The procedure for measuring this clearance requires some patience and, depending on the installation, some contortions. First, the coupling bolts must be removed. These should be stowed in a safe location, preferably where they cant be knocked into the bilge. Inspect these bolts carefully for excessive rust or signs of stretching and fatigue. Now separate the coupling from the output flange. The shaft should spin freely, but with some effort. If the prop is installed, this adds quite a bit of mass to the shaft. If it is a folding model, its flopping blades can make spinning even more difficult. Loosening the packing nut on the stuffing box may also ease resistance. The surface of the flanges should be clean and free of rust. They should be sprayed with a silicone-type lubricant and buffed lightly with a scuff pad, then wiped dry and sprayed again. It must be possible to push the shaft coupling tightly against the output coupling to carry out the measurements. Once the two halves have been pushed together, attempt to fit the largest feeler gauge possible into the joint where they meet. Once a feeler gauge has been found that will slide into this space, again, be sure that it is the largest one that will fit with a slight amount of resistance. The gauge should penetrate only a portion of the joint. If it fits in through 360 degrees, the two halves were not pushed together tightly enough. If the largest feeler gauge that fits into any portion of the joint is three-thousandths of an inch, for example, and the coupling diameter is five inches, then no further alignment is necessary. However, if the clearance is nine-thousandths of an inch some engine movement will be required. If this clearance is at the 9 oclock position on the flange (viewed from aft looking forward) then the front of the engine would have to be moved to port. If the clearance is at the 4 oclock position, then the front of the engine would have to be moved to starboard and the back of the engine raised, or the front of the engine moved to starboard and lowered.