The bellows on the 86 Mercruisers are a huge pain to install. I used many very very bad curse words while doing that. The things mentioned about cleaning properly are generally true specially if using the glue. But the glue is tricky. It usually ended up all over my hands and some where it should go. I came up with a decent solution. It goes like this :
Get a ring or washer that is bigger than the housing that the bellows has to go around and make sure that it is narrow but big enough to stretch the bellow so it will go around the bellows housing without touching. Now throw the bellows in very hot water so it becomes a bit soft and stretchy. Now after you take the bellow out of the hot water ,dry it and put the glue on both surfaces and let it dry a good bit so that it feels dry but still ever so slightly goopy. with the help of the ring inside the bellows slide the bellows around the housing and carefully remove the ring. Now the bellows sits where it is supposed to be. Let it sit a bit so the glue can do its thing . Then finish what you would normally do. I find that doing the side away from the engine first and then the side closer to the engine works best for me. All of this for the bellows that are real tight and hard to install. I mean the type where you usually scratch your knuckles bloody. I believe that nowadays there are some more flexible ones, but there this system also helps
