Let's talk about Radial Engine TBO's (time between overhauls). I hear a lot of discussion a the convention relating to this subject. The following statement is one set forth a the introduction in the 1340/985 overhauls manual, - it reads:
"Time Between Overhauls" Where engines are consistently cruised at relatively low power, such as in operations involving long range aircraft, and where conservative cruising outputs can be consistently maintained, periods of 1000 to 1600 hours (helicopter engines 500 to 800 house) (agricultural aircraft 600 to 1000 hours) between overhauls are feasible, but such extended periods between overhauls should be determined by experience with the engines of a particular model in the given type of operation, and their condition at overhauls. It is suggested that overhaul period increases be made in increments of 100 flight hours. Oil consumption is usually one of the best indications as to whether or not the engine requires overhaul, provided the engine is no indication of possible trouble or irregularities requiring more than normal line maintenance attention. A sudden increase of oil consumption, or a gradual increase of oil consumption to double that which has previously been average, is usually as for overhaul."
You can tell by this statement that Pratt & Whitney did not hard-line this subject. Maximum TBO for the 1340 is generally considered 1600 hours; however, there is a fairly dramatic rise in the number of major parts rejected in the agricultural service. It is interesting to note that Air Tractor, Inc. set the TSMOH limit for warranty on their new crankshaft at 1200 hours. A.D. note 68-09-01 requires the replacement of the flyweight liners and rework of flyweights in the crankshaft of the R-985 at or before 1200 hours TSMOH when using Hartzel Propellers Models HC-93Z30, HC-B3Z30. This also affects engines operated with a combination of these props and any other eligible propellers. Engines operated with other eligible propellers (not light Hartzel's), the flyweight liners must be changed prior to accumulation 1600 hours TSMOH. This rework requires a complete disassembly and removal of the crankshaft. This effectively limits a 985 to 1600 hours TBO as it would be foolish not to overhaul the engine at disassembly!
If you have a good relationship with a reputable overhaul shop, I would recommend that you strongly consider overhaul between 900 to 1200 hours. This will give you your best economy (regarding over-haul cost/parts replacement cost) in the long run. If you are doubtful about the integrity of the last overhauler, or don't know who last overhauled the engine, you are usually money ahead to determine time to overhaul solely on condition, performance, oil consumption, etc.
Have a safe and prosperous season!
