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Hi Don,
Here's the story on my rocker arm woes:
Both before and after my engine overhaul, I had oil dripping out of the throat of my carburetor after shutting the engine down and letting it sit overnight. Before the overhaul, I verified that this oil was in fact running down the inside of the intake tubes. This oil was leaking past the valve stems, since this is the only place that oil can enter the intake tubes. The oil was making a mess inside the engine cowling, but was not so much that it was noticeable as far as oil consumption was concerned. (I've been using about 1 quart every 15 hours or so, which is when I normally change the oil anyhow.)
A friend of mine had the same problem with his C-90-16F engine, and he told me how he cured the problem on his engine. It seems that these engines can be equipped with a "new style" or an "old style" intake valve rocker arm. The "new style" arm has an extra oil hole drilled in it that supplies extra oil to the interior of the rocker box. The "old style" rocker arm does not have this extra oil hole. Both types are suitable for installation on the C-90-16F engine. My friend installed a set of the old style arms on his intake valves and that cured his problem with the carburetor leaking oil.
I discussed this with my mechanic, and he agreed to change the rocker arms on my engine to the older style arms. The engine shop that overhauled my engine was consulted and they said that they had no problem with this change, and that it would not affect the warrantee on the cylinders. They said that these engines ran for years and years without the extra oil hole, and that both parts were approved parts for the engine. They supplied four reconditioned, yellow tagged, old style rocker arms to my mechanic, and we installed the arms and did the required logbook entries. We also changed the oil and inspected the oil filter at this time. We found no metal at all in the oil filter.
I flew the plane and immediately noticed that the oil leak problem was gone, or at least it was 95% gone. I had very little oil in the engine compartment after changing the rocker arms.
I flew the plane 13 hours and changed the oil again. This time when I changed the oil and cut open the oil filter, I found lots of very tiny bronze or copper colored metal particles in the oil filter. These particles were so small that you could see the reflection from them in the oil and on the oil filter element, but if you tried to get some on your finger they just about disappeared. They were almost the size of powder or dust.
My mechanic and I discussed all of this, and we decided to fly the engine a few more hours and check the filter again. We thought that this metal might have been coming from the new rocker arm bushings wearing into the used rocker arm shafts. The amount of metal that we found was less than would typically be found during the first oil change of a newly overhauled engine. We were comfortable with flying the engine a few more hours since the particles were so small, and they were obviously soft bushing material, and the oil filter was catching them all.
So I flew the plane 9 more hours and on the next oil change, I had just as many tiny bronze colored particles. We disassembled the rocker arm assemblies and inspected the rocker arms and shafts. On the first inspection efforts, we really couldn't see where any metal was coming from on the rocker arms. The following day, I returned to the hangar determined to find where the metal was coming from. I was sure that it had to be coming from the rocker arms, because changing the rocker arms is the only thing that we did just prior to the engine starting to make metal. Also, there are only a few parts in the engine that are made of that type of metal, and it didn't make any sense that any of the other parts could be generating tiny little flakes of metal like we were seeing.
What I found on the second inspection was that the rocker arm bushings on the newly installed intake rocker arms were not installed properly by the engine shop. The bushings had been pressed too far into the rocker arms such that the ends of the bushings were protruding beyond one end of the rocker arms by a few thousandths of an inch. The bushings are supposed to be flush with, or slightly recessed from, both ends of the rocker arm shafts. In operation, the rocker arm shafts in these engines are free to move side to side on the rocker shaft. The thing that stops them from moving out of alignment with the ends of the valves and the valve push rods are the flat machined steel bearing surfaces on both ends of the rocker arms. These surfaces bear on the flat surfaces of the aluminum rocker shaft bosses that are cast into the cylinder heads. Normally, the smooth steel faces on the ends of the rocker arms rub or bear on the flat aluminum surfaces. The rocker arm bushings, being flush with or recessed into the surface of the rocker arms, are not supposed to bear on the aluminum rocker shaft bosses. But because the rocker arms had the bushings protruding from the rocker arms, the bronze (or oilite) bushings were wearing on the aluminum rocker shaft bosses. The bosses (and the rest of the cylinder as well), had been cleaned during overhaul of the cylinders by bead blasting them, so the flat bearing surfaces of the rocker arm shaft bosses had a sandblasted finish to them. This rough surface was filing down the protruding portion of the bronze bushings every time the rocker arms actuated. I found a definite wear pattern on the ends of the rocker arm bushings where they were being worn down by this filing action.
I also found that the clearance between one of the newly reconditioned rocker arms and the rocker arm boss was out of spec. The overhaul manual allows a certain amount of clearance between the arms and the bosses (maybe 0.019 max - I don't have the spec in front of me), and one of the new rockers had too much clearance. Further measurement showed that the width of that particular rocker arm was quite a bit narrower than any of the other rocker arms. We had to return that rocker arm to the engine shop and get it replaced.
After discussing all this with the engine shop, we replaced the one narrow rocker arm, installed factory new rocker shafts (just to be on the safe side), polished the protruding rocker arm bushings down flush with the ends of the rocker arms, removed all the valve springs and checked the clearances of the valves in the valve guides by moving the valves around in the guides, and then reassembled the valves, rockers, shafts, etc with plenty of STP lubricant.
So now we are in the process of reassembling the rocker covers, baffling, cowling, etc, etc. I'll let you know how it does on the next oil change in 10 hours or so, but I am not expecting to find any metal. I've flown the engine about 50 hours since the overhaul was finished at the end of August, and I'm hoping to put another 50 hours on it in the next 3 or 4 months. (The guys at the airport where I buy fuel actually looked my number up on the Internet and called me the other week to find out what happened to me! I think their revenues have been down since I've had the plane grounded for maintenance!) I'd like to get 100 hours on it without any problems so I can feel like I'm not in "test flight" mode any more.
Lessons learned:
1. The fact that a part has a yellow tag and is legal doesn't mean anything except that it has a yellow tag and is legal. The part can still be junk. But we already knew that.
2. Never make the mistake of assuming that just because a part was overhauled or reconditioned by a certified shop that the part is good. It's up to the owner and the installing mechanic to verify that the part is good. Again, we already knew that. In this case, both of the defects (the protruding bushings and the narrow rocker arm) were impossible to notice unless you knew to be looking for it and checked the part with vernier calipers or a micrometer. My mechanic and I did look at the rocker arms and inspect them, but you can't really notice something being a few thousandths out of place with the naked eye.
3. If my engine didn't have an oil filter kit installed, I'd be sweating bullets about that metal going through the engine. The metal particles were so small that they probably would have gone right through the oil screen. (I removed the oil screen when the filter kit was installed.) The fact that there were no metal particles in the oil that I drained out of the oil sump, and the fact that no metal particles came out of the sump when I flushed the sump out with kerosene, tells me that very few if any of the particles got past the oil filter and into the bearings. Actually, these particles were so small and soft that they probably would not have caused a problem anyhow.
4. One of the many advantages of changing the oil often is that it gives you more frequent opportunities to catch a developing problem. In my opinion, trying to extend oil changes to save a few dollars is false economy. It's better to spend the extra time and money to change the oil more often and have a look at the oil screen and/or filter.
OK, Don, that's the deal with the rocker arms!
Gotta go put an engine back together.
Best Regards,
Wayne DelRossi
Alon N5618F
Alon N5618F
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