Valve wobble test out of spec.

[lr172]

Originally Posted by scsmith

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.

[flysrv10]

Originally Posted by Walt

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.

Thank you.

[flysrv10]

Originally Posted by Gusmax

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?

[flysrv10]

Originally Posted by rag

I would call Continental/Titan James Ball or Mark Cummins. They have been very helpful to me.

Thank you for the contacts. I will call on Monday.

[flysrv10]

Originally Posted by rv6agrbk

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.

[Scott Hersha]

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.

[flysrv10]

Originally Posted by Scott Hersha

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.

Thank you. I will have them in mind.

[flysrv10]

Originally Posted by lr172

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.

[BillL]

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.

[Ted RV8]

Originally Posted by flysrv10

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.

This article is a very interesting read. Couldn?t find a date to when it was written and wondering if there is any updated information.