Well I finally got the 1972 johnson 20hp running good. I was taking it on a test drive and after about 3 minutes the lower unit made a sound like I had hit a small tree branch. I kept driving and it made the sound again, I kept driving and on the thrid time it made this noise the shear pin broke. In all actuallity I don't think I was hitting anything in the water, but it was making that same kind of noise. It would clunk, like maybe it was skipping a gear in the lower unit. When I first got this motor last fall, the guy told me that the lower unit was shot and I thought he was full of it. The shear pin was broke then, so I figures that he thought the lower unit was shot because he couldn't go anywhere. What does this problem sound like to all you pros. Could the lower unit gears be bad, or the clutch dog? Why would that break the shear pin. I have never actually been into the gears of a lower unit, but I am ready to learn.!! It cant be that hard can it? Does it take any special tools.