I have found that the OMC Cobra bellows will last about 10 years before small cracks start forming in the folds, that's a good time to do it before you have a failure.
Suggestions:
Use an OEM bellow, I think they fit better and last longer.
While some people try to avoid removing the pivot housing by removing the exhaust bellows first, I don't think that's a good short cut, because you really want to make sure that the end of the bellows that fits on the gimble housing is on correctly.
so this means you have remove the pivot housing....there are 2 half in hex bolts (IIRC) that have to be removed, and there are plastic washers between it and the swivel housing.
What I did to avoid having to re-adjust the shift cable:
I go up top on the engine bracket and just pull the cotter pins that are holding the shift cable in the engine shift bracket, and tape the threaded cylinder in place so the adjustment can't change. This allows you to pull the pivot housing back without stressing the shift cable but leaving the adjustment as it were.
Start with removing the exhaust bellows, watch out for the snap ring on the stern end, wear eye protection. Remove the other end of the exhaust bellows and put it aside.
Then next, I remove the stern end of the D/S bellows from the pivot housing then I tie a length of twine around the opening in the pivot housing and run it up to the ski rope hook up on the boat, that will hold it in place but let you move it as needed. Then remove hex pins and pull it out from the swivel housing. Now you can get at the DS bellows at the gimble housing end and unscrew the clamp
after you get it off clean up the flange on the gimble housing. You'll see a groove in it and there is a raised lip in the bellows flange that fits into that groove.
also the bellow is asymmetrical, there is one more fold on one side, this should face down, so it gives it more stretch when you tilt it up, makes it last longer.
I put some Evinrude gasket sealer on the flange for the bellows and oriented the screw clamp at about the 2:00 position.
Then use triple guard grease on the other end of the bellows and pop in into the pivot housing.
Next install the gimble end of the exhaust bellows with the clamp
put some grease on those plastic washers to hold em in place and maneuver the pivot housing back in place so you can reinstall the hex bolts. Tighten up the hex bolts and re-install the other end of the exhaust bellows with the snap ring.
go back up by the engine bracket and re-install the shift cable.
This way works pretty well, just take your time to make sure the DS bellow is installed right on the gimble flange end.
Here's some pix from the last time I did it.
in the last pic you can see the ridge in the lip for the bellows, that fits into the groove on the gimble housing flange.
