just want to add two bits of info, one is that the Jett 60 thru 90 size engines have the same mounting hole dimensions as rossi and OS, second you can find the jtec/tatone mounts on fleabay as well that are predrilled and tapped for the OS 61 engines. I am using one on a tiger 60 right now and I swap out the Rossi and Novarossi and Jett engines as needed to break them in. I dont think the mount will need ny clearance work for a rear exhaust engine.