Leak Down Gauge Search

[Bart]

Ok, the normal context of leak down on a 2 stroke chain saw is piston tdc thru spark plug hole for rings, or block carb/exh and check case halves, gaskets and crank seals, impulse line or say tank and carb pressure holding or venting. On a newer 4 mix the valves come into play too. On a car it's intake, exhaust valves, head gasket rings. So normally you have inlet and outlet gauges in psi and you calculate percentage leakage. That's not in the sealed pressure/vacuum maintenance context which is slightly different, but same principle of checking sealing.

My current dilemma is on Briggs small engines the closest thing to a published spec is an unlabelled leak down tester #19413 which I surmise to have a 100 psi full scale inlet dial with target inlet pressure ticked off at about 60% full scale and the outlet gauge looks like 60 psi full scale, but is uncalibrated except for at about 50% full scale it goes from green to red (good/bad). So my question is, does any one have one and know if the 100 psi and 60 psi full scale assumptions are correct? If so, surprisingly Briggs is a-ok with a 50% leaking engine!! Could be, but its a tough one to believe.

thanks

my bad if this is a stretch from directly chainsaw related.

 

[Marc-Antoine]

The different engine compartments have nothing as a real airtightness, but only a relative capability of holding the pressure (positive or negative). So you have to put the time in play, because 50% can be very acceptable if the test protocol asks for a long enough duration.
You get a slope with the datas. If it doesn't reach the reference level after a certain amount of seconds/minutes (I don't have a clue about the value), you're good to go.
The other way is to find the time needed to reach the say 50% loss for example.
Either ways, you have to find the value(s) mandated in the test protocol to get a valuable result.

 

[Bart]

Thanks Marc, I get where you're coming from. My Mighty Vac is for crankcase, carb and tank testing where you hold vac/pressure for a specified time period My two-gauged leak down tester is meant for situations where it wouldn't hold real steady e.g. a 10% leakage rate spread between intake, exhaust and rings is common in the 4 stroke world. There's a calibrated flow orifice involved in the second tester.

 

[sawcollector]

I put this leak down tester, 5910 850 0300, in Service for STIHL back in 2004. It is calibrated for small engines so it doesn't over pressure the engine like an automotive tester would.
See attached for instructions on using it for the STIHL 4-Mix engine as well as how to use it on two-stroke engine when doing a pressure vac test on the crankcase.

 

[Bart]

Maybe I'm a bad reader, but what is the inlet pressure set to the gauge by the regulator? My guess 100 to 150 shop air, tool regulates applied to about 50 psi? Bit smaller orifice in the tool to match smaller cylinder and valve sizes? I can see results dialing as low as 40 psi applied sometimes on mine.

 

[sawcollector]

If you are doing a leak down test on a four stroke it doesn't matter what the shop pressure is, just zero the gauge before you hook it up and then read the percent of leakage and locate where with your ear. For a two stroke that is failing the vac pressure test on the crankcase, hook it to the spark plug hole with no air pressure applied, then slowly add pressure until you can hear where it is leaking. Does that answer your question?

 

[Bart]

My question really is what psi is your "zero the gauge"? My unit is in psi after the regulator e.g. where you set your testing pressure. The FAA spec most equipment is based on is 80 psi applied and a 60 degree inlet 0.040"dia x 0.250 long calibrated orifice for 5" dia or less bores, although it allows to test as low as at 60 psi applied. The Briggs unit looks like it tests at 60 psi applied. On a calibrated second gauge a reading of 54 psi would be down by 6 psi, 6 psi/60 psi applied = 10% leakage, just using numbered gauges.

 

[sawcollector]

Sorry Bart but you are getting this way more complicated than my little brain can handle. The STIHL tool is made by Thexton and I adopted it because it is calibrated with one gauge for percent of leakage and the other to see the incoming air as adjusted by the regulator. It is designed for use on small displacement engines and won't dump full shop line pressure through the hose the way an automotive tester does. An auto leak down tester is filling a much larger volume and if it was an ex valve leaking, for instance, you would have to listen at the tailpipe to hear the air flow so that is why more volume and pressure is needed.
Back when the STIHL four-stroke was introduced we found that any leakage less than 10% really did not matter, but anything over 10% would cost you power and top end RPM so this why we needed a gauge with an actual percent scale instead of the red - yellow- green or Good - poor - bad type of scale.

