One of the bolts holding my bow rail is coming loose; which allows water to seep into the cabin. I tried tightening the allen head on top; but, it just turns the whole cylinder thingy below. I assume there is a nut or something on the underside; but, gaining access would require cuting my headliner up. Any ideas on how to tighten this?

You could try to take the bolt out, coat it in 4200/5200 and reseat it. Won't help with the structural integrity, but may stop the leak. Just don't back it out too far where it drops the nut underneath.

There in lies the problem. I can't get the bolt out and don't really know what lies below in the cabin. I suspect there is a hex head bolt; but, without cutting the headliner out, I can't tell.

You just drop the headliner. You don't have to cut it.

Not to hijack your thread, but have the same issue on the aft most screw or bolt in the rail on my 272. Does anyone know if it's a Philips head screw, or a hex bolt? Was gonna try to tighten it through the headliner. Thanx!

Mine were 1/4x20 hex head ss. Standard 7/16 socket, but it's 1990 boat.