Spark plug "caddy" layouts for 4 cylinder and 6 cylinder opposed engines are shown below. Swap the long lead spark plugs with the short lead plugs as shown at each overhaul to equalize wear caused by constant polarity and high capacitance.
Note: 4 cylinder engines equipped with single drive dual magneto fire with constant polarity, therefore, it is not necessary to rotate plugs to maintain even electrode wear. However, you may wish to rotate top to bottom to counteract deposit buildup.
For users of REM37BY sparkplugs. While providing excellent protection against lead-fouling, the projected nose core design on the REM37BY lends itself to an unusual wear pattern. This wear condition can result in centre-electrode "bottlenecking" (erosion adjacent to the ground electrode as a result of electrical erosion and chemical corrosion) and ground-electrode erosion (ground electrodes eroded to knife-edge condition). In either case plug replacement will be necessary.
The foregoing information provided courtesy of Kori Ibey of Maximum North Aviation.