Lycoming O-320 Compressions

[walkman]

What is the consensus on an O-320 E2C (wide deck) that has been "converted" to a B3A by putting in high compression pistons, that had all cylinders "honed" at the overhaul, all the valves, guides, seats etc replaced, and after 600 and change hours has gotten compressions in the 68-74 range year after year. The compressions on each cylinder have hovered in that range for 18 years, with one cylinder occasionally jumping up one year but other than that consistently down in the high sixties to low seventies.

I have not put a compression tester on it myself, or seen it in the flesh, so I can't say whether or where there is leakage.

[RV10inOz]

Leak downs are just one indicator of health, and most folk read far too much into the result.

Run the engine, best to take it for a quick circuit (pattern) and then rip into it.

Obviously the leakage past the rings is expected, and so long as it is not massive it should be a good indicator of their health.

As for valves, well a static valve at 80 PSI is hardly representitive of the actual running pressures etc so it really is hard to make much of it, if you get an exhaust valve leak, their MAY be a problem, but maybe not. You might find running it and testing again yields a different result. This is the time for a boroscope and have a good look at how the valve seats when you pull the prop through. Is the valve sliding excessively into place? What is the colour patterns on the valve showing you?

And engine with all cylinders in the 40-50 range is not necessarily a shot duck, in fact it could be quite healthy and pulling full rated power on a Dyno.

[NickAir]

Originally Posted by RV10inOz

Leak downs are just one indicator of health, and most folk read far too much into the result.

Run the engine, best to take it for a quick circuit (pattern) and then rip into it.

Obviously the leakage past the rings is expected, and so long as it is not massive it should be a good indicator of their health.

As for valves, well a static valve at 80 PSI is hardly representitive of the actual running pressures etc so it really is hard to make much of it, if you get an exhaust valve leak, their MAY be a problem, but maybe not. You might find running it and testing again yields a different result. This is the time for a boroscope and have a good look at how the valve seats when you pull the prop through. Is the valve sliding excessively into place? What is the colour patterns on the valve showing you?

And engine with all cylinders in the 40-50 range is not necessarily a shot duck, in fact it could be quite healthy and pulling full rated power on a Dyno.

My aviation years agrees with "David" above. Lycoming does have minimum compression numbers that are a standard for further investigation/resolution on certified engines. Also, learned from a commercial A&P, a reciprocal engine compression check should be done after fresh recent operation, on every test, then tear into it for testing. You can still get different compression numbers at each test. One surprise is when the rings happen to align the gaps. This will indicate a low compression although it is recommended that you go operate the engine and return for an additional new test. The rings will move, rotate, during operation enough to change the gap alignment between the rings which will then give a compression you are used to. There are many issues that can change the numbers with compression. Not all are a problem. You don't want to just jump to a top end overhaul.

[Bob Axsom]

Originally Posted by RV10inOz

Leak downs are just one indicator of health, and most folk read far too much into the result.

Run the engine, best to take it for a quick circuit (pattern) and then rip into it.

Obviously the leakage past the rings is expected, and so long as it is not massive it should be a good indicator of their health.

As for valves, well a static valve at 80 PSI is hardly representitive of the actual running pressures etc so it really is hard to make much of it, if you get an exhaust valve leak, their MAY be a problem, but maybe not. You might find running it and testing again yields a different result. This is the time for a boroscope and have a good look at how the valve seats when you pull the prop through. Is the valve sliding excessively into place? What is the colour patterns on the valve showing you?

And engine with all cylinders in the 40-50 range is not necessarily a shot duck, in fact it could be quite healthy and pulling full rated power on a Dyno.

Every so often you come across a message that is so honestly straight forward and helpful it is stunning - I think this is one.

Bob Axsom

[RV10inOz]

Now back to engine leak down tests (so the thread doesnt get broken up, that would ruin it) I thought that we should add doing a leak down test is very dangerous, you don't want to end up being killed or suffer a severe injury from a prop strike. So here are some simple rules as best I can think of them. Others might want to add some more.

1. Never connect the air hose until the person holding the prop says CONNECT
2. Have an agreed word, STOP is a good one, immediately disconnectthe air supply
3. Using the already fitted leak down hose listen or use your cheek to feel the air flow as you roatate the prop to find TDC of the comp/fire stroke, then allow the prop holder to get a good grip and be is a safe position before connection.
4. Watch your buddy all the time, not the gauges. the gauge can be read quickly, but you need to be seeing of he is struggling with the prop blade, even before he yells stop.
5. Dont commence a test with a plane taking off, no loud music and no talking to others. Any of these things could kill.

[dealfair]

Thank you all for the great information and the laughs!