Leak down test kit

[old man emu]

8 hours ago, skippydiesel said:

The type of devise is not as important as consistency of measurement

I was once given the advice that any measurement, no matter how dubious , is better than no measurement at all.

 

Skippy's comment is correct. The usual reason for including leak-down testing in a scheduled maintenance regime is to obtain data to identify a trend. If you are servicing your own aircraft, and using the same device all the time, scientific degree accuracy is not required. Every instrument has its own unique inaccuracies, called instrument error, but they remain constant throughout the life of the instrument. It's no use getting an instrument like this gauge calibrated against a known standard. As soon as you throw it back into your tool chest, or knock the gauge on something, it can wander from that calibration. 

 

At times there is a need for a one-off test. That is when a normally good engine suddenly goes bad. Then you do the leak-down test to identify the dud cylinder, and go from there. 

 

And as with all maintenance - Record, record, record and RFGuy said. If ever you want to sell your aircraft, comprehensive maintenance records return more money than the cost of the time spent writing them.

[Thruster88]

Most maintenance organisations will have a master orifice tool. This allows checking against a known leakage.  

[old man emu]

A measurement instrument has two features: resolution and accuracy. Imagine if you zoomed in on this image  and there were markings/subdivisions instead of just one big red dot bullseye. That is one way to think of resolution.

Accuracy (resolution) relates to how small a difference between two identified points an instrument can show. The diagram shows the same position (the point in the center of the bullseye) measured by two instruments. The two grid patterns represent the smallest objects that can be detected by the instruments. The pattern on the left represents a higher-resolution instrument.

 

 

For a leak-down test the optimum would be a device that is "accurate and precise", but since we are looking for trends, "precise but not accurate" is sufficient. Note also that "accurate and precise" does not mean the "Robin Hood shaft splitter" shot.

 

 

[RFguy]

My observations are there is a fair spread of leakdown results in the manner at which they are done.  They can can be certainly made to look good, so best  (most applicable) method as as described in the relevant engine manual from the manufacturer.  Depending on the ailment, some will be worse just before or just after TDC (maybe ring related) , some will be worse static  at TDC (maybe valve related) . some will be worse during rocking the crankshaft around TDC (maybe ring related) , there are other variations (cylinder -head interface leaks etc, cracks etc) AND COMBINATIONS !

Edited March 2, 2023 by RFguy

[old man emu]

9 minutes ago, RFguy said:

My observations are there is a fair spread of leakdown results in the manner at which they are done. 

A I said, a bad measurement is better than no measurement. If the manufacturer says that the result should be YY/XX and you get KK/XX, you know that you either have to redo the measurement to eliminate technique error, and if that is OK,  go through a logical sequence as indicated by RFguy.

[RFguy]

arm yourself with a good stethoscope (1.5m length of soft silicone  (or  not too hard PVC ) tubing jammed into your ear hole)  (note that's EAR hole)  and some soapy water (cylinder -head interface leaks / intake gasket/exhaust couplings  - Jabiru) 

Edited March 2, 2023 by RFguy

[old man emu]

RFguy, you are beginning to sound like an Enema of the People.

[facthunter]

An inaccurate tension wrench is worse than none at all. Proficient mechanics can JUDGE tension to about 5% error and you don't just keep adding till the instrument CLICKS IF your brain tells you have gone too far.   ALL the good tooling in the world won't make a clueless fiddler a proficient fault finder/ analyser/ trouble shooter. You have to know what to do . (Look for).. It can be referred to as "Nous".  Some have it . Some never will.  You'll see it all around you if you look.   Nev

[Tasmag]

16 hours ago, MartyG said:

Thanks for all the info guys, Found this on a Aussie site but doesn't say what size the orifice is but I will call them. 

https://premium-diagnostics.com.au/products/cylinder-leak-down-tester-compression-leak-detector-petrol-engines

I purchased this exact kit, total POS. (Not from there, and not that price though)

 

The orifice size looks like it was blown out with Oxy!

 

Been meaning to fix it but haven't bothered yet. (Using a loaner instead)

 

Mike

[kiwiaviator]

1 hour ago, Tasmag said:

I purchased this exact kit, total POS. (Not from there, and not that price though)

 

The orifice size looks like it was blown out with Oxy!

 

Been meaning to fix it but haven't bothered yet. (Using a loaner instead)

 

Mike

I made the mistake of buying one of these. I have finally retrofitted the POS % gauge with a real gauge and it seems to work ok. Didn't check the orifice in the block. Maybe I should....

[Tasmag]

Might be worth checking, I swapped the gauge out and was getting really good leak Down numbers, but when tested against another, and checked the orifice,  it was always going to give 80/78's😀

[facthunter]

The leakdown test won't identify whether it's either valve or rings causing the rate of leakgage  IF it leaks faster turned one particular way it's the ring gaps both on one side. ONLY the compression rings do this..  Nev

[Thruster88]

A good ear at the engine breather, exhaust or intake will identify the source of a cylinder leak.   

[facthunter]

Exactly.  and all you have to do is turn the prop a bit the appropriate way, and listen.. You can't tick the number in your book but you might be able to rectify the fault and Isn't THAT what is wanted?  nev

[Old Koreelah]

Finally decided which one to buy, mostly because both its gauges match, unlike the crap automotive kits which have % on one dial.

Ordered on Tuesday, arrived Friday at my address in the sticks. Impressive, but the plurry fittings are not compatible!

Time for another rant about lack of standardisation. All my compressed air stuff uses Nitto fittings; this one aint. Another damned trip to the big smoke to buy compatible couplings.