Not so good old bolts.
I saw something in your post that sent up a little red flag. In the Columbia 30, the bolts (studs) were originally epoxied into place. If there has been the beginnings of a failure there, the all-thread will not have anything good to “hang” onto. It might be a good idea to try some solvent like gamma-butyrolactone in there to be certain that you are only tapping into steel. (MEK, Ether, Mineral Spirits, Touline, and there ilk won’t have much affect on cured epoxy.) I’m not so sure that tapping is a good idea for two reasons: 1.) The keel is a steel-lead composite and does not have the same strength characteristics as steel. 2.) Because, the holes were originally oversized to allow for positioning of the studs while they were being epoxied into place. A high sheer strength epoxy might be a better way to go. 3M has some good folks to suggest a product. My Owner’s Manual has information regarding Keel-Hull attachment. I would be willing to photocopy and mail you a few pages. I might even have the relevant section scanned in already. I'm sorta’ in a similar situation. The bolts (studs) in Aquila (a Columbia 30) were terribly rusted and had actually stretched a-little. You could actually see it in the thread count in the space that had been between the nut and the keel. They were also of varying length. The shortest being less than four inches and penetrating the keel less than one inch. The studs were zinc plated mild steel threaded rod, and could not withstand the torque or tension without deforming. The studs had actually slipped and caused sheer failure in their mounting holes. The keel dropped about a half inch. This was due to water freezing in the hull-keel joint while on the hard. I am replacing the studs with six inch, stainless steel studs. The nuts and washers arell also stainless steel. I am looking for some stainless to replace the rusted base plates also. This is getting long so I will stop here. Good luck and be careful. Sincerely, Bentley Vaughan of the C-30 Aquila