Stuck on the Air Leak Test (MM-5)

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rick___4532
Posts: 1
Joined: Mon Sep 15, 2003 8:44 pm
First Name: Richard
Last Name: Flanders

Stuck on the Air Leak Test (MM-5)

Post by rick___4532 »

Hi,

My son and I enthusiastically assembled our first MM-5 coffee cup
engine and we were doing great until we got to the air leak test.
It failed. Obviously, we have an air leak. So we filled every nook
and cranny with silicon and it still fails. I used plumber's tape
to close up every possible gap, including around the brass crankcase
tube. We fail. I switched to our "spare" diaphragm. Fails. I am
at my wits end. It looks like most of you built these things in a
few minutes, with little trouble. So where could we have gone wrong?

Here are some questions I have: I am trying to understand how the
cylinder tube can be airtight if a wire moves freely up and down
through it. Obviously, that part is not airtight. So how could the
leak test possibly work? Also, the diaphragm is a bit wrinkled - no
matter how careful we are. And the brass ring easily bends when the
nuts are tightened. But I am not sure how to overcome these
problems. Any help would be greatly appreciated.

Thanks in advance,
Rick
stan.hornbaker
Posts: 532
Joined: Thu Jun 20, 2002 6:01 pm
First Name: William S.
Last Name: Hornbaker

Response to Stuck on the Air Leak Test (MM-5)

Post by stan.hornbaker »

Assuming that the displacer chamber joints were properly glued there are only two other places where leaks can occur.
First the tube and displacer rod are a sliding fit and have little leakage at the pressures in the engine. This is NOT the problem.
Second you mentioned the brass ring holding the diaphragm bends when the nuts are tightened. This is the area of difficulty. The nuts need be tightened ONLY FINGER TIGHT plus not more that 1/6 of a turn or one 'flat,' if that much. Now be sure that the brass washer is flat, and use some very fine 500 or 1000 grit wet or dry abrasive paper to gently remove any sharp edges from from the washer. Apply a very small amount of silicone GREASE to the black rubber gasket so as to make it look wet, also apply to the bottom of the brass washer.
The diaphgram, washer and nuts are assembled almost finger tight and the diaphgram pulled up 1/8 " and the nuts tightened 1 flat. Over titghtening will distort the brass washer and the seal is lost.
When pulling up on the diaphgram for the leak test the pressure inside the engine is still nearly atmospheric so the leakage at the displacer support rod seal is nearly zero.Hobby shops should have the fine abrasive and silicone grease.
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