I have 2 ST90's, one Chinese and one Italian. In my opinion the one manufactured in China runs better, is easier to tune and is stronger than the one made in Italy. I also have had two OS91 FX's, still have one, and, again, in my opinion both ST90's are stronger than the OS91's. I will say that the ST90 is a little more difficult to tune and takes much longer to break in. All in all it is an excellent engine and you can't beat the price.

Regards,
doubledee