Just found these two links describing how to make your own leak down tester. Both links describe virtually the same method...but slightly different instructions about how to use it. Sounds pretty straight forward...and a real $ saver if you happen to have a spare regulator and gauge (which I do) laying around in the shop.

http://www.xs11.com/tips/misc/misc3.shtml

http://www.650motorcycles.com/LeakDownTester.html


Dewey

Let's say you build one of each and one shows 10% leakage and the other one shows 5% - which one do you think is correct and why?

Looks like one has an orifice and one depends on the compliance of the regulator to establish the pressure drop. How about measuring the air flow directly with a variable orifice flowmeter?

I would think, (that right there qualifies anything further I have to say), That the tester that has the .040 orifice is the correct one. I am not sure what the 5 or 10% is in reference to. I assume the sized restriction inbetween the gauges the is the benchmark against which the leak(s) in the cylinder are measured. With out the sized orifice I don't think I would trust the regulator to stay consistant with varying flows. It would appear that the amount of air allowed to pass through the .040 restriction is the maximum allowable to leak past the rings,valves,etc. Any more air that is allowed to pass will show up as a pressure drop that can be measured. I would suppose that a smaller restriction in the tool would show a larger pressure drop when used on the same cylinder. Does that sound right? This leads me to believe that if you tested 2 cylinders of different bore sizes that were operating with the same efficiency, and you used the same tester, you would get 2 different readings. I have seen on the internet,(so I'm sure it is the truth,haha), leakdown testers that have a larger orifice that are designed to be used with motors over 1000 CID. (also saw it in my airframes and powerplants manual) I'm going to do a test soon on a couple of vehicles using both my Mac tools differential pressure tester and a 5 to 50 cfm flow meter, and see how compareable the outcomes are.

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I would assume the model with the .040" orifice restriction would be off on cylinders that leak more than the .040 will allow.