2009 yz250 break in.

[TotalNZ]

It will run ok exactly as you say totalnz but the primary reason for heat cycling is to allow the piston shape to more exactly and gradually conform to the cylinder shape.The differences and gains are small but they exist.This generally gives a longer lasting and more powerful engine as compared to just starting it and riding straight away.Pistons of course,aren't exactly round so the better fit you have,the better off you are.This also helps with ring sealing.The truely anal set the engine at bdc between cycles but I don't bother.

I'm no metallurgist but i don't buy that at all. There's no way i believe that the operating temps of your motor are enough to permanently change the shape of your piston, anything like that has already happened in the manufacturing process. Maybe back in the day when manufacturing and machining process's weren't as good.
Also it's not the pistons job to be a tight fit and conform exactly to the barrel, that's the job of the rings.

[graham472]

Putt around for a while, then sting it!!
Feel that 2 stroke rush

[ford832]

I'm no metallurgist but i don't buy that at all. There's no way i believe that the operating temps of your motor are enough to permanently change the shape of your piston, anything like that has already happened in the manufacturing process. Maybe back in the day when manufacturing and machining process's weren't as good.
Also it's not the pistons job to be a tight fit and conform exactly to the barrel, that's the job of the rings.
[/quote]

Strange but true.Metal is in a constant state of change in one form or another though the biggie is at break in.As for your engine temps being enough to change the shape of your piston,have you ever seen pistons go egg shaped?I have.How about a warped head-or any other engine part for that matter.Engine temps are more than enough.Have you ever put a case in the freezer then literally dropped a room temperature bearing in it?That's a differential of what, 50' or so?I know thats not a permanent change but it shows how little it takes.How about a cold seizure?Heat changes the structure of aluminum.If you bend a lever and try to bend it back you'll break it but if you heat it and then let it cool completely you can bend it back in to shape.If you then try to bend it more you'll break it-unless you heat it and cool it again.I'm no metallurgist either but I'm told it's the effect hot/cold has on the molecular structure.
As for the seal being the job of the rings,you're right but the better the piston/cyl fit the better seal the rings will have.

[mxaniac]

The shape of a piston at room temp will not be the shape of it when hot, that is a given.  Manufacturers compensate some with the shape but can't completely.

Nonetheless power comes from compression.  Getting the best ring seal is the primary goal of a break in.  Still I must confess, I've never been able feel any seat of the pants distinction from a slow easy break in or a fire it up and drop the gate approach.  Same with longevity, without keeping a detailed log I don't know that you could determine any particular approach that makes an engine last longer.  I'm basing this all on experience with 2-stroke single cylinder bikes.  Planes, trains, and automobiles may vary.

[ford832]

OK guys,I surrender.I'll continue to do it this way as that's what I've found in my 25 years as a mechanic works the best.To each his own however,so whatever works for you,go for it.

[Chokey]

Interesting, no ones keen to explain the need for heat cycling. or what it's supposed to even do.

Heat-cycling allows the trapped internal stress in manufactured parts to reach equilibrium. It's only really an issue if you are running a forged piston. When breaking in a new two-stroke with the OEM cast piston, the primary concerns are:

1.Allowing any high spots to wear in without putting the parts under high load, which will cause micro-welding, pitting, and galling.

2.Ring seating. It only takes a minute or two for the rings to seat fully, but it requires fairly high cylinder pressures to do so. It's best on a new bike to give it one heat cycle, and after it cools completely, ride it, using fairly large throttle openings, but avoiding high rpms.

Now if you're rebuilding and using a forged piston, this is the ideal method.

From Moto814

1) Assemble the engine properly and torque all fasteners to specs.

2) Start the engine with the bike on a stand and allow the engine to come up to operating temperature (top of the raidator hot to the touch). Do not allow the engine to run at one RPM at all. Constantly vary the RPM and do not allow the engine to idle. When then engine reaches operating temperature (about 3 to 5 minutes of running time), shut it off.

3) Let the engine cool completely (at LEAST one hour). You want the engine to be dead-stone cold. Longer is better.

4) Start the engine with the bike on a stand and allow the engine to come up to operating temperature (top of the raidator hot to the touch). Do not allow the engine to run at one RPM at all. Constantly vary the RPM and do not allow the engine to idle. When then engine reaches operating temperature (about 3 to 5 minutes of running time), shut it off.

5) Let the engine cool completely (at LEAST one hour). You want the engine to be dead-stone cold. Longer is better.

6) Start the engine with the bike on a stand and allow the engine to come up to operating temperature (top of the raidator hot to the touch). Do not allow the engine to run at one RPM at all. Constantly vary the RPM and do not allow the engine to idle. When then engine reaches operating temperature (about 3 to 5 minutes of running time), take the bike off the stand and put it in gear. Take it for a ride. During this ride you want to keep the engine under a load at all times. Do not coast. Do not let the bike idle. Do not allow the engine to stay at one RPM. Riding on a mild slope is fine for this, as is slightly dragging the rear brake the entire time. Do this for about 15-20 minutes. Then shut the bike off.

7) Let the engine cool completely (at LEAST one hour). You want the engine to be dead-stone cold. Longer is better.

8 ) Re-torque the head and base nuts.

9) Go ride.

The cool-down steps are crucial to this operation. You must let the engine cool completely for the break in process to work properly.

Also, do the warm up procedure I outline here before EVERY ride. Your top ends will last much longer if you do.

-Steve