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Valve wobble test out of spec.

flysrv10

Well Known Member
Just did a Valve Wobble test and all 6 ECI cylinders are out of spec. Max wobble per lycoming Service Bulletin 388C for my valve is .035. Cylinders 2,4 and 6 were between .045 and .050. All other cylinders were near or at .035 limit.

I do not have any issues but decided to do the test due to the hours on the engine. The IO540 engine has 1400 hours from brand new ECI cylinders and has been running well. Oil consumption has been 1 qt per 10 hours and borescope shows good symmetrical pattern on all valves.

So my question for the engine experts is what to do next? Rework or replace the cylinders that are out of limit? I hate to do major surgery on a perfectly good running engine but want to the right thing. The SB, although it indicates upper limits for valve wobble was issued to check for tight clearances and sticky valve issue.

Thank you.
 
Great post. Glad you were pro active on looking into the health of your engine. I would not call it a good running engine. You are wearing out valve guides like crazy. Why? I am no expert but something should be done. New valve guides seems like a good idea. Good luck Florida neighbor.
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I'd keep flying it if it were mine, plan on a top in a few hundred hours.
 
How comfortable are you that your jig or your technique is not the source of the high readings? Just a little looseness of the jig on the valve stem, or a little too much force when applying the side load can make a big difference. It seems a little odd to me that 3 cylinders would go bad at the same time. (The ones are limit are not bad. They could have left the factory that way).
If it were me I would fly another 20 hours and then re-test them. At the same time I would remove the intake and exhaust and do a visual inspection on the valve stems. If the first test is correct I would fly another 50 hours and see if the wear continues.
The test is meant to find tight valves due to carbon/lead build-up. A stuck valve is the real enemy. You have good oil consumption. Worn valve guides should show an increase in oil consumption.
 
. Worn valve guides should show an increase in oil consumption.

While I have no first hand experience, given the horizontal orientation of the valve and the anemic oil flow to the rocker box, I doubt that worn guides would show noticable increased oil consumption like they would on most auto engines.

Larry
 
How comfortable are you that your jig or your technique is not the source of the high readings? Just a little looseness of the jig on the valve stem, or a little too much force when applying the side load can make a big difference. It seems a little odd to me that 3 cylinders would go bad at the same time. (The ones are limit are not bad. They could have left the factory that way).
If it were me I would fly another 20 hours and then re-test them. At the same time I would remove the intake and exhaust and do a visual inspection on the valve stems. If the first test is correct I would fly another 50 hours and see if the wear continues.
The test is meant to find tight valves due to carbon/lead build-up. A stuck valve is the real enemy. You have good oil consumption. Worn valve guides should show an increase in oil consumption.

The test was done by a shop. They disclosed that they had only done the test once before but they did get second opinion from other techs in the shop before concluding that the test was valid. Hence, I am here asking for advice.

The stems felt loose to me but I have nothing to compare them with.

If I remove the valves, I might as well do the guides since it requires removing the cylinders?
 
I'd leave it alone. The main purpose of the Wobble Test is to identify insufficient clearance, which leads to stuck valves. Your oil consumption is great and the valves don't show signs of distress.
 
From what I recall in the Lycoming engine school, when the valve opens / closes, it doesn't move in a straight line; rather there is a little side action to it and if you were to watch it close you'd see it kind of snap into position as it closes. As the engine ages, excessive valve guide wear could cause the valve to seat incorrectly leading to the valve failing.

In this case, I'd suggest a look at the valves with a borescope to see if there are any telltale heat issues. AOPA has a good chart with illustrations at https://www.aopa.org/-/media/files/...fn0000-asi-valve-safey-poster_final.pdf?la=en . If you see any evidence of green tint to the valve or if the heat signature is obviously uneven, then the risk of a valve failure is very high, and I wouldn't fly the airplane in that condition as I expect the valve would probably fail sometime in the next 8 - 10 hours.

Dan
 
From what I recall in the Lycoming engine school, when the valve opens / closes, it doesn't move in a straight line; rather there is a little side action to it and if you were to watch it close you'd see it kind of snap into position as it closes. As the engine ages, excessive valve guide wear could cause the valve to seat incorrectly leading to the valve failing.

In this case, I'd suggest a look at the valves with a borescope to see if there are any telltale heat issues. AOPA has a good chart with illustrations at https://www.aopa.org/-/media/files/...fn0000-asi-valve-safey-poster_final.pdf?la=en . If you see any evidence of green tint to the valve or if the heat signature is obviously uneven, then the risk of a valve failure is very high, and I wouldn't fly the airplane in that condition as I expect the valve would probably fail sometime in the next 8 - 10 hours.

Dan

Thanks for the info. I have checked the valve pics and compared with AOPA's article. My valves look good for now when viewed form inside of the cylinders.

In addition too this forum, I am planning on calling Lycoming to see if they have anything else to add.
 
