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Exhaust Valve Concerns?

Ironflight

Ironflight

VAF Moderator / Line Boy
Mentor
It's amazing what thread ideas you can come up with while cruising along VFR on a long cross-country with nothing to do....

For instance, I know that there are many ex-Grumman drivers here int eh RV world, and I know that a lot of them used to (and/or still do) read the Grumman Gang emails. One constant for all the years I was in that world was the unrelenting concern over exhaust valve guide wear and subsequent breakage. A week didn't go by when someone didn't get a lecture on the necessity of doing the "valve wobble test" every 500 hours, and many a story was written about people sucking a valve and making the resulting forced landing.

OK, so here we are in the RV world, flying the same O-360's, with essentially the same FWF configuration as the Grummans. Yet nowhere have I heard people talking about valve trouble, the need for wobble checks, etc.....Are we immune for some reason? Or are we just more alarmed by other things?

I'm not trolling for trouble, just curious if anyone else has noticed this? (Here, all the arguments are just over flying LOP or not... :rolleyes: )

Paul
 
Paul,
One of the problems relating to the valve guide wear in the early Grummans was the poor cowl design. Oil temp almost always runs right at red-line.
 
Mel said:
Paul,
One of the problems relating to the valve guide wear in the early Grummans was the poor cowl design. Oil temp almost always runs right at red-line.
Click to expand...

Yeah, my old AA1B's oil was always hot...and in the RV, the oil temps are nice and tame...but man, just looking at the RV FWF, it's hard to see that it is much different! I don't know why it cools better....

Paul
 
Yeah, Paul, I can relate to your angst. I've thought about the wobble test myself, and then something else more pressing enters my noggin and off it goes into hibernation for a while. Certainly cooling and leaning patterns (managing lead) have a lot to do with it all. As a hedge against the guides fouling up, I splash some Marvel oil into my fuel whenever I can remember, and run it lean all the time here in Nuevo Mexico where we never run full rich anyway. Even so, I recently bought a valve spring compressor tool from Spruce and plan to open up the rocker covers and see what I see.

Maybe. Someday.
 
Paul:

I'm a perfect example. I can't tell you how many cylinders I pulled on our Tiger in the course of nearly 30 years and three engines. It was at least 20! I sometimes got only 300 hours on a number 1 or 3 before excessive exhaust valve leakage occurred. 2 and 4 rarely gave problems. In contrast my -8A has 750 hours with 78 compression on all jugs. Same pilot, same leaning technique but very different results.

In retrospect, I think two things are involved. One is that the Tiger's fixed pitch prop had to be running full out to get book performance. I used to fly above 10,000 feet most of the time at close to 2700 RPM. That was only about 60% power but it simply did not allow the valves to shed heat adequately. That fact, coupled with the low oil flow to the copilot side rocker boxes that Bill Scott and I discovered and wrote about extensively, combined to doom the exhaust valves on that side of the engine. With my RV, I run at 2300 rpm in cruise with a CS prop and even if I had a fixed pitch prop I believe I could still get great performance at 2500 or so. The lower RPM gives the valves longer to cool and reduces the need for oil to aid in that process.

The second thing is that Van's cooling design is simply much better than the Tiger. The engine sits farther back in the cowling than it does in the Tiger, allowing a full plenum to exist. In the RV the forward baffle is excellent but in the Tiger was almost an afterthought that I believe allowed a great deal of cooling air to escape. While the Tiger often had 380 to 390 CHTs in cruise (with well trimmed and pliable baffle seals), the RV sees only 330 or so. Out here in CO in the winter I have all but 1/8 inch of my oil cooler covered just to keep the OT at 180. Even in the summer I only have about 1/3 of the cooler exposed to the airstream.

To me then it comes down to two things in explaining the lack of engine problems in RVs -- lower RPM operation and better cooling system design. Both combine to get heat out of the exhaust valves more efficiently and that makes them run cooler. Cooler valves means longer life.

Bill Marvel
 
Great Info Bill!

Bill,

Your thoughts and info are greatly appreciated, as I know that you and B. Scott did a huge amount of work on the problem back in the Grumman days. Your arguments make sense - along with the comment that the speed is higher, so the cooling mass is that much greater. I had never really thought about the effects of RPM's in the way you mention, but it sure makes some sense.

Paul
 
What I've always wondered is how to interpret "valve temperature" given certain engine parameters.

In which scenario will an exhaust valve tend to be hotter?

1) CHT=290, EGT=1500, FF=7.5 (typical LOP cruise)
2) CHT=370, EGT=1280, FF=15.0 (extended steep climb)

I've wondered if Deakin & Watkinson & Braly have instrumented valves somehow on their test stand engines.
 
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Dan:

The answer is unknown because a critical ingredient has been left out -- heat dissipation from the cylinder head in general and the valve/guide in particular.

Consider two extremes -- one is where there is absolutely no cooling air provided at all and a minimal amount of oil flowing over the valve stems in the rocker box. I think we can agree that this will toast the engine in short order in either scenario you mentioned.

