Re: OMC Stringer transom seal replacement?
Maybe someone replaced the boot once before? I bought my boat new in 1975 and pulled the motor and outdrive assy in 2006 to replace the floor and redo the mount stringers. When I unscrewed the boot from the transom, there was absolutely no sealant/adhesive. When I removed the boot from the IH housing, there was absolutely no sealant/adhesive there either. I cleaned up the IH, repainted it, and reinstalled it with the same boot. I am not 100% positive what sealant I did use but it was most likely Gasgacinch, which is a very light adhesive. No reason the the 'right stuff' would not work but I think it is overkill. You do, however, want the ability to rotate the boot once you have it on the IH because you never get it on in the exact right location, to line up with the holes in the transom. I actually marked my boot and IH housing with pink chalk to make the lining up easier but I still had to rotate the boot a little bit once I got it on.
I think if your mounts had settled you would see substantial evidence in the fiberglass covering them. I've seen them rot to where they would not hold lag bolts but there was no settling. Just make sure that you have your front mounts as low as you can go. That will raise the IH mounting flange in relation to the transom opening.