As I said on another thread, I've just had an enforced o/haul and the engine was found to have serious corrosion. I'd just changed the oil filter a few hours before the strip down and there was very little contamination in it.
You can determine corrosion by borescoping the engine which I regularly do. Also I send in an oil sample for analysis after every oil change (which I do on a 25 interval). I don't think that there is much about engine health that I could miss with that approach.
Look at the statistic a little further down in this column.
It doesn't follow that engines should never be overhauled. It merely shows the relative risk of flying behind a "recent overhaul" engine.
I don't think there is any way to see the camshaft etc without pulling a cylinder
No but pulling a cylinder every now and then when the engine reaches a certain age isn't all that wrong. Also you can easily measure the travel of the valve which tells you something about the lifter's health.
I had an exhaust valve guide replaced this year (and while at it the valve + piston rings), just as preventive maintenance because it had a lot of play (still within Lycoming limits) and I found exhaust stains under the rocker cover. That is not expensive but gives you an opportunity to take a closer look inside.
There can be an interesting debate over whether an engine that needs a cylinder pulled has made TBO 
I still would like to know more about Ben's two engines. It is extremely unusual.
An engine that "needs" to have a cylinder pulled in order to remain airworthy probably hasn't made TBO (unless it's a Conti TSIO-520 where pulling cylinders is kind of like an oil change). However, sometimes it can be smart to perform preventive maintenance. Especially the big Lycomings pretty much all suffer from stuck valves ("morning sickness") at some point so I like to be very careful about this. A lot of them continue for years with the stuck valve issue until at some point they swallow a valve.
I am not sure if you mentioned this before here, but were they done by a particular UK company?
No Peter, the engine was o/h in the US by a shop that is regarded by many to be one of the best.
Both times ? Can you email me the name ?
OK Peter, just came back from Israel so I will release you of your agony. The engine gave up at 2740h and was sent to the US, the shop was Zephyr Engines in Florida. They have opened the engine and found the cylinders to be corroded and pitted, the lifters were pitted and the central shell bearing of the crankshaft broke its dowel (the pin that holds it in place to the crankcase) with some heat checks to the crankshaft itself. The case was repaired as well as the crankshaft and the engine was o/h to new spec. with new cylinders and camshaft from ECI, new lifters (that were tuned) all accessories (other than the alternator and CSU that were very low time) being replaced with new, the fuel servo, spider etc. were o/h. On the aircraft the engine mount was striped, X-rayed and re-coated while the fire wall was cleaned, inspected and re-protected. The engine re-fitted and I started flying it according to the instructions given and changing oil per instruction. I did not carry oil analysis as that was waste of time and money. The engine temps were high for the first 5-10h as well as the oil consumption that did not drop until about 45h. at that point I changed to the Elite W20-50. Sometime later I went to fly and noticed that the oil level was low so toped it up. At about 50h I met Rufus Heald and did some training with him where he showed me how the RAF performs a forced landing. When I returned home I was short of 4 quarts of oil on a flight of 3.5h, this turned the warning lights on. By then I have moved the aircraft to Marshal (why is another story) and we started to watch the engine, I can tell you that the filters were full with metal, so at 75h we decided after getting the OK from Zephyr to remove 1 – 2 cylinders and check the engine. We found that cyl 6 was a goner with 4 not far behind it. The exhaust valve guide was loos and moving with the valve inside the cyl. head, the rings were worn out and very sharp the camshaft had two flat cams and the lifters were damaged too, so Zephyr asked us to remove the engine and send it back. They have re-overhauled the engine to new spec. and when I wanted to pay for it (by the warranty terms all they had to do was a repair) they refused but asked me (but only if it is not too hard) to cover the FedEx. charges. I again ran the engine as needed and this time things seemed to be better. Then, one day, on the way back from Project Propeller I heard noise which was similar to a fast machine gun coming from the direction of the engine, otherwise all indications were normal so I continued home which was another 15min. away and the following day I flew the aircraft to Marshal to see what was wrong. After some head scratching one of the engineers noticed that at a certain point the engine requires more force to turn it and that this happened every 720 degrees of rotation. Marshal was on the phone to Florida and at the end we were asked to send the engine back so they can start to strip it bit by bit and see what is going wrong. Very soon they found that the double gear broke a tooth, yet they have opened the engine and inspected all components before sending it back. Since then no problems and oil consumption is ~ 1qt for every 7h flight. I have no complains about the shop, I think that they have treated me with honesty. They might have some bad parts coming from ECI or even made some mistakes themselves, I will never know, but they stood behind their work and solved the issue without even trying to shift any blame to me. If this shop was in the UK I would have used them again but when you face such issues you want to have shorter communication lines and you want to avoid shipping the engine around the world as often as I had to do. By the way, Zephyr Engines just won for the second or third time the 'Aviation Consumer' title of the Best O/H shop in the US.