Now look at the other extreme -- massive amouts of very cold, dense air forced around all of the cooling fins coupled with a large volume of oil flowing across the exposed exhaust valve stem and guide in the rocker box. You can imagine that neither scenario would cause any grief because of excellent heat dissipation.

The real world is a combination of the above -- heat input versus heat rejection. The problem we have in operating recip engines is that the exhaust valves are the weak link (certainly so in Lycs) but we have no way to know exhaust valve temp. Neither EGT nor CHT alone tells the full story. My Tiger ate valves at a CHT of 390 or less and EGTs of only 1250. This is nowhere near any red line limitations. Hence we shoot for the best cooling system we can get and not leaning more than recommended amounts. This keeps us in a safe area with fairly wide margins. Once you wander over to the edge with both high heat input from the engine and poor cooling you are in a no mans land of unknowns.

As to their test cell I also would like to know how they extract valve temp. I suspect they bore a small hole through the head all the way down to the OD of the guide and put some type of temp probe in there against the guide. In fact, Lycoming does exactly this in a couple of their engines in order to cool the OD of the exhaust valve guide with oil. They actually have an external oil line running to each cylinder head and carrying oil down to the exhaust valve guide where it circulates around the guide in an annulus and then flows down a slot to the rocker box like the rest of the oil in the upper end. If you happen to see a Beech Duke, take a look in the air inlets on the front of the cowl. You can easily see this oil line that most people confuse with a fuel injection line. In the Duke the oil line is on the top of the cylinder and the fuel injection line is on the bottom.

Bill Marvel
 
7/16 exhaust valve

Seems like the 500 hr wobble check was for the early engines with the 7/16 inch exhaust valve stem. Lots of problems until they went to 1/2 inch stems.
 
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John:

FWIW, the SB388 wobble check is considered mandatory by Lycoming on every engine they have ever made regardless of whether or not they have 1/2 inch valves or sodium filled stems. In the presentation I occasionally give on the work Bill Scott and I did, I point out something that almost no one knows --

SB 293B was issued in 1967 and is still active. It describes an earlier version of the wobble test and says that it only applies to O-320 engines and only if they do not have sodium filled exhaust valve stems. If the O-320 engine was upgraded to have exhaust valves with sodium filled stems, this SB no longer applied. The clear implication is that sodium filled exhaust valve stems "solve" some sort of valve problem.

Now, almost 40 years later, the SB388 wobble check applies not to just the O-320, but to every engine the company has ever made. It applies regardless of whether they have sodium filled valves or not. So much for progress in this matter during the past 4 decades.

Bill Marvel
 
bmarvel said:
John:

FWIW, the SB388 wobble check is considered mandatory by Lycoming on every engine they have ever made regardless of whether or not they have 1/2 inch valves or sodium filled stems. In the presentation I occasionally give on the work Bill Scott and I did, I point out something that almost no one knows --

SB 293B was issued in 1967 and is still active. It describes an earlier version of the wobble test and says that it only applies to O-320 engines and only if they do not have sodium filled exhaust valve stems. If the O-320 engine was upgraded to have exhaust valves with sodium filled stems, this SB no longer applied. The clear implication is that sodium filled exhaust valve stems "solve" some sort of valve problem.

Now, almost 40 years later, the SB388 wobble check applies not to just the O-320, but to every engine the company has ever made. It applies regardless of whether they have sodium filled valves or not. So much for progress in this matter during the past 4 decades.

Bill Marvel
Click to expand...

SB388 mandates the inspection every 400 hours. If anyone needs a copy of the bulletin, it can be had at

http://www.lycoming.textron.com/sup...nancePublications/serviceBulletins/SB388C.pdf

dd
 
What Lycoming says...

Yukon said:
Seems like the 500 hr wobble check was for the early engines with the 7/16 inch exhaust valve stem. Lots of problems until they went to 1/2 inch stems.
Click to expand...

Not quite.... the Service Bulletin 338C that Bill is referencing is at 400 hrs. if you don't have hi-chrome valve guides. They can be identified with a "C" in a circle stamped near the drain back fitting on the cylinder head just outside the valve cover.

If you have these guides, then they say 1000 hrs. (half way to TBO), for the check...

http://www.lycoming.textron.com/sup...ePublications/serviceInstructions/SI1485A.pdf

The basic SB 388C is here, detailing the procedure...

http://www.lycoming.textron.com/sup...nancePublications/serviceBulletins/SB388C.pdf

The 400 hrs. interval is with a caveat....

All Other Engines ? 400 hour intervals or earlier if valve sticking suspected until exhaust valve guides are replaced with guides made of improved material. (Refer to latest revision of Service Instruction No. 1485.)

