Chris and Gnu, thanks for your thoughts on the subject, all are very valuable points of interest. I will probably build a new fuselage and mount the ST Series X 25 on it and see how things go. Chris, as you mentioned earlier the use of ST engines, I have a no. of A/B classic designs with ST 23's on them and they do quite well. My only concern is that generate quite a bit of vibration!!! I fly them with an 8x4 APC prop. I've been told that allot of the older loop scavenged engines weren't design to run at the rpm's that we run them at in FF. I was thinking of trying a 9x3 APC to see if I can slow down the engine a little bit and reduce the vibration without diminishing the pwr. output. I have also discussed balancing the ST 23's with Bob Mattes and I will be sending him an engine to see what he can do!. The ST series X engines are great engines and they are at a par with the K&B's, i.e. 3.5, 4,9,6.5, etc.