How do I balance a crank shaft for my yd100

When you do it you should put a piece of metal the thickness of the space between the 2 lobes.
This will help some with causing more trueness issues.
Even with a plate between the crank lobes, it should be trued after drilling it.
In my experience with this it’s amazing how little pressure is necessary to open or close the lobes so they spin parallel.
The reason I said a hand drill isn’t up for the task.. even though the Chinese metal is quite soft, it takes a fair amount of pressure on the bit to cut the hole. Even a cheap Harbor Freight press is way better than trying with a hand drill.

Is possible to do it with a hand drill?

Yes… but even with new bits it’ll be a pita. I doubt a hand drill will cut squared up holes with no wallowing.
YMMV
 
Wait a minute, if it’s so easy for cranks to become out of true, why don’t they loose trueness under high rpm? Especially on modified motors the cranks would just fall apart
That's a good question and a good reason to have a one piece crank. Two stroke engines are something else in so many ways. When removing metal from the crank you can make it look pretty or pretty ugly. Either way the result is the same.
 
Wait a minute, if it’s so easy for cranks to become out of true, why don’t they loose trueness under high rpm? Especially on modified motors the cranks would just fall apart
that is a good question and the answer is... yes... they do sling themselves out of true if you spin em at high rpms a lot.
They don't fall apart but the pin does get fretted...a trench worn on the power stroke side...when that happens the rod bearing cage rubs the pin and eventually wears through the cage and pieces go thru the engine.
Modified engines really need a stronger crank. The kits recommend stronger cranks and 12mm wrist pins.
 

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This is the general idea of how it’s done.

A drill isn’t gonna do it. There’s a good chance the crank will get screwed up using a hand drill.

Boy, I didn't realize how many balancing videos are on U-Tube. This one is basically the same as above except the guy does a much better job of explaining the proceedure.

Having worked with the machinist who ballanced all of our shops race car engines I know there is a lot more to determining these factors than what you will learn in these simple videos. I was surprised to see this fellow actually does it correctly. The time spent to create his calculator must have been near eternal. Even if you don't understand it you will now have an apreciation for what is actually involved in a true dynamic balance job.

Is this too good to be true? This machine shop has done thousands of balance jobs. Is it this simple?
 
I’ve found drilling and balancing is part of the vibe thing. Getting the crank to spin true and a good tune helps a lot with vibes too.

2 stroke stuffing has a video showing how to true a crank.
 
If any of you old farts like me were into racing back in the 60's you will really enjoy the story of this crankshaft balancer. Do any of you remember the 180 degree crank for small block Chevys? I do.
 
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