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Low Compression, Worn Exhaust Valve Guide, Decisions, Decisions

Steve Pierce

BENEFACTOR
Graham, TX
So I participated in the STOL Demo at Oshkosh and had a lot of fun. I noticed that my engine started to hesitate a bit when i pushed the throttle forward on take-off so when I got home, like I did prior to going to Oshkosh I changed my oil and checked my compression. Cylinders 1 through 3 were all 76-78/80 like they have always been over the almost 5 years I have owned my super Cub. However, the best I could get on cylinder #4 was 58/80 with the air coming out of the exhaust. Borescope showed everything looked normal. I flew the airplane around the pattern a few times and staked the valve but still no change. I recently realized that I was just putting the tach time in my engine log when I changed the oil and checked compression and I better total up the time since major overhaul. After all I have had it almost 5 years and it had almost 1600 hours on the factory overhauled engine when I bought it. Well after I totaled the numbers a few times I realized I have 2124 hours on the engine and it still only burns a quart of oil every 10 hours or so and all I have had to do was general maintenance on it. When I pulled the rocker cover to stake the exhaust valve I could see where it looked like combustion gasses had been getting into the rocker cover area so I got out my valve wobble test fixture and printed off Lycoming Service Bulletin 388C. Well low and behold I had a little over .050" of clearance vs the service limit of .030" Lycoming says I should have. Mystery solved, major dilemma started.

Once I knew the issue and how much time was on the engine etc I had to start seriously thinking about my options. Now we are talking some serious dollars for a complete overhaul and not chump change to top it. I gave Ken Tunnel a call to see what he thought. He figured I'd find a bad cam and lifters when i pulled a jug until I told him it was a roller cam engine. I also pull the filter and check it regularly and know it has been clean. He said other than the top end that is usually the only thing he finds wrong on an O-320 with these times. Kinda my experience as well although he sees way more engines than I do. So now what? Buy 4 new cylinders and top it, get these 4 rebuilt since they are first run cores, put a guide in this cylinder and keep going, install the narrow deck engine I have until I can do a complete overhaul? Sell it and get a less expensive hobby? :lol: Decision, decisions, decisions. :Gwhoa: Exploring my options$.

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Well you can't just sell it since it is part of your advertising.
The answer in my eyes is to let the other 3 cylinders talk. Where is the guide wear on them? Is this just one worn guide, or a few more ready to show their fangs?
What are the bores like?
To me your engine will give you the most honest answer.
 
Pull the jug, leave the Piston on the rod and replace the guide and try lapping the valves. If valve won't lap grind valves and seat, lap valves and re assemble jug. Don't touch rings or remove carbon from Piston.

Sucks the be poor Steve but you'll open up a can of worms if you do more.

Glenn
 
call sals or jj and see if they have a used one, bare, ready to go. sal he probably wont sell a bare cylinder but can try. when you send your used ones in to rebuild, if you have my luck 2 or 3 out of the 4 will be cracked, there goes postage and the inspection fee. harrison sometimes has bares ready to go. sometimes one of these guys are pretty good on price. and then run er.
 
Your engine just smelled the new flaps and got jealous, so minimize the project (because its not like you don't have anything else to do) and go flying before something else falls off.
 
What Glenn said. Not a lot of labor to pull the jug and have a new guide put in. Of course check for cracks while it’s off. I’ve run O-320s over 7000 hours without a problem on banner tow and glider tow airplanes. Not much harder use than towing.


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May be a good idea to periodically remove the valve covers to look for exhaust blowby as TBO approaches. The insides get covered black and leave heavier tracks near the exhaust. Take some pics to compare with next time. Oil turning black soon and increased vent leaks have several causes.

Gary
 
There is a SB that has you put an airspeed indicator in the oil fill tube with a plug, then run the engine. If the “airspeed” (pressure) in the case exceeds a specific value, then you have excessive blow by. I forget offhand what the airspeed is, but seem to recall about 40mph.


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Sending cylinders to a shop - we get about 90% rejection. One poor little 65 hp has been down for three months. They went through all my spare cylinders. I want them back; I am not as picky as an engine shop.
 
Which is worn, the one valve or guide or both. Just replace the culprits and keep on flying. Do you have access to valve seat grinding equipment? Don't turn this into a project where there is none. The guide just presses out. Then heat the head and slide in a new one. They come in oversize if needed. Match the new valve to the seat and put it back together.
 
I have access to the equipment to replace the guides and dress valves and grind the seats, have done it on many cylinders over the years and it is not that big of a deal. The thing I have been contemplating the most is pay now or pay later. I work usually at least six days a week and like to fly on Sunday and a trip or two a year. I pulled the rocker cover off #2 while I was there and could see some blow by at that exhaust guide but not like #4. Had work to do so didn't get any further into it. I could do the wobble test on all cylinders and repair as needed. I don't expect any bore issues from what I see with the borescope, oil consumption and what I see in the filter. I have a new set of pistons to bump my 150 hp wide deck engine up to 160 hp. I am not to concerned about cracking as these are first run factory cylinders and by the date of manufacture don't fall in the dates of some poor head casting a few years later.

