood » Sun Mar 02, 2008 7:00 am

Yesterday I finished assembling my first Stirling engine; the MM-5.
Great experience and it runs very well even though it has a leak that
caused the Air Leak test to fail repeatedly. After much trouble
shooting, I found the leak source to be at the displacer rod tube.
The engine seems to run fine (ran for 35 min. yesterday on a cup of
boiling water), but I think it would run better if this leak was

Also the displacer on it's uppermost travel makes contact with the
power piston at it's lowermost travel. I can see the contact cause
the foam displacer to bend, and I see this makes the displacer rod
shift, pressing it against the displacer tube. This surely causes a
reduction in engine performance.

I'm sure that others have experienced the same situations with their
MM-5's. Does anyone have suggestions on how to minimize the leak and
prevent the displacer/power piston contact?

Also, if anyone has an MM-1 (pre-assembled engine), does it have the
displacer/power piston contact problem?

Thanks in advance for your help,
Bud Wood
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Joined: Sat Jan 26, 2008 1:51 am
First Name: Bud
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Response to MM-5 leak, displacer/power piston contact

Postby longboy4 » Sun Mar 09, 2008 4:58 pm

I don't have a MM5 motor but looking at the sales photos it looks like there is a black tube that crimps on and joins the two halves of the connecting rods for both the power and displacer connecting rods. You will have to shorten the power piston rod so the piston doesn't travel far enough to make contact with the other piston. This will also make that piston rise higher on its up stroke. Hopefully you have enough room in the power piston bore to center the piston stroke so it rises and falls without hiting the displacer piston on the down stroke.......or hits the top end cap on the up stroke. Air leaks thru the push rod gland is acceptable so if you are up and running.....don't fix it! Dave.
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