Ageless Radial Engines
Field Service Note # 1
Piston Ring Break-in With Mineral Oil
The information in this service note is also in all the current engine plans but is included here for use by the model engine hobby.
I found that with multiple cylinders the piston rings did not seat well, on a microscopic level, to give good compression and oil control.
Over a long period of development, I found that running with mineral oil for the first few hours greatly improved the engine compression and oil control.
This was later confirmed by Lycoming in their recommendation SI-1014M for non-supercharged piston engines. The technique is to run the engine for 50 hours, or until oil consumption stabilizes, using Mil-L-6082 or SAE J-1966 ( Mineral Oil ).
This is available from local fixed base airport operators as Aero Shell 80, mineral oil for piston engine break-in. Pharmacy grade mineral oil should not be used.
Conventional motor oils have various anti-wear additives to protect the cam and gearing from metal to metal contact under high loads and temperatures.
Mineral oil is also a petroleum based oil but without the anti-wear additives.
Current production automotive engines and motor cycles are manufactured using the best equipment possible and therefore do not need the break-in running that was recommended during the 30's, 40's and 50's.
When the piston rings, made by the home machinist, are first installed they ride on a few high spots that are quickly worn down until the ring force per unit area has reached a minimum. The rings are then seated along their entire circumference.
It is important that the engine parts initially be assembled with mineral oil to keep the anti-wear additives from becoming involved during the break-in period.
During break-in running, the oil should be checked often and drained as it becomes cloudy. The engine should be allowed to stand and cool naturally. This process should continue with short running times and natural cooling until oil consumption stabilizes.
As always, be vigilant for oil accumulation in the lower cylinders, prior to starting. Drain the lower cylinders by removing the spark plug or hydraulic lock can cause engine damage. Never force an engine to turn.
After the rings have seated, the oil system should be drained and fresh synthetic oil be used. This is also available from Aero Shell as W15-50 ashless-dispersant, semi-synthetic.
Keep these recommendations in mind when planning to run your radial engine for the first time.