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How to measure cylinder compression, which tool do we need?

swisseagle

Well Known Member
Hello

I'm still building, but my buddy's around me are flying since a while. We want to buy the service tools that we need to keep our RV's in good condition.

One is cylinder compression ... ok, but there are many supplyers and different styles like simple ones, differential pressure tester with or without orifice ...

1. ACS, simple one.http://www.aircraftspruce.com/pages/ep/test/comptestkit.php

2. ATS, differential pressure tester http://www.aircraft-tool.com/shop/detail.aspx?PRODUCT_ID=2E

3. ATS, differential pressure tester with master orifice http://www.aircraft-tool.com/shop/detail.aspx?PRODUCT_ID=2EM

Which one should we buy and how does they work?

Do I run the engine by hand to get the compression up on the manometer, or do I use the starter for this?
Do I need compressed air to get the tester running?

We run Lycoming 320 and 360.

Any help to make us smarter are very welcome :eek:
 
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You want the differential pressure tester with the .040" orifice. Although AC43 says use the .050 orifice on piston diameters more than 5", the O360 is only slightly larger and everybody I know sticks with the .040 (you'll get a slightly 'better' reading with the .050").

There are plenty of instructions on-line, but basically you warm up the engine, remove 1 plug per cylinder, position each cylinder in turn at TDC on the compression stroke, pressurise the upper chamber of the tester to 80psi then read the resulting pressure in the downstream chamber.

If the leakage area is less than the equivalent of the orifice then it will be choked at the point of leakage and the two gauges will both read 80. As the leakage area increases due to ring wear, valve leakage etc, then the downstream chamber will read a lower pressure. 70-80psi is fairly normal, below 60 may indicate a problem, but there are many caveats, so don't pull a cylinder just on the basis of one low reading.
 
Most folks use the second one you listed, or one similar.
2e.jpg


To use this is a two person job-----one holds the prop so the piston is at TDC, and the other works the gauge. I suppose it could be done solo, but there is a bit of a hazard doing so, the prop can whack you pretty good if it slips loose.

Instructions should be included with the unit.

This unit does not really check the compression like most folks think of, instead it checks how well the cyl assembly holds pressure. You feed it a measured pressure of 80psi, and it gives you the pressure that the cyl maintains.

That is why you see folks quote a figure of 77/80, or 75/80 etc for an engine with good compression.

You can also find videos on the internet as to how to check this. U tube.
 
To use this is a two person job-----one holds the prop so the piston is at TDC, and the other works the gauge. I suppose it could be done solo, but there is a bit of a hazard doing so, the prop can whack you pretty good if it slips loose.

Just to add to Mike's comment, it can do more than "whack you pretty good". Many years ago there was a young A&P killed at SBA while doing a compression test solo. The prop got loose and fractured his skull. The lesson here is to, as always, treat the prop as if it is "hot" even with the plugs out and never stand in the prop arc.

John Clark ATP, CFI
FAAST Team Representative
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
 
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You want the differential pressure tester with the .040" orifice. Although AC43 says use the .050 orifice on piston diameters more than 5", the O360 is only slightly larger and everybody I know sticks with the .040 (you'll get a slightly 'better' reading with the .050").

......

They stick with the 0.040 orifice because that is what Lycoming calls for, and the manufacturers instructions trump AC-43....:)

The error is a bit more than "slightly better".

70/80 with the correct 0.040 orifice will read 73.5/80 with the larger orifice.

http://home.comcast.net/~r123rs/Documents/SI 1191 Cylinder Compression.pdf
 
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And if you get an anomalous low reading on one cylinder, rock the prop a bit and try again. May be that the valve or rings didn't seat quite perfectly (kind of like gear lash) when you stopped moving the prop. If I get a low reading, I'll move the prop slightly back and forth and often then get a good one.

Greg
 
Thank you!

Whow, all these response!

Thats all what I need to know, I will go with the No. 2.

Thanks alot and best regards,
 
As to moving the prop around to try and "bump up" the reading ... it doesn't take super-human strength to hold the prop when it's under pressure right at TDC. But, if you get it very far off, I don't think I know anyone (or two, or four for that matter) who could keep the prop from getting away.

I'd suggest bringing up the pressure slowly to be sure you have it under control and being ready to quickly dump the pressure on the cylinder if necessary.

Dan
 
Dan's advice is good. I still sneak up on the pressure on some engines. Start with like 30 psi. Go to TDC and hold. Then turn on the air. Then you can carefully move the prop around to see just how much force there is a few degrees off center. It's tremendous. Go back to TDC, then dial up to 80 psi.
 
Suggestion

For SwissEagle: If you have someone nearby who has done a compression check before, it would be my suggestion to get local help for the first check. After that, you will likely be comfortable doing it yourself.

Nothing better than someone to show you all the little in's and out's of the process.
 
Will do!

Thanks Pete

I see, it can be dangerous. I understand the function of pressure on such a lagre piston and which force it can generate!

Nothing to play the first time ourself!

We will book an A&P or someone which has done it allready.

Thanks for all your inputs folks, your great!
 
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