Yes the rudders were predrilled. One was even pre-chipped.

7 of the 8 bolts needed to secure the new gudgeons were installed without a problem. The 8th one just hits a solid stop inside the transom rather than a hole. I have read (here of course) that there is an aluminum plate secured to the inside. I think I must have dislodged it when I backed the boat and trailer up and the rudder dropped (don't do that by the way) as the old gudgeon had a noticeable twist in it.

I read this "The stock 1/4" bolts will not take the additional stress of the new rudders over time. Replace with 5/16", usually not a problem to carefully drill and tap. The al backing is pretty thick if it is still anchored to the transom. DO NOT Peel off deck. With patience and ingenuity you can work from the gap at the bottom of the bulkhead. If you want to go faster or through-bolt, holesaw a handhole through the bulkhead." by Steve Blevins a few years back. Hoping he will be reading this.

I am short on patience and lack the ingenuity to think of a way to work through that small hole at the bottom of the bulkhead, short of borrowing a surgeon or GI doc with a fiberoptic endoscope.

I could just drill through and tap what I assume is the Al plate that stops the hole but would like to know exactly what is going on back there.

Anyone have an idea how to get a look at it withoud cutting a hand hole in the bulkhead. Fiberoptics anyone?