RE: 3d for .91 4 stroker??
What 91FS?
The Saito 91 is identical weight to the OS 70FS and makes a fantastic engine for the U Can do 3D 36, or the Funtana 40 or the other 40 size 3D birds mentioned here.
The OS91 is 3 or 4 oz heavier, so you need to watch weight