1/8 or 3/32--gonna need to beef up the front after cutting so much out of the bottom to get the muffler to clear--it will fit now if I use 1/2 " standoffs with a little extra room to spare--will measure distance again as 1/2" standoffs might put engine out a little further than I want ( that's a lot of weight up there for this plane )-- I'm going to need some lite ply to make the battery tray with also--it looks pretty good so far--I need a Futaba pcm rx also