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To ream or not?

Lycoming IO540-C4D5D with 2100 hours. Runs smooth, and for the last 500 mostly operated hard between 70% – 75% power at or slightly below max EGT. Engine consumes about a 1.5 quarts of engine oil every 10 hours. Compression: 79 78 78 74 76 72
Had Cyl#4 pulled at 2000 hours due to start of uneven pattern on #4 exhaust valve. See picture. My goal is to get to 2500 hours and then do a top overhaul. Never had any indication of a stuck valve, or fouled plug for the last 500 hours (Tempest Massive). But stuck valve (besides magneto and fuel servo failure) is one of my worries, since it can lead to complete inflight engine failure. I have never done the wobble test and never have reamed the valve guides.

Question: Would you preventatively ream all six exhaust valve guides for my particular case?

The theory of the uneven sides of the valve stem in the picture below is that there was uneven pressure from one side, ensuring contact between stem and guide and thereby kept stem shiny, while the other side without contact coked up. The valve would probably had failed the wobble test. This asymmetry led to uneven heat pattern on the exhaust valve. The uneven pizza pattern was still benign and I probably could have flown for another 100 to 200 hours, i.e. no green color, but I decided to fix it, since it doesn’t matter whether to do it now, or 100 hours later.

Thank you for any advice?

United States

I thought the valves were supposed to rotate. At least in the VW beetle engine I built they do. The rotation is forced by the swivel pad being off center (still fully seated) on the valve stem. It’s one of the very important details to make sure is correct on that engine.

From what I know, rotating valves are normal for all/most? engines. Don’t know about Lycoming, but if it’s supposed to rotate, you got a stuck valve.

The elephant is the circulation
ENVA ENOP ENMO, Norway

How many hours a year are you doing?

The cylinders when fitted 2100 hours ago were they new or overhauled?

The IO540 valves are supposed to rotate.

This is interesting. I bet at least 99% of Lyco/Conti valves are never borescope inspected

Administrator
Shoreham EGKA, United Kingdom
Looking at the valve photo I cannot see a rotation feature in it. My guess is that the type of valve cotters/collets are clamping on the valve stem when seated in the valve spring cups, so no rotation possible – unless there is some Rotocaps device somewhere, in the upper or lower valve spring caps/seats. Judging from the stem wear and carbon I´d say the valve is no longer concentric to its seat ring, maybe due to warping of the cylinder head or from manufacture from the start – or a bent valve. So a bit of seat cutting is required to correct the offset. But then valve guide wear plus stem wear might lead to the decision for better having a replacement cylinder assembly instead. Vic
vic
EDME

Bathman wrote:

How many hours a year are you doing? The cylinders when fitted 2100 hours ago were they new or overhauled?

Cylinders were overhauled 2005, and plane has flown consistently over 100 hours per year.

Where the valve has uneven pattern and the stem is shiny, lines up with shiny spot at the valve seat edge. With regards to valve rotating, I agree I would expect a symmetric pattern. According to the Mike Busch’ presentation (fast forward to 9:30), one should see a nice shiny ring around the circumference of the valve. With this valve the shiny area is only at the spot highlighted. The Cyl#4 overhauler did mention that the guide was in a bad shape, but I failed to ask him what exactly he meant by that.



If it wouldn’t have been by the benign looking, slightly uneven pattern of the exhaust valve as a result of the boroscope inspection, nobody would have done anything about it. It confirms the importance of boroscope inspection for Lycoming engines exceeding TBO (and Continental’s when exceeding half TBO).

Last Edited by Lucius at 24 Dec 17:36
United States

I got myself confused above when I said the valves rotate, but actually it is the cam followers that rotate, in the thread I linked. Fortunately the valves are also supposed to rotate.

One of the things about Barrett Precision (who did two engine builds for me) is that they have developed some special way of grinding the valve seats.

Administrator
Shoreham EGKA, United Kingdom

Overhaul the effected cyclinder. Then replace all with New as planned at 2500 hours

Last Edited by Bathman at 25 Dec 12:55

Bathman wrote:

Overhaul the effected cyclinder. Then replace all with New as planned at 2500 hours

Thank you. That is my plan. Cyl#4 has already been IRANed (not overhauled). My question is, whether I should ream or not, whether I should do the wobble test or not, etc… My inclination is not to, despite never having done the wobble test, because I never had a stuck valve or morning sickness. On the other hand, I am at 2100 hours, and not babying my engine to improve longevity. In general I subscribe to Reliability-Centered Maintenance philosophy (if it works, don’t fix it), but also don’t want to gamble.

United States

Lucius wrote:

In general I subscribe to Reliability-Centered Maintenance philosophy (if it works, don’t fix it), but also don’t want to gamble.

Do nothing unless not operational you mean. Well, every book written about maintenance will tell you it’s the most expensive method. But then expenses are the cost of maintenance + the cost (loss of income) of downtime (scheduled or otherwise). For a private plane used for leisure by the owner, there are no costs of downtime, only cost of maintenance. It’s still the most expensive though, because the aircraft is left to gradually fall into a state where every component has a high probability of failure. Essentially a worn out wreck in constant need of being fixed, and the cost of man hours are large compared with the cost of components.

Only when you do all the maintenance yourself (no cost), and components are cheap, will your subscription work financially. The downside is only an airplane left into a state of maximum unreliability.

Just my 2 cents

The elephant is the circulation
ENVA ENOP ENMO, Norway
12 Posts
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