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By Dave W
FLYER Club Member  FLYER Club Member
#1832159
What do people use as an affordable and accurate compression test kit? There are lots of automotive ones out there - are they to be avoided or are they any good?

Engine is a Lycoming O-235, and aircraft on a Permit, if that makes a difference.
By riverrock
FLYER Club Member  FLYER Club Member
#1832198
I don't have advice on a specific kit ( offers in my group do this), but for aviation a differential pressure tester is normally used, with an air hose attached ( providing at least 80psi) with leakage measured. They will have two gauges - incoming and measured pressure.
Automotive would often use a more simple system, with pressure produced when engine is turned over ( and one guage).

If you get an aviation kit, you should s also get the right spark plug adapters for your engine.
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By Dave W
FLYER Club Member  FLYER Club Member
#1832203
Thank you - it was differential pressure kits that I was looking at, from automotive suppliers.

I'd anticipated perhaps needing to buy a separate plug adaptor if the chosen kit did not include the correct one.
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By Flying_john
FLYER Club Member  FLYER Club Member
#1832223
I have access to an approved differential pressure test rig for Annual testing, but wanted my own device. So I bought one from a well known on-line auction site and found that the gauges were no good !

So I changed the gauges to ones that read to 150psi and did some comparisons to a "proper" test rig.

It works with sensible readings, but since the calibration is all about the precise size of a "pin hole" which allows leakage air to pass through, thus lowering the pressure on the output gauge side, its never going to be as accurate as a proper one.

Having said that, if you are only ever going to use it to determine if there are any year on year changes then a cheap one is more than adequate to determine any trend.
#1832250
Dave W wrote:What do people use as an affordable and accurate compression test kit? There are lots of automotive ones out there - are they to be avoided or are they any good?

Engine is a Lycoming O-235, and aircraft on a Permit, if that makes a difference.


As an owner (and Inspector) I use an aviation cylinder differential compression tester like this one:

https://www.lasaero.com/products/article/I02EX3VH1

You do need a 100psi compressor to provide the differential (hence the name).

Use a borescope too. A compression test tells ~ 25-50 % of the story.

Looking at the valve train helps too (part of the engine inspection).

An long-term periodic oil analysis (non-mandatory) would provide trend analysis of engine condition.

The usage, history, age etc of the engine all provide clues as to its condition. No one of the above can alone define its condition.

Hope that helps.
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By Dave W
FLYER Club Member  FLYER Club Member
#1832287
Thank you everybody. @Charliesixtysix , that's a very kind offer.

We do have access to a calibrated compression test set via our Inspector, so we get a check at every Annual.

My question was based around our thinking that we wanted the capability to also do it ourselves in order to monitor trends at each 100hr check (in normal times it's not been unusual for the aircraft to exceed 200h/yr.) We do already have a boroscope - thanks @Rob L.
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#1832647
Of course proper accurate equipment is the right way to go, but a friend had a poorly performing engine and we didn't know whether it was cylinder/valve related. electrical or fuel.

One plug out from each cylinder and quick whizz of the starter hand pressing in a basic automotive tester showed good sensibly the same compressions so we looked elsewhere.

Refurbished magnetos and the engine was back on song.
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