Valve

I have seen posts from people who like the shop in Sebring. When Cessna resumed production on piston single's they plumbed the oil pressure line to the front of the gallery on the right side. This will result in a reading 10# lower that at the normal position for the oil pressure line. The oil pressure relief valve is then set to read middle green or higher. So 70# is now really 80#. There are experts who believe that this is very beneficial to the valve train, especially the guides.
 
I would call Continental/Titan James Ball or Mark Cummins. They have been very helpful to me.
 
This is music to my ears. What is your recommendation based on? Thanks.

Nothing scientific, just experience. As has been mentioned the test is mostly to spot stuck valves, as long as the valve faces don't show any signs of distress I'd keep running it.
 
For what it's worth in your decision making, I recently had a momentary power loss over some very inhospitable terrain in the Coast Mountains. As oil consumption had also started to increase recently, I had a mechanic help me with an inspection of the engine (Lycoming O-360 with about 1400 hours on the cylinders). When I pulled the bottom number 2 sparkplug, you could pour oil off it. Further inspection revealed that there was very excessive wobble in the exhaust valves of two of the cylinders (we didn't have the official tool to measure, but it was very obvious). The other two cylinders were questionable as well. Compressions at the most recent annual about 50 hours earlier had been good. We sent all four cylinders to the engine shop and they replaced all exhaust valve guides and two exhaust valves. While it might be early to consider pulling the cylinder, I would definitely monitor for awhile to determine if it is getting worse.

Rob
RV6A C-GRBK
 
While I have no first hand experience, given the horizontal orientation of the valve and the anemic oil flow to the rocker box, I doubt that worn guides would show noticable increased oil consumption like they would on most auto engines.

Larry

Physical orientation (horizontal vs vertical) will have far less effect on oil consumption through the intake valve guides than the pressure differential that exists (primarily on intake stroke at low manifold pressure) between the ambient pressure in the rocker box and the intake chamber.

Not clear to me what mechanism would promote oil consumption through the exhaust valve, since the chamber around the valve stem is normally higher than ambient.

It seems to me that if the guides are worn enough to cause seating errors that might risk breaking a valve, it would show up in a compression test.
 
Physical orientation (horizontal vs vertical) will have far less effect on oil consumption through the intake valve guides than the pressure differential that exists (primarily on intake stroke at low manifold pressure) between the ambient pressure in the rocker box and the intake chamber.

Not clear to me what mechanism would promote oil consumption through the exhaust valve, since the chamber around the valve stem is normally higher than ambient.

It seems to me that if the guides are worn enough to cause seating errors that might risk breaking a valve, it would show up in a compression test.

During a differential pressure test, the valve is seated slowly as the motor is turned by hand. The valve will almost always seat. Running at 2300 rpm the valve has many opportunities to hit off center and fail. All prior DP tests were normal.

I lost an exhaust valve at night...... the motor does not run smooth (shakes like he**) on three cylinders. What saved me was the fact that 1/3 of the head stayed with the stem and the other 2/3 broke in two pieces and was pushed out the exhaust port.

I would not mess with a sloppy exhaust valve.
 
During a differential pressure test, the valve is seated slowly as the motor is turned by hand. The valve will almost always seat. Running at 2300 rpm the valve has many opportunities to hit off center and fail. All prior DP tests were normal.

I lost an exhaust valve at night...... the motor does not run smooth (shakes like he**) on three cylinders. What saved me was the fact that 1/3 of the head stayed with the stem and the other 2/3 broke in two pieces and was pushed out the exhaust port.

I would not mess with a sloppy exhaust valve.

Point taken. It just seems like 0.050" of misalignment on seating is not enough to create a significant moment, and is more likely to just guide the valve into its seat. But I suppose if it did it the same a bunch of times, it could fatigue the valve stem and fail. And of course, "a bunch of times" can happen pretty quickly at 2300 rpm.
 
Point taken. It just seems like 0.050" of misalignment on seating is not enough to create a significant moment, and is more likely to just guide the valve into its seat. But I suppose if it did it the same a bunch of times, it could fatigue the valve stem and fail. And of course, "a bunch of times" can happen pretty quickly at 2300 rpm.

The valve material is pretty hard and doesn't like bending. Lyc has gone with some pretty exotic materials on the exh valve. And don't forget that it is hollow so they can put sodium in it. The crazy valve design, along with anemic oil flow to the rocker box makes the exh valve a serious weak point on these engines. For comparison, when was the last time you heard of an auto engine swallowing an exhaust valve. Quite rare these days. It seems to happen routinely on Lyc's. A loose guide allows the valve to hit on one small area of the face and creates a bending moment. The stronger the spring the worse it is. Over 1000's of cycles it causes fatigue. As you mention, the looser the guide the greater the bending moment. But I do not believe that it just gracefully slides into place. The real question is how loose is too loose. Given no other meaningful data, we need to go with Lyc's recommendation, though they are likely conservative. Rest assured that over time a loose guide (arguably unclear how we define loose) will cause either face/seat wear or a crack. The hollowed stem makes it a more likely candidate that the face and likely why lyc's swallow exh valves instead of developing cracked faces. Given all of these issues, I would not take risks with loose guides on a lycoming. I suspect Gasman others others that have experienced a swallowed valve would agree.

