How to breakin a new engine is an extremely contentious topic. In this video from the MC Garage we bust the engine breakin myth by building up two engines, breaking them in using drastically different methods, and then tearing them down and comparing the components at the heart of the discussion.



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 breakin? Fresh fromthefactory 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 breakin is the seal between the piston rings and cylinder walls. And rightfully so. Ring seal is the key condition that’s going to affect performance and longevity, so it’s worth thinking about.

How Should You Break In A New Motorcycle Engine?
What’s the best way to get a good seal on those piston rings? If you follow the breakin procedure outlined in your owner’s manual, it’ll recommend a 600, 1000, or even a 1500mile 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 breakin is a waste of time and not an effective way seal the rings, and that a more condensed and aggressive breakin—some would say brutal—is the way to go.

How We Conducted Our Engine BreakIn Test
To answer the question once and for all, in this MC Garage we assembled two identical Honda CB300F motors with fresh topend 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 BelRay semisynthetic 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 leakdown tests—which is a standard way of checking topend health—then disassembled, measured, and inspected the internal parts. And the results, well, they might surprise you.

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Engine BreakIn Specs

Engine 1, “By The Book BreakIn”
Installed at 1,828 miles, removed at 2,861 miles
BreakIn 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 BreakIn”
Installed at 2,861 miles, removed at 3,890 miles
BreakIn 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 highspeed 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

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