Two Different Heads

The morning I was to leave for my last camping trip, a discovery was made as he spark plugs were being changed. The spark plugs on the right side of the 440 engine are different than the left. WHAT? Well, so I must have a least one head that did not come with the motor. The right side has plugs with a compression washer, the left has a beveled edge, much like a lug nut. So, the left side did not receive new plugs. This is not horrible news, just an inconvenience. I wonder what year they changed from one to the other?