Finally got around to rebuilding this beast. I have a couple questions tho. My current piston is marked II-B, and my new one is marked I-B. What are the differences and do I need to return these for the other type?

Also, the manual states the arrow should face the rear of the engine. This one was facing toward the front. Could that have caused the wear on the middle ring? It is pretty considerable.

[2015 Z Force 800 53/Trail EPS]

I can't direct you on the IB and IIB situation but it is critical that the piston goes in the proper direction as the wrist pins are not in the very center, they are off set ever-so-slightly. I do know that on another forum a member did install one backwards in a Kawasaki V-twin and it did run fine (he thought)..for about 3000 miles, then the piston and cylinder was so scared they both had to be replaced.
On direction, this might help. The manual had a mistake:

That is the page I saw. Cylinder 1 is at the rear of the machine, correct? In that case the arrow points at the intake, as mine was. But it states front and rear would be reversed and my piston was in backwards. I have yet to take apart the front (#2) cylinder as I wanted to tackle only one at a time with zero chance of mixing up parts.

[2015 Z Force 800 53/Trail EPS]

That is the page I saw. Cylinder 1 is at the rear of the machine, correct? In that case the arrow points at the intake, as mine was. But it states front and rear would be reversed and my piston was in backwards. I have yet to take apart the front (#2) cylinder as I wanted to tackle only one at a time with zero chance of mixing up parts.

Yeah..#1 is at the rear. I always look for the the CVT air intake to tell me which way they are thinking. On the older CForce 800s it's in the other way.

I thought so, but good to have a second opinion. I'll put it back in with the mark facing the intake, as it was. I just wish I could find something regarding the IB and IIB markings. I haven't been able to find any explantions of differences. OEM parts diagrams show 2 options, but no reason for the options. I did order these from a dealer, so they had my machine info.

[2015 Z Force 800 53/Trail EPS]

I thought so, but good to have a second opinion. I'll put it back in with the mark facing the intake, as it was. I just wish I could find something regarding the IB and IIB markings. I haven't been able to find any explantions of differences. OEM parts diagrams show 2 options, but no reason for the options. I did order these from a dealer, so they had my machine info.

Might Mic the two and see if there is any size differences. If not maybe its just an updated version.

Looking at the cylinder diagram, there is also 2 options for the cylinder. I am going to guess they have to match. So looks like I'll be calling the dealer Tuesday to get the right pistons.