Leak Down Procedure (Pic Heavy)

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seabee

seabee

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Leak Down Procedure (Pic Heavy)

Did a leak down test on the 273 today and took some pics on how I did it for the next guy that searches it. Please add any , corrections, advice, or tips.

1. Find Top Dead Center on Piston #1. I used a piston stop to make sure the balancer hadn't shifted.
View attachment 1714983303

View attachment 1714983304

Using a small level I made horizontal and vertical marks:
View attachment 1714983305

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Each 1/4 turn of the crank will put the next cylinder in the firing order at top dead center. Firing Order: 1 8 4 3 6 5 7 2
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Next I checked the gap (solid, non hydraulic lifters) to make sure the valves were fully closed. I could also just remove the rocker arms completely.
View attachment 1714983309

Then I used the piston stop on the cylinder opposite the firing order to prevent the crank from turning under the air pressure. Opposite pistons: 1:6, 2:3, 4:7, 5:8
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Then Applied 100PSI to get the first reading. Number 8 cylinder is losing 20%.
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Then I put the spark plugs into the neighboring cylinders to help rule out a head gasket breach between cylinders.
View attachment 1714983312

Next I use a piece of hose to my ear and listen to locate where the 20% of air is going to.
Carb: Intake Valve
Exhaust:Exhaust Valve
Dip Stick (Crankcase): Past the piston rings (Normal)
Radiator: Cracked head or cylinder wall.

I wrote down all the results:
View attachment 1714983313

Might be time for a rebuild, what do you think?
Click to expand...

Read more about this article here...
 
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Curious, what's up with the retainer next to the valve you're checking gap on?
 
Only thing I know to add:

With valve gear in place, you must check piston at TDC. If you have something like a rough/ cracked cylinder wall, this wont' show up. Better to run at TDC, then pull rockers and run again all cylinders at BDC

Also, this test will show different results hot and cold.
 
seabee said:
The spacer shims? (circular thin washers)
Click to expand...

On the on the spring retainer, runs around the top. What is that?

Leak Down Procedure 7-1.jpg
 
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Marks from manufacturing I guess. Several of the retainers have them, which is why I didn't notice.
 
67Dart273 said:
Only thing I know to add:

With valve gear in place, you must check piston at TDC. If you have something like a rough/ cracked cylinder wall, this wont' show up. Better to run at TDC, then pull rockers and run again all cylinders at BDC

Also, this test will show different results hot and cold.
Click to expand...

Del, just curious as to what the difference would be BDC vs TDC?

Great thread, thanks Seabee!
 
Question. Is the test performed when the engine is cold or after warming to operating temperature?
 
First, excellent post ! THX added.
Second, I (with Your permission Sir). would like to add an note on the hot/cold test
parameters.
Pistons are, always have been, and always will be of an 'cam ground' design--- read the larger mass across the pin bosses , when heated, 'pulls' the smaller mass of the casting/forging into a circle.

Apologies, suffice to say, do the test no more than 10 min after normal operating temp has been reached.

Now Sir on to My humble answer on Your rebuild 'Q'.

seabee said:
I wrote down all the results:


Might be time for a rebuild, what do you think?
Click to expand...


Left bank total % = 56.
Right bank total %= 60.

total variance 4%...bank to bank anyway. further,

#1-#2 delta , 4%.
#3-#4 delta , 4%.
#5-#6 delta , 5%.
#7-#8 delta , 1%.

'Delta' an term used for 'total' overall engine 'sealing' health. as it were.

In Your case Sir 14% total delta = changing #8 when it oil fouls, probably
after 3-4k miles

Thx to the board for My long winded reply !!!!
 
A low cost leak down gauge can be built from two 0-100 gauges, a pressure regulator, and a plug between the gauges with a 0.040" hole. This is what the piston engine aircraft rebuilders use to check leak down.
It can also be used to check manifold leaks by pressurizing the intake. You will need to make a plate to cover the carb side of the manifold and tap for a fitting. Pressurizing the intake and using the leak down gauge will help identify if there is an intake leak and where it is. You will need to remove the rocker arm assembly to make sure all intake valves are closed.
 
RLF Cuda said:
A low cost leak down gauge can be built from two 0-100 gauges, a pressure regulator, and a plug between the gauges with a 0.040" hole. This is what the piston engine aircraft rebuilders use to check leak down.
It can also be used to check manifold leaks by pressurizing the intake. You will need to make a plate to cover the carb side of the manifold and tap for a fitting. Pressurizing the intake and using the leak down gauge will help identify if there is an intake leak and where it is. You will need to remove the rocker arm assembly to make sure all intake valves are closed.
Click to expand...

pressurizing the intake you would be able to check the condition of the valve guides and check if there was any leaks from cnc porting also?
 
RLF Cuda said:
It can also be used to check manifold leaks by pressurizing the intake. .
Click to expand...

jimjimjimmy said:
pressurizing the intake you would be able to check the condition of the valve guides and check if there was any leaks from cnc porting also?
Click to expand...

..............Hmmmmmmmmmmmmmmmmm.......... Now there's one I had not thought of..........

Might be better, though, to come up with a vacuum tester rather than high pressure.. Not too sure the intake gaskets would reliably hold 100 psi, and certainly not something like the "bottom pan" design of the magnums.
 
