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Compression leak down test

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Plain
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Compression leak down test

#1

Post by Plain » Sat Jan 21, 2006 6:33 pm

Description: One of the more important tests, to me, that I have used over the years is a compression leak down test.

Post by Plain on Nov 16, 2004, 2:16pm

This has to do with the thread - "leaking oil, sounding awful, running bad" started by Kowalski0A5599. Rather than trample on that thread or lead it off in a new direction I am starting a new one, but this thread pertains to the discussion going on in that thread.
One of the more important tests, to me, that I have used over the years is a compression leak down test. A decent engine will have a 6% leakdown. A finely crafted engine will show a leakdown of 2%. As an engine wears, the % will rise and used in conjunction with a compression test will give a very good indication of what state the engine is in. If you do not have a leak down tester, one can be made for not a lot of money.
At the very least, after a compression test, charging the cylinder with 150 psi air, in a quiet shop, and listening carefully at the carburetor, exhaust pipe and timing hole plug will help to pin point a problem. Any good threaded compression tester can be modified to allow charging the cylinder. I should not have to say this, but I guess I must. If you do not have the bike in gear and the wheel blocked, or the engine suitably locked, hitting the cylinder with 150 psi will result in strange and exciting things happening.
I never see a leak down test or pressure charging the cylinders advocated in conjunction with a compression test on this board. Do the rest of you not consider this an important tool in diagnosing an engine problem? Seems to me that after the compression test, but before Kowalski pulled the heads, that charging the cylinder with air would have helped to predetermine whether the intake, exhaust or rings on each cylinder were most likely the problem. Maybe I am just stuck in the past.

Adios-----Plain

Post by 57pan on Nov 16, 2004, 5:00pm

Plain,

What is the % in reference to? I assume it means that a certain percentage of psi is lost over a certain period of time. How long should it take to drop 6% ?

Post by Plain on Nov 16, 2004, 5:53pm

57Pan:

This is complex, or at the least would take a lot of space to explain. Try this - http://www.xs11.com/tips/misc/misc3.shtml You should find the explanation sufficient, and it gives a good description of where to buy or how to make your own tester. Also some additional tips on different methods for using the charged cylinder to find other problem areas. If the link does not work, or you have additional questions, let me know.

Plain

Also http://www.vclassics.com/archive/leakdown.htm for a clearer explanation of leakdown percentage

Post by FastEd53 on Nov 16, 2004, 11:05pm

57Pan, I just used a old pair of refrigeration gauges to make my leakdown tester and it works very well. Take one gage to 100# and take the reading on the other gage. Haven't worked up the nerve to put it on the old pan yet, she runs fine and dosent smoke to bad so I dont want to know how loose she is! Just curious if any body has leaked a fresh rebuild with some breakin miles on it and what kind of readings they got. If so please let me know how far out there bored. Sorry Plain to jump ahead like this but I do believe it is a very good tool to help locate problems in a engine, I've used it for years on chevy engines. Ed

Post by Billy on Nov 17, 2004, 1:19am

Plain-
I agree a good leakdown tester is a good addition to anyone's box. I use it more for mutiple cylinders. 4,6,8
Better to 'isolate a single problem'
My reason for using Comp. test & then adding oil is that it will reveal if it's valves or rings. If it's valves I 'm not gonna pull off any OHV HD head & only do 1 valve.
Or for that matter only 1 piston. So I just need to know valves or rings & all in that 'related area' will be thoroughly gone over, on both cylinders....
Not taking anything away from "leakdown testing" which is very accurate.
I guess I'm used to this way on HD's..
But that's just me. :Smile

Post by Plain on Nov 17, 2004, 3:15pm

FastEd:

There should be a lot of different opinions to your question. I will give you my criteria. If you test just about any production Harley engine, new or with break in miles they will average 5% to 6% leakdown (excludes Showerheads, I have never tested or worked on a Showerhead). A carefully rebuilt engine by a good engine builder will test 2% or less. If you paid someone to rebuild your engine, and quality components were used, when you get her home you OUGHT to test at 2% or less. By the time an engine reaches 10% or more, it is going to be noticably down on power. Generally, when an engine reaches 12% my opinion is that it is due for a rebuild. That being said, the friends 54 Pan that I mentioned in an earlier thread has a 20% leakdown, and purrs down the road each day. So it is somewhat relative. Leakdown testing has no hard and fast rules, but rather lets you know more of what is going on in the engine to enable you to decide what to do, or not.