 

[Bart]

Thanks for the reply. For info, the automotive/motorcycle testers have a regulator and drop the line pressure to whatever you prefer. Some folks test at lower pressures, some prefer higher. The only formal, standardized spec I can find is the FAA as airplane folks like their engine maintenance activities CYA mode. Someone with a flow bench or similar might calibrate those standardized % leakages into CFMs. Maybe a welding gas flow meter would do the trick...

On my once-new fancy dirt bike the valve sealing check was place the head upside down, pour solvent on the closed/seated valve and see if it leaked. At all. Pretty high standard, but it is a racing engine. On a utility engine its looser " can you hear it hissing?" Then, "Ahhh, that's not so bad, ought to be useable..."

 

[Bart]

So I rigged up my tig flowmeter to feed my leak down gauge, taking the needle valve and pretending it was an engine cylinder by sticking it at the end hose.

I made two diy orifices, one .040 and one .060 each .250" long with nice conical input. I also measured my original one at .040 but only 0.160 long.

I'm not quite sure what to make of the results as JBWeld is not precision surface machined when you drill it. Also I measured air with an Argon calibrated flowmeter, at 0 degrees C not 20 degrees. I found correction factors which I'm mostly convinced on. Main result was I could raise the ball to a full 70 SCFH with what would not be considered catastrophic leakage. Even 10 or 20% leakage was a huge airflow to my perception. And the correction factors said I was under-reading it. It also didn't vastly change readings between .040 and .060, nor with different applied pressures to the orifice, maxing at 70 psig as that's what my reg put out to feed the OTC reg. Head scratcher. Only other unknown I can come up with is compressor refresh cycles raising the inlet air temperature higher than 0 degrees C, unmeasured and unknown. Still haven't graphed the data.

 

[Magnus]

Most think that saws are tight with 0 leak, but that is inaccurate. There is always some leakage, there is tolerance for it.
The fluxing pressures (Both negative and positive) in a saw engine is at times a lot higher than we would like for all kinds of reasons.
Most important is the saw runs OK and perform as it should. I taken apart saws that had pretty large leaks and still ran some what ok.
Others that have not so high leak, but did not work ok.
Others I tested, brand new strait out of box has not been very tight and ran ok.

I try not to put too much weight in the test results itself, bore focus on the condition of the parts so all is right.
If it then perform ok it is not so important with a small leak. Next time I test it may not leak much at all..

 

[Bart]

A bit of trivia I ran across in my research was that racing engine guys, big block etc, do what they refer to as static leak down (which is the testing we are discussing) but because they're 4 stroke engines they also measure dynamic leak down at full rpm and loaded on a dyno. In that case its basically the rings and blowby measured in CFM instead of percent, which if your valves are up to snuff is the bulk of leakage. Some guys found in spite of a good static test they had a lot of blowby under load. Conversely some guys had less than great static results but lesser blowby under load. It seems that sometimes the rings seal better under load.

I know that has limited application to a 2 stroke, but I found it interesting nonetheless. For further confusion on managing blowby, take a look at all the different schemes for PCV on car engines. Lotsa valves, hoses, logic etc. I used to think it was simple.

I also just noticed that the zero reading on my gauge is obscured by a 5 psi wide mark. So the first 5% of the reading is in doubt, which in some cases would make a difference to somebody. I have a big mirrored anti-parallax scale 2 revolution pressure gauge which I occasionally use to calibrate small gauges. I was debugging a fuel injection problem recently and found a small typical bourdon gauge was out 2 psig at a reading of 40. Still workable if I understood the offset. The saving grace for small engine equipment is if it has two identical gauges, if they read identical at stopped flow you'll still get a correct percentage because I found that the applied pressure level doesn't affect the results much. (not to be confused with 2 stroke case and seals testing at 1/2 atm).

We're still locked down. Cut our cases in half after a number of weeks. Time on my hands.