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Old Nov 18, 2012, 06:00 AM
OkiThumper is offline
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Vince, I've come to the following conclusion based on all inputs so far including those most intimate with the fuels, the manufacturers, model aircraft magazine writers, and long term serious engine enthusiasts.

The metallurgy of the legacy engines a generation ago utilize steel pistons or steel piston rings running in lapped steel liners. Cox engines utilize the same steel on steel and have steel connecting rods running in the steel ball and socket joints. For these, not using Castor oil in fuels could provide a shortened service life.

The following is stated in the manual for the OS 10/15/25 LA plain bearing Aluminum piston running in a Brass liner that is Nickel plated (ABN) engines:

ADVICE ON SELECTION OF FUEL, GLOWPLUG & PROPELLER Fuel [....] Synthetic oils are permissible but are less tolerant of a "lean run" than castor-oil. If, therefore, a synthetic lubricant is used in the fuel, readjust the needle-valve to a slightly richer setting, as a safety measure, in case the fuel/air mixture becomes too lean through maneuvers in flight. If a higher nitro fuel is used, the engine should be checked out to make sure that it is sufficiently run-in to operate on that particular fuel without overheating. Do not use fuels containing less than 18% lubricant.

Basically, one can get away with using fuels with sufficient synthetic lubricant in modern ABN or Aluminum piston running in a Brass sleeve that is Chrome plated (ABC) engine, including those with a bronze bushing supporting the crankshaft if prevented from overheating. However, use of Castor oil will protect a hot running ABN/ABC engine (overly lean air/fuel mixture, inadequate cooling - cowled engine, and/or using an oversized propeller).
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Last edited by OkiThumper; Nov 18, 2012 at 06:09 AM. Reason: Cleaned up logic in last paragraph.
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