Is the 912 zero balanced?

What do you mean by Zero Balanced? An overhaul shouldn't do anything, but restore the engine back to factory specs.

I use to own a gyrocopter with a rotax ,2 cycle engine, 503 which they don't make anymore but parts may still be available.

The crankshaft on that was dynamically balanced at the factory. This means rotated on special machine with I believe connecting rods on it and balanced. Now there was a very very skillfully person with a aluminum hammer that hit the crankshaft which is pressed parts with bearing to bring it into specs. He knew where to hit it. These days maybe there is a machine that does it all. On the 503 engine you bought a new crank with connecting rods attached. You requested it be delivered in a wood box. After receiving it you checked both ends with dial indicators on home made equipment to make sure it was not damaged during shipping. I did this once on my 503. 

I don't know if shops overhauling 912 have such expensive equipment. Usually a new factory one is installed but I may be wrong.

As to pistons spec is checked. Clearance within cylinder I believe is zero to ten thousands and weight may have limit spec to others. 

Rotax four cycle crank is totally different than say a lycoming and continental engines. Those cranks are one piece and can be filed to spec and oversize bearings put on after dynamically balanced without connecting rods. 

You can always call technical person at Lockwood aviation in Florida. They are in the internet for phone or perhaps by email. I can think of no other even though there may be other good companies regarding your question. 

Very good question and hope others will provide way better answer. 

I'm thinking by "zero balanced" he is referring to what in the auto world is called "blueprinting an engine". 

When factory auto engines are assembled, there is an "acceptable tolerance" of weight differences between pistons and connecting rods and other moving parts. Sort of like "good enough for highway use". If you want to race that same engine and get much higher RPM out of it, you "blueprint it" - meaning you bring the weight of the pistons and the connecting rods, etc. to a much finer consistency. By doing this, you have contributed to a more "balanced" engine that can rev much higher and produce more power. "Porting and polishing" is another thing they do to decrease resistence to the flow of intake and exhaust movement. They also seek to make the crankshaft more perfectly balanced and "true" along its length.

There are loads of other things they do but basically the idea of "blueprinting" is to bring the engine closer to its theoretical optimal design for maximum efficiency. They do the same kind of things to the engines used in the race planes at Reno and other venues.

I realize this does not answer the question. But, I would suspect Rotax engines are closer to "blueprint" than other engines due to their overall tighter tolerances compared to many other "factory tolerance" engines.

Brian, I think Rotax engine are not balanced at the factory which is one of the things that causes it to have higher internal vibration than it needs.

Blueprint includes clearance and stuff and Rotax does have very tight clearances and strict tolerances but it is not balanced as well as it could be.

Well Joe, if that is the case, it looks to me like you answered your own question. Right?