By Steve Scott
Great job, you have tightened the last bolts and fittings on your freshly rebuilt engine. It is filled with coolant and oil and ready to run, but if you miss the next step, you may be very disappointed.
Breaking-in the engine.
Breaking-in an engine without a dyno or load bank is an essential process often overlooked in the Original Engine Manufacturer’s service publications. Although using a dynamometer (dyno) and load bank are the preferred methods for this process, they are not always available. Nonetheless, breaking in the engine is crucial. The term “break-in” refers to the process of applying sufficient load to the engine to generate the necessary combustion temperatures and operating conditions to ensure proper seating of the piston rings and cylinder bores.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2025EPQ1/#p=88
According to Cummins® Bulletin #014-004 for On-Highway Engines: “Operate the vehicle pulling the heaviest available trailer allowed for the first 80 to 160 Km [50 to 100mi] after rebuilding. Operate the vehicle in the highest gear possible within the normal operating rpm range of the engine. It will be necessary to operate the engine at or near full throttle at 75 to 85 percent of maximum horsepower rpm indicated on the data plate. Do not idle the engine for more than five minutes at any one time during the first 160 Km [100 mi] of operation.”
For Off-Highway Engines, this bulletin advises: “Operate the engine under the highest load possible at full throttle within the normal operating rpm range of the engine for the first three hours of operation after rebuilding. Do not idle the engine for more than five minutes at any one time during the first three hours of operation after a rebuild.”
Consequences of an Incomplete Break-In
Failing to properly break-in your engine will lead to increased oil consumption and blow-by that are often irreversible. Idling the engine will not generate the temperatures and forces needed to properly seat the rings. Diesel engines are designed to work hard, and if a freshly rebuilt engine is “babied,” it is likely to never be weaned from using oil.
Simple formula:
Idling at break-in =
• Blow-by
• Oil consumption
• Unhappy customer
By design, piston rings apply a certain amount of tangential force outward by themselves. However, compression rings rely on greater combustion pressures to force them down against the bottom of the pistons ring lands and outward to the cylinder wall. Without this combustion force, these rings will not seat or seal properly.
Oil control rings regulate the amount of oil film left on the cylinder wall. Compression rings (top and intermediate) remove some amount of this oil film resulting in proper oil control, but their primary function is to seal compression.
To seal the compression, an adequate load must create enough combustion pressure and temperatures to seat the compression 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 are not enough combustion pressure or heat to seat new rings.
Break-In Strategies
Break-in can be achieved without a dyno or load bank. Once an engine is up to temperature and there are no leaks, rebuilders tend to have their own methods for break-in.
For example, these methods can range from:
- Drive it like you stole it.
- Run it up to the speed limit as fast as possible, and then while still in gear, let off the throttle and let it coast to a near stop—repeat as often as needed.
- Use the heaviest trailer and steepest hill you can find.
- Wet stacking – a process of restricting the exhaust to cause back pressure.
Rebuilders can be secretive about the processes they have worked out over their years of experience, but what they have in common is they focus on getting the rings to seat by using approximately 75% of full load on the engine for three to four hours and keeping idle time to a minimum.
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 buildup on the cylinder walls and prevent the rings from sealing. Once glazing forms, it can be difficult, if not impossible, to remove without disassembling the engine.
Break-In Oils and Additives
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 amount of hours/distance. Their effectiveness is not always supported by OEMs. However, much like the individual break-in processes and some rebuilders use them faithfully and others don’t like them at all.
Historical Practices
The “old-timers” reading this article are thinking: “Bon Ami it.” Bon Ami® household cleaner was their substitute for the old Caterpillar 7F5225 “Break-in Powder” (other manufacturers offered something similar). Back in the 1970s, Caterpillar serviceman’s reference guides gave the instructions for using 1-1/4 teaspoons of their 7F5225 powder per cylinder to get new rings and liners to seat. That’s not to say that these miracle powders didn’t have some side effects.
Intentionally introducing abrasives is not only outdated, but also like nails on a chalkboard. With the tighter tolerances, precise surface finishes, and specialty coatings, you can only imagine the amount of damage this can cause in today’s engines.
This article emphasizes that a proper break-in procedure is critical to the success of a rebuild. Consult your rebuilder or review OEM service publications for recommended break-in procedures. How the engine is treated immediately after leaving the shop can significantly impact the effectiveness of the rebuild.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2025EPQ1/#p=88