Busting The Engine Break-In Myth

Getting a new motorcycle is an exciting prospect, but once you fire it up for the first time you’re left with what many deem a great responsibility: breaking in that new engine.

What is engine break in? Fresh from-the-factory parts appear smooth, but they actually have microscopically rough surfaces that need to rub against their counterparts and bed in, and that happens during those first miles of use. Once the components are polished smooth and broken in, friction is reduced, sealing is improved, and you’re ensured good power, fuel economy, and reliability.

There are a lot of sliding and rotating parts within an engine, but what everyone gets riled up about when discussing engine break-in is the seal between the piston rings and cylinder walls. And rightfully so. Ring seal is the a key condition that’s going to affect performance and longevity, so it’s worth thinking about.

Related: Mineral Oil vs. Synthetic Oil. What’s The Difference?

What’s the best way to get a good seal on those piston rings? If you follow the break-in procedure outlined in your owner’s manual, it’ll recommend a 600, 1000, or even a 1500-mile process wherein you limit throttle and revs and constantly vary the engine speed. At the other end of the spectrum, there are people that say a gentle break-in is a waste of time and not an effective way seal the rings, and that a more condensed and aggressive break-in—some would say brutal—is the way to go.

To answer the question once and for all, we assembled two identical Honda CB300F motors with fresh top-end parts, broke them in differently, and then compared the results.

The first engine was installed and run in gently as per the manual, which meant painstakingly limiting and varying the throttle and slowly ratcheting up the revs over the course of 1,000 miles. Then we swapped out the babied motor for engine number two. While engine one didn’t kiss redline until that final 1,000th mile, this second engine was given a minute to warm up and then taken right to the limiter and ridden at or near WOT on the highways of SoCal for the entire 1,000 miles. Both engines were initially filled with Bel-Ray semi-synthetic oil, and we changed the fluid and filter at 600 miles as recommended.

After breaking the engines in using drastically different methods, we performed compression and leak-down tests—which is a standard way of checking top-end health—then disassembled, measured, and inspected the internal parts. And the results, well, they might surprise you.

Drumroll, anyone?

The truth is, there was no significant difference between the two engines. There was hardly even a discernible difference. The compression and leakdown numbers were stellar and identical on both motors, and all the measurements of the internal parts, including the piston diameter, cylinder diameter, piston-ring end gap, and valve clearances, were all within spec and inline with each other. Check them out for yourself below. The ring end gap was slightly wider on the engine that was broken in brutally, but that’s it. Similarly, there was no obvious difference in the color or debris content of the oil at that first 600-mile change.

Related: How Much Debris Should You See In Your Oil After Break-In?

So we’ve more of less shown that it doesn’t matter how you break in an engine, the results will be the same, right? Well, not exactly. The CB300F, even when operating at WOT, just isn’t working that hard because it’s in a mild state of tune. We certainly wouldn’t recommend going to WOT with a new CBR600RR that revs to 15,000 rpm. Each motor is different, with different cylinder materials and compression ratios and redlines, but the lesson here is that there doesn’t appear to be a night-and-day distinction between break-in methods, so don’t sweat it. Motorcycles are meant to be enjoyed, so just go ride it and enjoy it.

That being said, the long and drawn-out recommendations found in all owner’s manuals persist for two reasons. First, the methodology is a holdover from days of yore, when metallurgy, machining technology, and lubricants weren’t nearly as good as they are today.

Second, taking it easy with a new bike is just a good idea. After all, it’s not just your engine that needs to break in. You need to scrub in those new tires, bed in those new brakes, and overall get familiar with how your new bike is balanced, how it turns and handles, shifts and stops. Modern motorcycles are amazingly reliable, but failures still happen, and most major issues are likely to happen within the first few hundred miles. Wouldn’t it be better to have something break or come loose while trundling along at 25 mph instead of 75? There are lots of good reasons to give yourself and your bike a day or two of gentle riding to shake things out.

Engine Break-In Specs

Engine 1, “By The Book Break-In”

Installed at 1,828 miles, removed at 2,861 miles
Break-In Notes: Ridden gently. Didn’t exceed 50% throttle or 5,000 rpm for the first 600 miles, and didn’t exceed 75% throttle or 7,000 rpm until 1,000 miles. Didn’t experience WOT until 1,000 miles was accumulated. Ridden with constantly varying engine speed and load, no easy task on a slow bike in a busy, urban environment! Oil and filter changed at 600 miles.

Piston OD 2.9910” at install, 2.9910” after 1,000 miles
Cylinder ID: 2.9925” at install, 2.9930” after 1,000 miles
Top Ring End Gap: 0.0130” at install, 0.0145” after 1,000 miles
Compression: 235 psi hot after 1,000 miles
Leak Down: 4% hot after 1,000 miles

Engine 2, “Brutal Break-In”

Installed at 2,861 miles, removed at 3,890 miles Break-In Notes: Went to WOT within moments of first starting up, and was ridden hard for 1,000 miles. Did everything we’re told not to—lots of throttle and high-speed droning at steady throttle. Oil and filter changed at 600 miles.

Piston OD: 2.9910” at install, 2.9910” after 1,000 miles
Cylinder ID: 2.9925” at install, 2.9930” after 1,000 miles
Top Ring End Gap: 0.0130” at install, 0.0150” after 1,000 miles
Compression: 235 psi hot after 1,000 miles
Leak Down: 4% hot after 1,000 miles

Source: MotorCyclistOnline.com

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