Break In Why break in? WHY?

[G-Superior]

So after lots and lots of reading i dicided to start this thread.
MOST people out there(not in MBc) say that there is no point in doing break in and never have problem and many people that say that they doing or did break in did have problems
my local machinery shops says it is not needed and i worked there for a long time and he has 100% feedback and never blow or seized an engine, every new engine that he gets out off the box he just puts fuel in then start and rev the **** out of it(Im always scared when he does that to a 800$$$ machine)
My local scooter shop dosent know what break in is and never did to any bike and nothing never happens to him
BUT here in MBc every talks about it as if it was the most important part of an engines life and i have seen lots of then seized
Im not saying that we doing anything wrong but is it just the quality of engines we use or what?
and when i talk to my dad about 500km break in periods he goes crazy!
He raced 2 stroke go karts(not toys :helmet and racing modified 4 stroke hondas back in the 70s 80s and 90 and the break in peorids used to last 15min with the revs up and down around the track and then after that just rev it full and he never seized one up(he did that to tonnes of then with sucess)
So what is the real break in time suposed to be?
and does an engine really improve after break in?
I dont mean to sound stupid in this thread and i believe break in are essencial but it just dosent make sense how other peoples engine dont get damaged including those kids on youtube that get their minimotos and HTs out of the box and rev then to red line all they long with no problems at all
and what about extra oil in the mix for 2 strokes or only mineral oil or fully syntetic for four stroke?
IS IT REALLY NEED?(i know it is but that will probably help someone in the future when they read this thread)
Has anyboby ever had sucess running an engine with out doing break in?
I can really awser most questions that im asking because all the engines that i get are old and worn out but i read alot of stuff saing yes its vital for the engine BUT also read alot of people saing WHAT IS THAT ALL ABOUT?
(Hope you didnt get bored after reading all that)

G-Superior

 

[kerf]

From my understanding, what the break-in does is allow the ring(s) to seat to the cylinder walls. The cylinder wall is honed in a cross hatch pattern to allow the metal to wear together easier. Staying off WOT and varying speed, helps keep the rings wet with oil during this process. My particular engine is the Chung Yang R460 and the supplier recommends using mineral oil two stroke lube rated for air cooled engines. Running the engine about ten minutes and allow it to cool completely, then go through 4 of these heat cycles and it's done. They then recommend synthetic two stroke oil.

 

[GearNut]

kerf pretty much hit the nail on the head for the Chinese engines, and any other engine that uses ball or roller bearings instead of bushings or plain bearings.
Plain bearings are also called Babbitt bearings or shell bearings. They are just metal formed into the appropriate circular shape. When new, the very close tolerances that they are fitted to during manufacturing can cause a heck of alot of friction if subjected to strong loads. If you break in an engine easy, you are not straining it and therefore not placing strong loads on the bearings or bushings. They really need to break in slowly.
The act of breaking in an engine is actually allowing the parts to work out any anomalies created during manufacturing.
In simpler terms, fine polishing of any high spots and creating a perfect wear surface.
The slower this is done, the finer the polish.
In some circles this is called burnishing, although burnishing is usually done dry, outside the assembly. Just imagine what would happen if you burnished the bearings in your engine by running it dry! It would overheat from friction and seize in a matter of minutes.

Race engines are typically assembled to looser tolerances due to the fact that they are built then raced nearly immediately. They do not have the time for a long break in period.
Also consider that a race engine is only ran for a few hours then rebuilt again.
They are not built for years of reliability. They wear out faster because they were not assembled with tight tolerances in the first place, and ran very hard.

Petroleum oil when broken down to the molecular level is made up of larger particles that synthetic oil. It will not fit into the teeny tiny places on the surface of a bearing as easily as a synthetic oil. This allows the parts to come into physical, frictional contact with each other and break in faster than using synthetic oil.
You can break in an engine using synthetic oil, it will just take longer to get the job done, and some components, like the cylinder finish, will not stand up to that length of time. The mating component will not polish out properly.

That is a condensed version.

 

[AussieSteve]

Great info, kerf and GearNut.
I took it pretty easy during run-in with only occasional WOT bursts.
I pulled the top-end down after a couple of hundred kilometres and the top ring was only just starting to bed in properly.

When the rings do start to seat well, compression and therefore performance will increase noticeably.

I think that some of these engines still use a bush for the top-end, too, instead of needle rollers.

 

[machiasmort]

Agree with everything here, and well written. The only thing I'd like to add is the best way to warm a motor up, is under very gentle load. You can do Kerfs "heat cycle" and put the bike in gear w/ rear wheel suspended above the ground.

The slight load takes the slop out of everything and makes a huge difference. I let my car idle down the street all of the time and people look at me funny because I'm going slow... All I'm doing is taking the slop out of everything and not letting the parts "free float" which could be a lot more dangerous to the motor. This is why "free tacking" or reving your motor is not good for it!