Proper break-in is critical to a new or rebuilt engine’s performance and longevity.
Proper break-in is critical to a new or rebuilt engine’s performance and longevity. It allows the internal engine components to correctly seat, preventing premature component wear and failure while maximizing compression and controlling oil consumption. It also allows the engine to produce consistent horsepower.
Unfortunately, many boaters I speak with don’t realize the long-term importance of proper engine break-in. There is nothing mysterious about the process, and like most techniques there is probably more than one way to do it correctly. What I will do here is provide some technical information that should help you understand the details of how to properly break in an engine, as well as the importance and benefits. The trick is to avoid the mistakes that will reduce the performance and reliability of your power plant.
Although engine technology differs between diesel, gasoline, 2- and 4-strokes, the theory I am describing applies to internal combustion engines in general. Each engine manufacturer and engine configuration may have its own subtle differences, but the basis of proper break-in will be the same.
To better understand the break-in procedure, we need to accept that even with today’s technology, manufacturing tolerances can stack up to be either excessively tight or loose. Although there isn’t much that can be done by the consumer if tolerances are too large, the opposite can create genuine problems if the engine isn’t correctly broken in. Proper break-in gradually wears down the high spots that result from the machining process and manufacturing of such items as piston rings and skirts, cylinder walls and valve train components. This process allows tight spots to bed without overstressing.
Perhaps one of the most misunderstood aspects — and in my experience, the most critical — involves seating of the piston rings to the cylinder wall, which when correctly accomplished results in good compression, proper heat transfer and long engine life. A freshly honed cylinder bore appears smooth, but its surface actually has microscopic grooves, with sharp peaks and valleys. During break-in, the sharp peaks and high spots of the cylinder bore are gradually worn down, allowing the rings to mate to the bore.
When piston rings are considered properly seated, it means that the sharp edges of the cylinder bore peaks have been reduced, providing more surface area to support the rings while leaving the bottom of the groove intact to retain enough oil to keep the cylinder surface lubricated. Traveling up and down over this grooved surface, the piston rings — with either a tapered or barrel face — begin to wear evenly and eventually are in full line contact with the cylinder wall, allowing for proper compression sealing and oil control.
During normal engine operation, each microscopic groove acts as an oil reservoir, holding oil up to the top edge of the groove where it then spreads over the peak surfaces. During break-in there is only a minimal oil film on the high spots of the cylinder wall, allowing for the mating of the piston rings and cylinder bore surface.
Additionally, a calculated number referred to as BMEP, or “brake mean effective pressure,” plays an important role in the break-in procedure and seating the rings. Basically, BMEP is the force or pressure created within the combustion chamber. The BMEP within the cylinder makes its way between the rings and the piston, forcing the rings outward against the cylinder wall. The higher the cylinder pressure (created by high loads placed on the engine), the more the rings are pushed against the cylinder wall. This is why it’s important to carefully control the loading of the engine during break-in.
The friction associated with the wearing of metallic particles creates heat, which is a primary deterrent to break-in. Excessive heat causes the lubricating film to break down, glazing the cylinder wall surface, annealing the piston rings and preventing further seating.
If too little throttle is used during break-in, the BMEP won’t expand the piston rings enough, leaving a film of oil high up on the cylinder walls. The high temperature within the combustion chamber will oxidize this oil film, also causing glazing. (Check your engine manufacturer specifications for recommended rpm range.)
Now that you have a better understanding of what is occurring during engine break-in, the process should begin to take on more importance. My break-in recommendations should be used to supplement, not replace, those of your engine manufacturer. Various manufacturers may establish slightly different procedures to achieve the same result. Some use proprietary coatings and materials in component manufacturer and assembly, leading to slightly different procedures and time allocations for break-in. It is important to adhere to the specific recommendations for your particular engine.
With any engine, be certain to use fresh, well-
filtered fuel from a quality source. The better grades of fuel typically use higher-quality additive packages and burn more efficiently. Many2-stroke engines require that an additional volume of oil be mixed with the fuel during break-in, which will aid in cylinder wall lubrication.
Before getting under way, allow the engine to warm up thoroughly while in neutral, but keep in mind that during prolonged idling, when engine temperature remains below the normal operating range, the incomplete combustion of fuel will cause crankcase oil dilution and can lead to the formation of lacquer or gummy deposits on the valves, pistons and rings. To help reach the proper temperature quicker and to keep the cylinder BMEP high enough to slightly load the pistons, I run the idle rpm up to 20 or 30 percent higher than normal recommendations. With a gasoline engine, I slowly vary the idle speed by around 10 percent to allow flame front propagation, evenly heating the piston crown while reducing hot spots and more evenly distributing cylinder pressure within the combustion chamber.
As an example, let’s look at the break-in procedure Yamaha recommends for one of my 90-hp 2-strokes. Yamaha’s recommended idle speed for the outboard is 800 rpm. I would idle the engine around 1,000 rpm (25 percent over spec) and about once every minute vary that speed by 100 rpm both up and down. Once up to operating temperature, Yamaha recommends not exceeding half-throttle for the next 50 minutes. (For boats that plane easily, it recommends accelerating under full throttle to get on plane, then immediately reducing to half-throttle or less.) The time spent at partial load allows the components to seat without creating excessive heat. That heat would allow the internal components to expand, creating more friction and wearing more than the high spots on each part, resulting in premature component wear and increasing the tolerance beyond specification.
Still following Yamaha’s procedure, I spent the second break-in hour accelerating at full throttle onto plane, then reducing to three-quarter throttle and occasionally varying engine speed. Yamaha advises running the engine at full throttle for one minute, followed by 10 minutes at three-quarters or less. As previously mentioned, this varies the flame front position, allowing good cool-off time after the high BMEP created by getting onto plane and not allowing the engine to lug.
For the final third through 10th hours of break-in, Yamaha advises avoiding full throttle operation for more than five minutes at a time, and letting the engine cool between full-throttle runs. As I mentioned earlier — and this bears repeating — some engine manufacturers’ procedures could vary slightly, but the objective is the same.
I’m routinely asked about the use of synthetic engine oil during the break-in period. My experience with numerous new and rebuilt engines is that when using either full or blended synthetic oils, the time required to complete the break-in is substantially longer, due in part to the increased lubricity of synthetics. Representatives from Volvo Penta and Suzuki agree with my observation, though a Mercury Marine representative reminded me that its Verado engines use a synthetic blend at all times. Consult your engine manufacturer to be certain.
There are two more important oil-related issues to remember. First, oil consumption will be greater than normal until engine break-in is complete. Check the level prior to running the engine and again upon return. If breaking in an engine during an extended trip, I would check the level at least every hour initially. Lower oil level after operation is normal, but an increase in the level could indicate internal sealing problems. As the break-in process nears completion, there should be a noticeable increase in engine performance and a decrease in oil consumption.
The second issue is frequent oil and filter changes. During break-in, the metallic particles being worn away are carried by the oil, with the large particles hopefully trapped by the filter. And oil diluted by unburned fuel passing by the piston rings needs to be eliminated from the crankcase, as well. Some of the best insurance for your engine is to be obsessive with oil and filter changes. The manufacturers’ recommendations typically are the minimum. I would change the oil and filter at least two to three times while breaking in a new engine, and have the oil analyzed after the final change. After the first routine oil change following break-in, have the oil analyzed again. It should indicate fewer contaminants and a lower reading of combustion byproducts.