Engine break in !

[Vmax540]

Interesting reading !

http://www.mototuneusa.com/break_in_secrets.htm

[Bob Vehring]

I can tell you all that's true, we make our living building small race engines, and have done that for decades. The piston and cyl aren't round until you get them up to operating temp, you never get close to operating temp by letting an engine idle. It has to be run hard to get things round.
I know people building for a trail engine or dealing with a showroom new engine won't get carried away to the extent of race engines but I disagree about heat cycling. When a chunk of Alum is machined into a cyl, it is not done at 450 deg. It takes several cycles of heating and cooling for the new parts to take a set, the part will move around a little before it settles down. At that time its time to bore the cyl. Often if we have a cyl that has not been run before, we will heat cycle it in a oven several time up to 450 then let it cool down a few times. We also custom fit the rings to the piston.
Again, this is for race engines but if you want your engine to hold seal for as long as possible, all this makes a difference.