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How to break in a rebuilt engine

What process do you use, when there isn’t a process for breaking in a rebuilt engine?

One process that is often missing from Original Engine Manufacturer’s service publications is how to correctly break in a rebuilt engine, especially without the use of a dynamometer or load bank. Dynamometer or load bank testing are the preferred methods, but realistically it is not always an option. Regardless, breaking in the engine is critical. If this is not done correctly or completely, then the engine will most likely perform poorly, smoke, and consume oil. Often these symptoms are irreversible over time. The term, ‘breaking in’ an engine refers to the process where combustion temperatures and operating conditions force the rings and cylinder bores to conform to each other sealing the combustion gasses within the cylinders. Idling will not produce the temperatures and forces needed.
Piston rings are designed to apply a certain amount of tangential force outward by themselves, but compression rings rely on greater combustion pressures to force them down against the bottom of the piston ring lands and outward to the cylinder wall. Without this combustion force, these rings may not seat or seal properly.

Oil control rings regulate the amount of oil film left on the cylinder wall to lubricate the compression (top and intermediate) rings, and each compression ring removes some amount of this oil film resulting in proper oil control. It is important that an adequate load be put on the engine to create enough combustion pressure and temperatures to seat the rings. This is most critical within the first few hours of the engine’s new service life. Idling, increasing the RPM, and hauling light loads may not create enough combustion pressure or heat to seat new rings. Under load, you can obtain the pressure and temperatures needed.
Idle time and low load on a freshly rebuilt engine can result in “glazing” of the cylinder walls and prevent the rings from ever sealing correctly. “Glazing” is a condition where hard oil and fuel deposit build-up on the cylinder walls and prevent the rings from sealing properly. Once glazing forms, it can be difficult, if not impossible to remove without disassembling the engine. The ‘old-timers’ reading this are already thinking —“Bon Ami it.” Bon Ami household cleaner was available at their local grocery or hardware stores and was their substitute for Caterpillar’s 7F5225 ‘Break-in Powder’ (other manufacturers possibly offered something similar). Caterpillar’s serviceman’s reference guides gave the instructions for using their 7F5225 powder.

That’s not to say that these miracle powders didn’t have major side effects. Intentionally introducing abrasives into an engine sounds like fingernails on a chalkboard, as well as outdated. With the tighter tolerances, precise surface finishes, and coatings you can only imagine the amount of damage this can do to today’s engines. There is no piston ring manufacturer who would ever suggest using such products for any of the current modern-day engines!
Once an engine is up to temperature and there are no leaks, rebuilders tend to have their own methods for break-in.
These can range from:

  • “drive it like you stole it”
  • “run up to the speed limit as fast as possible, and then while still in gear, let off the throttle and let it coast to the stop—repeat as often as needed”
  • “use the heaviest trailer and steepest hill you can find and drive up it.”

These individuals can be secretive about the processes they’ve worked out from years of experience, but what they have in common is they focus on getting approximately 75% of full load on the engine for three to four hours and keeping idle time to a minimum produces the results they are looking for to seat the rings.
Outside of the operational processes, there are several break-in oils and additives on the market. These need to be considered with some degree of caution since many can only remain in the engine for a set number of hours, and most original engine manufacturers do not approve them. However, much like the individual break-in processes, some rebuilders use them faithfully, and others don’t like them at all.
The focus of this article is not to say every engine must be dyno tested, or that one process or fluid is better than the other. The purpose is to bring attention to a critical part of the entire rebuild process. Consult with your rebuilder, or original engine manufacturers publications (if available) about their recommended break-in procedures and service intervals. The first oil samples on freshly rebuilt engines tend to be high in metals, so they may recommend the oil be changed after a shorter number of hours or miles. What happens to a freshly rebuilt engine after it is picked up at the shop or driven off the lot can determine how satisfied you are with the service life of the engine.
Written by Steve Scott for Engine Professional.