MS 361 VS MS390 info from Stihl Tech rep
This message was sent to answer my question to Stihl
the MS361 is considered a Professional class chainsaw and the MS390 is considered a Mid-Range class chainsaw. The MS361 has a magnesium crankcase. The main bearings are pressed into the crankcase halves and the halves are pressed around the crankshaft. Then, the piston and cylinder are added and the rest of the parts are installed. The MS390 has the main bearings pressed onto the crankshaft. The piston is installed, then this assembly is put into the cylinder. The cylinder bottom is added, and then this assembly is installed into a polymer motor housing. This makes the MS390 less expensive to build but more expensive to repair. The MS361 is more expensive to build but less expensive to repair. I hope this helps and if I can be of any further assistance, please let me know.
Sincerely,
STIHL Technical Advisor