New Engine Breakin

I have a 2002 YZ 426. I just did a full engine rebuild. New top and bottom end, new crank, new water pump, new clutch. What would be the best way to break it in. I have heard you can break it in hard or soft. What would the details look like for the break in?

Warm it up well. Shut it down, check the oil level, and look for any problems. Ride it no harder than 50-60% for the first 10 minutes or so, but don't baby it. Run it at up to 75-80% for the next hour, and then almost as you normally would after that, avoiding only lugging it or over revving. Also, acceleration/deceleration is better for it than extended periods at speed. After 3 hours change the oil, clean or replace the filter, and you're done.

The key is to avoid being too easy on it without thrashing it either.

Maybe this is what Grayracer meant, but you should let the bike cool completely between runs (at least 15 minutes). i.e. warm it up, let it cool completely. Run it for 10 minutes, let it cool completely, etc. Definitely avoid lugging, over revving and steady rpm. When you're accelerating, it pushes oil away from the rings, and when you're decelerating, it pulls the oil back up to the rings.

Read the section in the manual about break-in.

I like Grayracers advice and thats what I do for break in also. I didnt break it in easy, but I didnt wring the snot out of it either.

My motor took about 5-6 hrs before the motor starts 'loosening' up and making decent power. You'll probably have to adjust your idle high at first and lower it as the motor loosens up.

Have fun!

Maybe this is what Grayracer meant, ... Run it for 10 minutes, let it cool completely, etc.
No, it's not. Running the engine for ten minutes will not allow it to completely reach full, stable, normal operating temperature throughout the assembly, which it needs to do to break in properly. The first actual ride should be at least 20-30 minutes long. Certainly, if there is a problem, stop to correct it, but otherwise, let it run.

I figured that the fist actual ride should be longer than 10 min. I have broken a bike in before but it was a two stroke. Is there any difference?

Not really. Breaking in a engine is an odd situation. The rotating assemblies would as soon be spun for 10 hours with no load or thrust applied, but the rings won't seat right if you don't lean on it a little, so you have to compromise a little. Just avoid the extremes and you'll be fine. Remember that it's a high performance engine designed to be run hard. As long as you don't brutalize it during break in, it's all good.

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