I have a 13.5 Wiseco Piston that I have been thinking of putting into my bike. My bike doesn't need a rebuild yet, but i got the piston direct from Wiseco on certificate that I won at a local race (50% off). I thought that it would be something to do while I am rehabing my knee. But, I am wondering if I should ever put it in. I know that some of you are thinking, "man, you can't use all of the power your bike has now". Well, that is probably true, but extra power off of the line is always better. The thing that makes me wonder the most whether it is a good move or not is that the '05 yamaha actualy lowered the compression on the new bikes. I guess this is an attempt to make them more subtle in their power delivery. I guess my concern is because I am happy with my bike now. More power would be great, but I don't want to turn my bike into something that I am unhappy with. I would like to hear from any of you that have changed to 13.5 pistons.