There is not much know about the differences between the Y-1 and Y-2 cylinders on a TZR125 4DL engine
But it looks like they have a 1,5HP differences between them.
What I can measure, it seems that the Y-1 has a slightly aggressive port timing
Its more in the 1,0mm's then in the 1,0mm's
Here you have the Y-1 and Y-2 cylinders
Here the Y-1 cylinder
You can see clearly the port's have been reworked
This is relative common by a fresh hone.
But this is done at the factory, as it is a stock piston
Here the Y-2 cylinder.
You can see clearly the port's have not been reworked
This one uses the first oversize piston.
So logical the oem made work is gone
To see if could find a differences between them, made a port map off the two cylinders.
There are many ways to do this, I use the piston to keep the paper on its place.
Make my finger a bid dirty and rub gently agents the paper until it leafs a mark.
After making it a bid dirty, it leafs a nice mark
This is what I found with the Y-1 cylinder
This is what I found with the Y-2 cylinder
Here the difference between a Y-1 and Y-2 cylinder.
It is difficult to say if these differences are good for a 1,5HP.
Measurement that precise are hard to do, but it looks like the Y-1 cylinder has the better papers.
Will see if I can find more info.