Hello, I just purchased my first four stroke (O.S. FS40S) and I noticed that there is a nipple for the crankcase breather on the back of the engine. What do I connect this to? I've searched through the manual several times and cannot find anything. Any help would be appreciated,thanks.

dont connect it to anything, the crankcase must be able to breathe, if not, early engine failure is sure to happen. I do have a short piece of fuel tubing on mine just to the outside of the cowl to vent gasses and oil away from the model and it will spew oil out, thats normal, its what you want.

As stated above, do not connect it to anything. However, there will be quite a bit of oil expelled from this orfice while the engine is running so, put on a short length of fuel line to vent it outside the engine area. Also, if you use after run oil (and I recommend it as you can not run a 4 stroke dry to clear the crankcase) you need to inject it here. To do that, you really need to use either a syringe or other device to inject the oil. Not just a drop or two but at least a teaspoon full. When inserting the after run oil, hold the engine so that the nipple is at the highest point and you may have to rotate the prop as you inject the oil to suck it into the crankcase. As the piston moves toward top dead center, you will suck in oil; BUT, as you continue turning the prop, you will blow it all back out again unless that nipple is at the high end relative to gravity. After getting a teaspoon full in, rotate the engine to distribute the oil over the two bearings inside the crankcase while also turning the prop over a few times.