Fact or Fiction: Modern engines produced to precision tolerances don’t require break-in; just set the needle valve a little rich, and fly.
In today’s plug ‘n’ play society, you may be tempted to heed this advice. After all, you can buy an ARF and have it ready to fly the next day. Is it really necessary to spend time breaking in the engine when you could be out flying?
When all else fails, we can look at the engine manufacturer’s owner’s manual. There does not appear to be a consensus among engine makers regarding the importance of engine break-in. Some manufacturers downplay the importance, stating that no break-in is required, while others stress its importance and give very detailed break-in instructions.
It is true that many engines produced today are manufactured using high-tech, computer numerically controlled (CNC) equipment. CNC equipment can produce parts to very close tolerances, but there will still be some variation (albeit very small), and the only way to achieve the optimum clearances between the cylinder and the piston is through engine operation. Metals expand with increasing temperatures; therefore, the engine must be at operating temperature for the piston to seat properly. A proper break-in will ensure that these clearances are established.
There is another reason to carefully break in your engine. During its manufacture, stresses are built into the individual parts. This applies to all engines, from small 2-stroke glow units to full-scale piston and turbine engines. Turbine engines, which are probably manufactured to the highest standards of all, are given a thorough break-in. The break-in cycle is carefully designed to relieve stress on the parts. The cycle consists of incrementally increasing the temperature, with cooldown periods between each increase. If this process is not controlled, the internal stresses could distort the parts and cause engine damage.
If you are really pressed for time or you live in an area where the engine noise will disturb the neighbors, there are still some things you should consider doing. If possible, mount the engine on a test stand so that you can become more familiar with it. Running it on a test stand also makes the initial mixture adjustments easier. For most engines, when the top-end rpm hold steady for a reasonable period, the idle is reliable and the transition is fair, the engine should be ready to fly. Continue the break-in during your initial flights by making certain that the needle valve is set on the rich side. Also, vary the throttle to thermally cycle the engine. Your engine will appreciate it!