I'm guessing that "valve sticking suspected" is the "morning sickness" at start-up, but am not sure.... would like to hear other folks opinions on what makes a valve suspect - other than a compression check...

gil in Tucson
 
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The physical properties and the material the exhaust valve guides are made of, changed a few years back.
If your ex. valve guides are the hi chrome style guides, the requirement to do SB 388C ("the wobble test") every 300 hours is extended to 1000 hours.
All current manufacture exhaust valves, form any of the three sources, Lycoming. Superior and ECI are high chrome guides.
Valve guide wear characteristics are markedly better with these guides installed.
This along with many other improvements, the parts and engine manufacturer?s make all the time, is another example of 1940's technology, that hasn't changed in 50 years :-}.
Good Luck and Happy New year,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
Valve Stick? temp bad, fix easy, avoiding easier

Good stuff, only add if you operate your engine within limitations (temps), operate it frequently, maintain it as recommended and lean it properly** your exhaust valves and guides should last 2000 hours.

Temp is the main driver of valve stick. At high temps the oil oxidizes and causes carbon deposits on the valve guides which causes the sticking. If you do suspect "morning sickness" which is valves sticking on first start (usually more in warm temps than cold) you should have it looked into immediately. The result of sticking valves is bad bad bad. The good news is the valve guides can be cleaned with the cylinders on the plane (Lyc service instruction SI1425A). You can actually push the valve into the cylinder after removing rocker, spring retainer and spring. The technique is called the "rope trick".

Thr "rope trick" allows a valve to be removed through the top spark-plug hole for inspection and cleaning, while the guide is inspected and reamed, removing deposits if necessary. A "lasso" inserted in the bottom spark-plug hole allows control and manipulation of the valve. After the cleaned valve is re-inserted in the guide a soft cotton braided rope is fed into the plug hole. The piston is brought up, compresses the rope against the valve to hold it in place while springs and keepers can be re-installed.

Here are some good refs.

Lycoming - How to avoid stuck valves


Textron Lycoming Service Instruction 1425A - "the old rope trick"

** I am a big proponent of aggressive leaning on the ground. Rich mixture and low RPM's (less than say 1100 RPM) for extend periods can foul your plugs. It also can cause more lead sludge in your oil. None of this is good for engine, plugs or valve. The lead scavenging agents in fuel do not work unless cylinder temp and pressure are high enough (above 900F at electrode tip). The main argument against aggressive ground leaning is you might forget for take off? Well that may be true. I have never tried it, but my guess is the engine will not run at full power with the engine leaned for 1100 rpm idle. The down side of high idle with RV's is taxi speed and brake wear. Sometimes you have to go to the throttles idle stop, granted, and high idle is worse with fixed pitch props which make more static idle thrust (with their greater pitch than a constant speed prop in low pitch/high rpm). Keep it in mind and whenever stopped, set idle to 1,100 RPM (no more than 1,200 rpm), your engine will be happier.
 
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gmcjetpilot said:
Good stuff, only add if you operate your engine within limitations (temps), operate it frequently, maintain it as recommended and lean it properly** your exhaust valves and guides should last 2000 hours.

Temp is the main driver of valve stick. At high temps the oil oxidizes and causes carbon deposits on the valve guides which causes the sticking. If you do suspect "morning sickness" which is valves sticking on first start (usually more in warm temps than cold) you should have it looked into immediately. The result of sticking valves is bad bad bad. The good news is the valve guides can be cleaned with the cylinders on the plane (Lyc service instruction SI1425A). You can actually push the valve into the cylinder after removing rocker, spring retainer and spring. The technique is called the "rope trick".

Thr "rope trick" allows a valve to be removed through the top spark-plug hole for inspection and cleaning, while the guide is inspected and reamed, removing deposits if necessary. A "lasso" inserted in the bottom spark-plug hole allows control and manipulation of the valve. After the cleaned valve is re-inserted in the guide a soft cotton braided rope is fed into the plug hole. The piston is brought up, compresses the rope against the valve to hold it in place while springs and keepers can be re-installed.

Here are some good refs.

Lycoming - How to avoid stuck valves


Textron Lycoming Service Instruction 1425A - "the old rope trick"

** I am a big proponent of aggressive leaning on the ground. Rich mixture and low RPM's (less than say 1100 RPM) for extend periods can foul your plugs. It also can cause more lead sludge in your oil. None of this is good for engine, plugs or valve. The lead scavenging agents in fuel do not work unless cylinder temp and pressure are high enough (above 900F at electrode tip). The main argument against aggressive ground leaning is you might forget for take off? Well that may be true. I have never tried it, but my guess is the engine will not run at full power with the engine leaned for 1100 rpm idle. The down side of high idle with RV's is taxi speed and brake wear. Sometimes you have to go to the throttles idle stop, granted, and high idle is worse with fixed pitch props which make more static idle thrust (with their greater pitch than a constant speed prop in low pitch/high rpm). Keep it in mind and whenever stopped, set idle to 1,100 RPM (no more than 1,200 rpm), your engine will be happier.
Click to expand...
Burning Premium Auto Gas if you can eliminates most of these problems. I have been running Premium auto gas in my 180 Superior for 3 years 500 Hrs. It may be the wrong thing to do. So far so good, I will let you know when I reach 2000 Hrs if it was detramental.
 
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