67Dart273 said:
..............Hmmmmmmmmmmmmmmmmm.......... Now there's one I had not thought of..........

Might be better, though, to come up with a vacuum tester rather than high pressure.. Not too sure the intake gaskets would reliably hold 100 psi, and certainly not something like the "bottom pan" design of the magnums.
Click to expand...

I was thinking in the 25psi range like the amount of a good turbo ?
 
So how much difference will there be (avg) in hot vs cold? I ask because my very fresh & very strong running 273 with no oil usage whatso ever cold has rite @ 20% (cold). I checked it only to compare with my 60 hp mercury outboard as a "new motor"base line. Out board (cold) was the same 20% using the matco dual gauge set up in carcraft mentioned above. Is there that much diffrence hot vs cold? The 273 runs flawless plenty of power no oil usage at all & keeps oil clean.
 
I've done them hot and cold on fresh engines with little measurable difference.But we are talking numbers in the zero to 3-4 percent!

On a used engine, there is a possibility for the pistons to be slightly cocked in the bore,due to the earlier mentioned cam-ground design,(post #13) which would tip the rings ever so slightly, and allow them to leak.Further more,with the cold used pistons, the worn ring faces may not be flat in the worn grooves, or centered on the piston, or there could be carbon involved or huge ring gaps.

However the amount of leakage is a concern. Here's a quote from Smokey Yunick talking about leakage over 8%.
" This doesn't sound like very much to worry about,but you have to remember that only 1/3 of the heat energy(pressure) developed by each cylinder is actually going to push the piston down. So, if 8% of the cylinder pressure is leaking away, you are theoretically losing 24% of the recoverable flywheel horsepower."
He goes on to say that around 5-8%, should be considered rebuild time.
If your tool is truthful...........There is a problem!
------------------------------------------------
2-cycle outboard engines are a whole different animal. These engines are cooled by sea/lake water. They go from idle to full load-WOT, and back. They spend huge amounts of time at full-load WOT. To cope with this abuse the pistons are laughably loose in the bores. When cold, there is no way to keep the pistons from rocking, and thus no way to keep the rings in full contact with the cylinder walls. I'm not surprised to see 20% leakage on an outboard. They also have huge head chambers and low compression, due to the 2-cycle design.They make power because they fire twice as often as a 4-stroker, and usually operate in a very narrow rpm band, allowing the designer to optimize the port layout.
 
I checked the motor on my very low hours go-devil mud motor (v-twin vanguard briggs) same deal it's saying 18%! That gauge set has got to be goofy! It's saying that my 273,my merc & my mud motor are all worn out! It's gotta be the gauge set! I havnt had eny thing ever read better than 18%! None of the them burn oil have great power. As for the 60 hp merc (or any 2 stroke) ring seal is a MAJOR factor in how they will run mainly @ idle. Without good ring seal it will not have the power/vacuum to draw in the next air/fuel charge. Until the Rpm is higher to cover it up. Like I'm thinking my gauge set has got to be somthin wrong with it. The big gauge when set is 25-30 psi it seems alot lower than any other set I've seen before. So it's putting in 25-30 psi & loseing 18-20% of that. The 273 is a year old with apx 3000 miles on it & runs great!
 
I think after doing some resurch that I'm getting diffrent readings is becouse of the psi my gauge set is testing @. His is testing from the more common 100 psi. Mine is testing from 25-27 psi! I bet of I get a gauge set that tests from the more "standard" 100 psi my % goes waaaaay down!
 
Hipo65 said:
I think after doing some resurch that I'm getting diffrent readings is becouse of the psi my gauge set is testing @. His is testing from the more common 100 psi. Mine is testing from 25-27 psi! I bet of I get a gauge set that tests from the more "standard" 100 psi my % goes waaaaay down!
Click to expand...

Even at 100 psi, gauge readings are not going to "compute" very well from gauge to gauge, as there are variables. What you need to do is to get clear back to the ORIGIN of leak down testing

These are sometimes called "differential pressure testers" and reference is made to a "master orifice tool"

7ts0.com/manuals/continental/Service%20Bulletins/MB%2084-15.PDF

Originally, the orifice........which is the key to the whole deal.........was a precision flowed orifice, not just a sharp cornered, rough drilled "somewhere around" .040" hole, with internal flaws and of unknown length, friction, and flow characteristics.

This thing is claimed to be such an orifice

http://www.skygeek.com/aircraft-too...aign=froogle&gclid=CN6AxsDP0cgCFRSEfgodcEEGQA

If you google the part no. in the pdf for the master orifice, it seems to be "unobtainium"

https://www.google.com/search?num=2....0....0...1c.1.64.serp..4.23.1325.oYwL9Vgm65M


A little 'bit from the FAA

http://www.avweb.com/news/savvyaviator/188758-1.html

and yet more

http://www.vansairforce.com/community/showthread.php?t=94870
 
The other factors which are probably going to have some effect on readings..........even from the same engine using the same tester.......

Temp, humidity, weather conditions.

Are you using dry air, or an air compressor?

Certainly, any other pressurized gas is going to give skewed results. CO2, EG is WAY more dense than is N2 or compressed air.
 
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