What engine? Do you mean the prop nut? For glow engines, the nut needs to be very tight, and for a four stroke both the nut and jam nut need to very tight. I use a 6" cressent wrench for my wrench, although a box end or an open end wrench of the proper size would be better. Snug down the nut, making sure the spinner screw holes are in alignment, then set the wrench so it is away from one blade, so if you squezed the two together, it will tighten. Watch the new plastic blades as they can be as sharp as a knife. By using one hand to squeze the blade and wrench to gether, you can get a very tight nut, but not so tight to snap off the crank nose. Just keep fliping the wrench over when the wrench and prop blade are nearly together, this gives you a fresh amount of movement. Usually just moving to the next flats is to much of a change for you hand to handle. 1/2 flats, or flipping the wrench works. If you are flying a four stroke with a jam nut, tighten it in the same mannor.

Always watch that your prop is thick enough that the nut and prop washer can fully tighten it before running out of threads on the crank nose. I have found that most of the 4" pitch props have to have a spinner back plate in order to fully tighten them.

Don