Originally Posted by DS Motorsport

Well basically I follow the same procedure with every engine and never had problems what so ever.
Break in, and the first race with the conrod the engine came with.
Mostly the engine is fully broken in after it's first race, at least in my case since I don't practice a lot.
After the first race I replace the conrod, and do a really good check up, clean up of the engine. With pretty much every engine I owned it wasn't needed to change the conrod a second time, because they were worn out before that point. My engines last around 10 liters after break in.

The bearings are really unpredictable, sometimes they become bad during break in already and sometimes they last 2 races.
When they start to feel different as when they were new, change them.
Don't wait until you see the (steel) balls become dull, then you have waited to long, and mostly you could see damage to the outside of your crank, it becomes hard to take it out.

When you change the bearings, put the crankcase in an oven, most of the time 45 min on 140C, then put the new ones in by using your crank to guide them in, and align them. I always immediately screw the flywheel on and rotate it. Since I have been doing it like this I feel the bearings last a little longer and the crank can spin more easily.
I wouldn't suggest using the cold-puller method, as this puts a lot of force on the components, and it's harder to properly align your bearings this way.