While the SB is mostly for tight valves, there are absolutely limits on max allowable guide clearance. I suspect that lyc's 035 wobble limit directly correlates to their overhaul service limit for guide clearance plus some expected wear.
 
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How old is the engine? Old exhaust valves can have corrosion issues.

Th engine is a narrow deck. 11 years since complete overhaul including new ECI cylinders. I fly often and I do not se any signs of corrosion anywhere in the cylinders during borescope. How else would I determine the condition without taking the cylinder off the engine?
 
For what it's worth in your decision making, I recently had a momentary power loss over some very inhospitable terrain in the Coast Mountains. As oil consumption had also started to increase recently, I had a mechanic help me with an inspection of the engine (Lycoming O-360 with about 1400 hours on the cylinders). When I pulled the bottom number 2 sparkplug, you could pour oil off it. Further inspection revealed that there was very excessive wobble in the exhaust valves of two of the cylinders (we didn't have the official tool to measure, but it was very obvious). The other two cylinders were questionable as well. Compressions at the most recent annual about 50 hours earlier had been good. We sent all four cylinders to the engine shop and they replaced all exhaust valve guides and two exhaust valves. While it might be early to consider pulling the cylinder, I would definitely monitor for awhile to determine if it is getting worse.

Rob
RV6A C-GRBK

Thank you for the great reference point.
 
JB Aircraft Engine Service, Sebring, FL. (863)655-5000. You can park your airplane on their ramp and hand Jimmy the keys, and he will take it from there. Jimmy is among the best in the business.
 
The valve material is pretty hard and doesn't like bending. Lyc has gone with some pretty exotic materials on the exh valve. And don't forget that it is hollow so they can put sodium in it. The crazy valve design, along with anemic oil flow to the rocker box makes the exh valve a serious weak point on these engines. For comparison, when was the last time you heard of an auto engine swallowing an exhaust valve. Quite rare these days. It seems to happen routinely on Lyc's. A loose guide allows the valve to hit on one small area of the face and creates a bending moment. The stronger the spring the worse it is. Over 1000's of cycles it causes fatigue. As you mention, the looser the guide the greater the bending moment. But I do not believe that it just gracefully slides into place. The real question is how loose is too loose. Given no other meaningful data, we need to go with Lyc's recommendation, though they are likely conservative. Rest assured that over time a loose guide (arguably unclear how we define loose) will cause either face/seat wear or a crack. The hollowed stem makes it a more likely candidate that the face and likely why lyc's swallow exh valves instead of developing cracked faces. Given all of these issues, I would not take risks with loose guides on a lycoming. I suspect Gasman others others that have experienced a swallowed valve would agree.

While the SB is mostly for tight valves, there are absolutely limits on max allowable guide clearance. I suspect that lyc's 035 wobble limit directly correlates to their overhaul service limit for guide clearance plus some expected wear.

Found this article http://precisionengine.home.mindspring.com/engine1.htm that in line with your post. Bottom line seems to be insufficient oil supply to valve causing premature wear.
 
A design perspective.

There assumptions here that all valve stems and valve guides are the same. Changes are made to engines constantly. You may just have a valve guide wear, but it could be valve stem wear too. As a general statement, design and overhaul specifications would dictate replacement for many parts that field technicians leave alone and have good experience to do so.

Walt is in the field technician category, with experience, and judgement to be trusted. As an engine design engineer representing the factory, it was always interesting to go into the field for investigations and talk the the guys who did maintenance and overhauls. It was always interesting and a learning experience to see the other side of a particular engine, of which I had intimate knowledge of its design and lab testing. I concluded that the brain power, judgement, and critical thinking of these guys were equal to the best engineers I worked with and certainly earned my respect, FWIW. They simply worked in a different world of experience and training.

So - a suggestion by Walt to continue, but also one should investigate the material properties of of the valve guides and valve stem coatings. Lycoming has some proprietary guide materials that were introduced after stem wear became a problem. The design of that area is a real balance of issues with oil consumption, carbon, stem wear and guide. All resulting in either sticking or loose valves. It is manufacturer choice for allowing one of the other in their assumption of liability and failure risks. Valve face and seat wear are another part of that design equation. Valve head separation is more likely when the the lash is too large and the valve seating velocity becomes too high.

Imagine, as the guides wear the valve will randomly seat a tiny bit off center. As it does there will be wear 90 deg to the rocker shaft, on lift and seating. This wear will work to alleviate the bending of the valve face (not 100.000%). If the valve stops rotating or the wear becomes excessive, it will begin to leak and progressively run hotter until it won't seat. Typically, the wear will change lash until the adjuster reaches it's limit and leakage and burning can occur when the valve can no longer reach the seat.

Visual inspection of the seats is in order to see if the wear is excessive at this point viewed by an experienced engine technician.
 
I wanted to thank everyone for their thoughtful suggestions. After talking to Continental, JB Engines and others, I have ordered 6 new Millennium jugs. CopaAir in Indiantown, FL will be replacing them next week.
 
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