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Re: Right way to do Leak Down Test?
[Re: HardcoreB]
#2849139
11/20/20 03:37 PM
11/20/20 03:37 PM
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Joined: Jan 2003
Posts: 25,791
Rio Linda, CA
John_Kunkel
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 25,791
Rio Linda, CA
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The cylinder leak down test is humorously referred to as "the A&P mechanics guaranteed employment test" because leak down of 25% or more in aircraft is considered "fail" even though the engine runs fine and makes plenty of power.
Consider the makeup of the tester, air flows through an orifice that is .040" in diameter...that is an opening of 0.00125 square inches in a cylinder that can be anywhere from 30 to 70 cubic inches in volume.
The INTERNET, the MISinformation superhighway
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Re: Right way to do Leak Down Test?
[Re: John_Kunkel]
#2849436
11/21/20 10:17 AM
11/21/20 10:17 AM
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Joined: Nov 2020
Posts: 348
Isle of Sheeps
Gtxxjon
enthusiast
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enthusiast
Joined: Nov 2020
Posts: 348
Isle of Sheeps
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To whom are you referring John. Are testers NOT allowed to wear make-up???  I've been an engineer mechanic for over forty years and NEVER done a leakdown test...  What does it prove? Compression is being lost past the rings and valves and/or the head gasket. A good mechanic would know if there is a problem by the signs? We always know, that when an engine is making good power, then its worn out and ready to BLOW!  The trick is, knowing when to make the LAST run before this happens... 
Last edited by Gtxxjon; 11/21/20 10:20 AM.
Carpe diem, quam minimum credula postero, thanx Horace!
There’s no point trying to fix stuff that ain’t broke,,, 'but if ain’t broke',,, you is not trying hard enough...
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Re: Right way to do Leak Down Test?
[Re: Gtxxjon]
#2849440
11/21/20 10:27 AM
11/21/20 10:27 AM
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Joined: Jan 2003
Posts: 8,026
Tulsa OK
Bad340fish
master
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master
Joined: Jan 2003
Posts: 8,026
Tulsa OK
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To whom are you referring John. Are testers NOT allowed to wear make-up??? I've been an engineer mechanic for over forty years and NEVER done a leakdown test... What does it prove? Compression is being lost past the rings and valves and/or the head gasket. A good mechanic would know if there is a problem by the signs? We always know when an engine is making good power, when its worn out and ready to BLOW! The trick is, knowing when to make the LAST run before this happens... A leak down tester is a great tool to diagnose something before it gets worse. I'll use my small block as an example. Car was running fine, ran a new best of 10.54 just a few weeks before I did a leak down and compression test just as a health test. I found 2 cylinders with leaky exhaust valves that amounted to about a 35% leak down number, the good cylinders averaged 3%. So off with the heads, new hardened exhaust seats and now the whole motor averages 3% leak down. I haven't beat that new best ET yet but I have added about 80lbs of interior back and my MPH has gone up so its making more power. I guess I could have just waited until it was backfiring from trashed exhaust seats but the leak down test seemed like a good idea. With a marked balancer its about a 10 minute test and can give you some great info to keep you from chasing your tail.
68 Barracuda Formula S 340
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Re: Right way to do Leak Down Test?
[Re: Gtxxjon]
#2849507
11/21/20 12:25 PM
11/21/20 12:25 PM
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Joined: Jan 2003
Posts: 25,791
Rio Linda, CA
John_Kunkel
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 25,791
Rio Linda, CA
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To whom are you referring John. Are testers NOT allowed to wear make-up??? noun Definition of makeup: the way in which the parts or ingredients of something are put together
The INTERNET, the MISinformation superhighway
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Re: Right way to do Leak Down Test?
[Re: John_Kunkel]
#2850874
11/24/20 11:17 AM
11/24/20 11:17 AM
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Joined: Nov 2020
Posts: 348
Isle of Sheeps
Gtxxjon
enthusiast
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enthusiast
Joined: Nov 2020
Posts: 348
Isle of Sheeps
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Apologies John, looks like my attempt at limey humour has failed...
Running without unleaded seats in 2020 Wow, how did you last this long lol...
American gas must be a lot better than UK fuel, (gas).
Built a 340 for a guy who ran it down the track for one day, seats destroyed...
Heads had lasted 40 years till then...
UK John without makeup...
Carpe diem, quam minimum credula postero, thanx Horace!
There’s no point trying to fix stuff that ain’t broke,,, 'but if ain’t broke',,, you is not trying hard enough...
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Re: Right way to do Leak Down Test?
[Re: John_Kunkel]
#2851330
11/25/20 12:02 PM
11/25/20 12:02 PM
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Joined: Jan 2011
Posts: 4,785
Utah and Alaska
astjp2
master
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master
Joined: Jan 2011
Posts: 4,785
Utah and Alaska
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As an aircraft mechanic I've done lots of these, the publication AC 43.13-1b outlines the correct procedure. The test is done on a warm engine and, to start the test, air is introduced into the cylinder while the piston approaches TDC and is finalized with the piston at TDC. The airplane has an advantage because of the prop, you can grab a prop blade and manipulate the piston position instead of just parking it at TDC. This assures that the ring is fully seated against to bottom of the ring land.
and to add to what John said, there are different orifice sizes based on bore size. Put in 80 PSI and you should read better than 60 for some engines, 42-43 psi for others as long as it is not through the exhaust.
1941 Taylorcraft
1968 Charger
1994 Wrangler
1998 Wrangler
2008 Kia Rio
2017 Jetta
I didn't do 4 years and 9 months of Graduate School to be called Mister!
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