Billy:

I wasn't knocking compression testing and using oil to quickly determine rings or valves. That is often all one needs to do. I had just never seen leakdown testing mentioned and thought that another diagnostic tool ought to be "aired" out on this board. Like every other tool, leakdown testing can give you fits and there is a learning curve to doing it right. Positioning the piston at TDC can sometimes allow the piston to rock over a little and unload the bottom of the rings from the ring land surface. This will give a different reading than if the piston had been stopped at two to three degrees before TDC to ensure that the rings are fully loaded. Same cylinder, two different readings. Took me a long time and a lot of butt scratching to figure that one out. So, it is not a perfect tool, just another tool.
There is another thread that could be started concerning rings rotating and ring gaps aligning or nearly aligning. That will seriously effect a compression test or leakdown test. And, I guess that somewhere out there is another thread concerning cams. Seriously aggressive cam with long overlap will show less compression on a standard compression test. Mechanical compression versus dynamic compression and all that that entails.

Adios-----Plain

Post by 57pan on Nov 17, 2004, 5:27pm
''57Pan:
This is complex, or at the least would take a lot of space to explain. Try this - www.xs11.com/tips/misc/misc3.shtml You should find the explanation sufficient, and it gives a good description of where to buy or how to make your own tester. Also some additional tips on different methods for using the charged cylinder to find other problem areas. If the link does not work, or you have additional questions, let me know.''

Plain

Also http://www.vclassics.com/archive/leakdown.htm for a clearer explanation of leakdown percentage
Plain,

Thanks for the links - I get it now. :)

Post by Billy on Nov 17, 2004, 9:22pm

Plain-
I wasn't meaning you were knocking. I meant the more techniques we have & use, the better the end result is.
You're not stuck in the past. If anything I am...

Post by Plain on Nov 18, 2004, 1:59pm

FastEd53:

It never ceases to amaze me as to the inspired things that people can come up with. Modified refrigeration gauges. Never would have come up with that myself, but heck you have two gauges, two long hoses, and a neat hang it all up hook. I like it!!!

Post by FastEd53 on Nov 18, 2004, 10:49pm

Thanks Plain, I did a leak test on my neighbor's stroked small block ford, give him the figures and his dad look at them and said they were not accurate. Long story short his dad showed up with a new moroso leak detector and run his own test, well what do you know they were almost identical ! I like making stuff out of old pieces. Gives me something to do in the winter. Ed

Post by Plain on Nov 19, 2004, 5:46pm

FastEd:

Made mine out of a modified mechanical compression tester. I was proud of it until I learned about yours. Now every time I look on the wall at my old freon tester I can hardly stop myself from converting it to a leak down tester.



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Re: Compression leak down test

#2

Post by 58flh » Mon Jul 11, 2011 1:50 pm

somebody asked what is the % for? Simply put ,it is what we call volumetric efficiency--a good rebuilt (tight) motor & im going to use a smallblock chevy for example. It will be or i should say run at 80% V.E.! The reason the %s are lower for a panhead is 2cyl. & lower comp. ratios. That 2-3% is = to the 2% the men are talking about. How to get 100% V.E.-- you cant! atleast not in a naturally aspirated motor! By that I mean carburated engines. Reason being you have crankcase pressures that are vented ,always a little loss at the pistons from slipping pressure past the ring gaps & your carb. Even artificially aspirated motors are not 100% V.E. but do live in the realm of 90-91% V.E. Which is very good! That would translate to that 2-1% for a pan,shovel,knuckle-etc. hope this helps anybody curious about what a leakdown test can help you locate or what your state your motor is in. & yes this is a very good test & shoud be done more often than many think!---I ride my 58 everyday its my main source of transportation.So I do it like every 2 months unless I think I have a problem. (Good-Luck to all 58flh)

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Re: Compression leak down test

#3

Post by kitabel » Mon Jul 11, 2011 3:01 pm

Has nothing to do with VE.

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