Still mulling things over in my head. I have always wondered at what point you risk damaging parts that cost you more money down the road like having to grind the crankshaft mains or rods. I am knee deep in finishing up a Super Cub rebuild (almost ready to push it out of the air conditioned shop, hange wings and tail and fly) and have a couple of annual inspections waiting on me to get motivated to do so I am not getting to my airplane anytime soon. Still contmplating. Appreciate all the comments and advice. I know there are others in the same boat.
 
Just sent a first run lycoming to overhaul this winter. Had 2100 hrs on it. The engine shop said they get them in for Oh with 3000+ hrs on it. Only problems they see is the bearings wear and then the cases fret and they end up replacing the crank cases at overhaul.
Throw your spare engine on and overhaul yours. Then you will have a nice new engine to put on your exp cub when you get around to building it


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A couple friends with Pawnee glider tugs went over 3000 hrs on their 540s without issues. A first run engine should still be good for a lot more hours.

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Just sent a first run lycoming to overhaul this winter. Had 2100 hrs on it. The engine shop said they get them in for Oh with 3000+ hrs on it. Only problems they see is the bearings wear and then the cases fret and they end up replacing the crank cases at overhaul.
Throw your spare engine on and overhaul yours. Then you will have a nice new engine to put on your exp cub when you get around to building it


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Case fretting is often due to incorrect torquing procedures on the through bolts or incorrect sealing. I’ve run O-320s over 7000 hours with no fretting. Once you torque the case, you need to let it sit for a day or 2, then go back and re-torque. You would be surprised how much the bolts stretch, the case relieves internal stresses, and sealant extrudes from the parting surface and you get another 3/4 to full turn on each nut to bring it to the correct torque.


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top it and put in the 160 pistons. I’d be sick to my stomach flying around with a A2B wide deck engine that was still 150 hp.
 
So I participated in the STOL Demo at Oshkosh and had a lot of fun. I noticed that my engine started to hesitate a bit when i pushed the throttle forward on take-off so when I got home, like I did prior to going to Oshkosh I changed my oil and checked my compression.

Wow....just think if you had 4 good cylinders @ OSH!

Engine "overhauling" pricing keeps getting crazy. A friend just looked into an 0-300. Through studs were $1000 each. If you go through the cylinders and run it for 500 hours or so and spend $2K of parts/time your extended run will be $4 per hour. So...that is how I would look at it. Keep it safe. Keep your heart out of it. Don't get whileweareinhereitis.

Here is a question: Bad cam with 900 SMOH. Do a bottom end overhaul or just replace cam and lifters?

Tim
 
Steve
You know all 4 exhaust guides will be badly worn with that many hours. You can wait until one of the others miss-seats the valve or snaps a valve and puts you in a tree or the river. So just suck it up and do all 4 guides, and take a hard look at the bottom while everything is off and you are sticking in the pistons. I have a few jars of SUCKITUP on the shelf, I can send you one.;-) Here is a you U-tube video thing of a loose valve miss-seating for those that have not had it happen to them over bad country. https://www.youtube.com/watch?v=x6OyfoV1Z2I
Behindpropellers
I just put new cam and lifters in mine with 1200 hours on motor. I had done all 4 exhaust valve guides at 500 hours so used the same top. Not sure what the right answer is but that is what I did.
DENNY
 
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If you've never removed a tired engine and replaced it with a new engine you don't know what you're missing. No, seriously, you don't know what you're missing.
 
I was in the same situation last year. Three out of the four exhaust valves were sticking on my 1800-hr first-run cylinders. New valve guides all around and four new exhaust valves, a bit of honing & new rings, & now it purrs like a kitten. Good time for you to install those 160 hp pistons.
windy


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Windy, what flavour of cylinders did you have on your first-run 0-360? DANG! I wish I flew as much as you do.

Thanks. cubscout
 
This is kind of a neat exercise in problem solving. Lots of different angles, mindsets and rationale. I’m one that looks at the problem from a poor boys perspective (duck tape and bailing wire) but hope to someday be able to look at it from the other side. Twenty something thousand for an overhaul these days sucks however, if you’d guarantee me 2000 trouble free hrs I’d sign off on that every time.....pay to play I guess
 
Windy, what flavour of cylinders did you have on your first-run 0-360? DANG! I wish I flew as much as you do.

Thanks. cubscout

I have Lycoming cylinders with nitride, ported & polished by Lycon. Now they have over 2000 hrs on them, at least 200+ since they came back from the hospital. No moss grows under my cub’s Bushwheels!
 
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