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Author Topic:   Leakdown Testing - How To
skookum point posted 04-23-2003 12:26 PM ET (US)   Profile for skookum point   Send Email to skookum point  
I'm trying to decide which of two motors has a better future. Assuming both have good compression, I have read that a leakdown test is the next step. I assume the test measures how long the cylinder can hold compressed air. Will someone explain how to do a leakdown test? Is a leakdown tester necessary or can a compression gauge be adapted to do the job? If a leakdown tester (tool) is required, where can it be purchased and what will it cost. And finally, what is the testing procedure?
Sal DiMercurio posted 04-23-2003 07:02 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Mac tools makes the best one, but they probaly cost close to $80.00 now, [ got mine brand new at a flea market for $10.00 still in the box ].
You can make one, but look at Macs first & copy it.
You will also need an air compressor.
Best to do the test on an engine thats just been started, [ warm ].
Pull one of the plugs, screw the tail into it, open the valve from the compressor till it reads minimum 50 lbs, be sure the piston it at tdc , if the piston dosen't get pushed back down, try 100 lbs.
When the desired preasure is reached, shut the valve from the compressor & both guages should read the same.
It should hold it for quite a while if the compression [ rings ]are good.
If it drops down right away, your rings are shot.
dgp posted 04-23-2003 07:55 PM ET (US)     Profile for dgp  Send Email to dgp     
Try this site for building a leak-down tester,
skookum point posted 04-24-2003 12:13 AM ET (US)     Profile for skookum point  Send Email to skookum point     
Thanks Sal and dgp, that's exactly the information I needed.


Bigshot posted 04-24-2003 09:55 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
A cheaper way of doing it is after you get a reading, squirt some oil into the cyl and see if the reading changes, not 100% but cheap and easy.
bsmotril posted 04-24-2003 10:01 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
I guarantee the reading is always going to change after you squirt in the oil. A leakdown Tester will pinpoint a lot of other problems besides bad rings. If you pull the other plugs, then pressurize each cylinder in swquence while checking for air coming out of other cylinder plu holes, you can identify leaky crankshaft seals. You can also find bad reed valves by looking for air coming out of the carbs. A lit punk or cigarette smoking away in front of the hole is a good air detector.

With a Fourstroke motor, you can do even more in the way of finding leaky intake or exhaust valves by looking for air coming from the intake or exhaust while you test each cylinder in sequence. Make sure you turn the motor over so that as you test each cylinder, it is at TDC with both valves closed.

mustang7nh posted 04-25-2003 05:22 PM ET (US)     Profile for mustang7nh  Send Email to mustang7nh     
what is the desired pressure for the leak down test to turn off the compressor at? Thanks
Sal DiMercurio posted 04-25-2003 10:59 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
You don't turn the compressor off, you shut the 1st valve.
5o is good, but I usually try for 100 if possible.
Even 50lbs is alot psi.
skookum point posted 04-26-2003 12:47 AM ET (US)     Profile for skookum point  Send Email to skookum point     
OK, I have assembled a leakdown tester and I'm ready to give it a try, but there is one thing I'm not clear on.
Sal - your instructions make it sound like the piston should move from TDC to BDC when the cylinder is pressurized. Is that right? Other sources seem to indicate the flywheel should be immobilized to hold the piston at TDC. Yet another source recommends locking the piston mid-stroke (2-stroke only). Can you shed a little more light on this?
Sal DiMercurio posted 04-26-2003 01:21 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Yes, lock the flywheel or the piston could be forced down [ to much preasure ] & the compressed air will go out the exhaust.
skookum point posted 04-26-2003 03:50 PM ET (US)     Profile for skookum point  Send Email to skookum point     
Thanks Sal !

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