Unlimited life with a caveat, fatigue, wear and corrosion
rjcthree said:
Is there an end of life rating for Lycomings? If parts are within limits, etc., should 6600 TTSN be a problem, with full logs, NDH?
While I think I'm going to pass on this engine - it's simply too early for me - I'm trying to get smarter.
Thanks in advance, Rick 90432
I asked Lycoming about life limited parts. The answer is NO. There is no life limit. However some parts may just wear out and be too undersized (like crank journals) to be usable. Clearly a used crank or cam that has been ground once has less meat on it, but fatigue life is not an issue.
Obviously "cycles" on the engine add up and affect the major components (expensive) items like the crank, rods, cam, case, but they have unlimited life. From a practical stand point they will mechanical wear and be out of limits at some point.
Is new better? Well let's say 100,000 hours is the designed life (just a made up number for example) and you have 6,000 hours; than your are fine. However the point about "re-work" material left on parts like cranks apply. Like any metal part subject to metal to metal wear, at some point you can't rework them anymore. (Corrosion is the killer but more about that last.)
As long as you meet all inspections, dimensions are with in limits you are good. Keep in mind as an experimental you can buy discounted engines. If you are buying some old engine, point out you can get a 100% all new engine (including most accessories) for less than $20,000. Why risk buying an old engine with a crank you might need to replace for a cost of say $5,000 for that one part. You pay too much for that "core" engine you might has well have bought a new "CLONE" engine (ECI, Superior). Same with cylinders. With competition price of new cylinders is fairly cheap. You can spend $600 reworking old cylinders and buying new parts (valves, seats, pistons, rings), or you can get a whole new cylinder with all new parts for $1000. Most people now just buy the new jugs and sell the old one to the airboat guys or some guy needing a replacement to milk some more time out of their engine.
As experimental we have a huge advantage. We don't need to pay certified prices for engines. We don't need to buy a specific model of engine. I find people want GOLD and Blood for old tired engines. Although there has been a price bump in the last year, new engines at about $20,000 are a bargain. If I was doing it today, I'd just go new engine KIT and build it my self (with help). You save a few grand, so it would be about $18,000. Other wise I would buy an engine Pro built from Mattituck, hang it and go fly.
Other items like
Cylinders and valves don't last forever or even more than 2000 hours, much less 6000 hours. Cylinders get worn out mechanically and from thermal cycles (fatigue). You can still use them but the chance is you will not make TBO without a crack or some problem. Cylinders have no fatigue or life limit either, it's just based on specs and inspections. However we know the heads just get beat to death. Exhaust valves? They just wear out, usually the stem. You are lucky to get 2000 hours. You would never use old exhaust valves that are barely in specs on a overhaul but you can, it just will not make TBO.
Lycoming does REQUIRE that some parts be replaced every overhaul or every 2000 hours, what ever happens first, like ROD bolts for example. When I say required TBO, that is recommended to you and Mme. If you are a commercial operator you can't go past TBO. For an experimental or general aviation plane you can go as long as you like or can get away with. I ran a pair of Factory Re-manufactured O320's to 2300 hours on a twin with out problem than sold the plane. They where still going strong when I sold them. The thing is the plane flew almost everyday and was operated carefully and maintained the same way (oil changes every 25 hours).
NOW HERE IS THE CATCH 22. It is not hours as much as how its operated and how much its flown. The big killer is corrosion. An engine sitting will waste an engine. An engine flown every day and operated and maintained properly will go to TBO. That same high time engine will also be less worn and in better shape at overhaul time. Now take the same engine but flown rarely with 500 hours in say 20 years for example. It may be destroyed. Sure it has less cycles (hours), but corrosion (pits) may have destroyed the cam or crank.
It has nothing to do with hours, it is condition.
MOST LYCOMINGS DIE FROM CORROSION. Very few Lycs die from fatigue.
However there have been fatigue failures of cranks and rods. Well what is up with that? These have mostly been from either a manufacture flaw or damage gone undetected or attended to, such has after a prop strike. Fatigue can be an issue! However if the parts are made per spec (which 99.999% are, notwithstanding the dark years in the 90's with the bad cranks) and maintained and flown properly, fatigue has been designed (conservatively) to be a NON issue. If there is a flaw or damage than you might not get 600 hours, much less 6000 hours out of a crank before failure. The reason Lyc recalled the second batch of cranks, in which NONE have failed is because they found they did not meet the unlimited life design criteria. They may be fine but there is a chance that some are below par from statistical testing of a sample in those batch's. The first group they recalled had a few actual failures in service. (When I say unlimited life there is no such thing, but in engineering terms its a time limit that is so large it is essentially unlimited. It also means that the part is good by condition not time or cycles.)
I bought a used engine and rebuilt it. My O360 crank (whole engine) left the factory in the early 70's. It now has about 4000 hours on it and two rebuilds. Keep in mind when you overhaul you don't need to carry the hours over to the new log book, but I happen to have the original log books of this engine and second log book after first rebuild before I bought it. I suspect I could go another 8000 hours with out problem from the rate of wear. At some point the bearing journals are too small and you need to retire the crank or rods. The cam wears down as well. I guess the case might go longer. All the big parts are original 1970's parts.
One thing about going new I just thought of is better design. My rods are fine, but use a torque bolt design. Newer rods use a stretch bolt design, which are suppose to be better, at least in assembly, where you get more consistent bolt pre-tension. They found mechanics under-torqued the rod bolts if care was not taken. It does not mean its bad, just the newer stretch bolt is more consistent and easier to control from what I hear. Torque verses pre-tension is dependant on thread / nut friction. Care has to be taken to use the proper lube and technique when using torque only method.
I recommend
"Sacramento Sky Ranch - Engineering Manual", for Lycoming & Continental. This book will explain ALL about fatigue and engine issues. It's a must read for any RV'er thinking about rebuilding/building an engine, making the used engine purchase or getting engine shop services. Frankly any RV'er would enjoy this book. If you can beg, borrow or steal a copy it is worth a read.
