I can only tell you how my centerboard is set up on my 1986 O'Day 222 and it is set up in the same way as the 86 O'Day 26. My pendant line pipe is "L" shaped and it fits into a hump in the cabin floor. The hump is nothing more than a recess for a single sheave U bolt assembly inside the keel slot where the pendant line rides though.
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I'm guessing that your pendant line pipe is straight and the pendant line or wire cable goes straight down to the centerboard and connects via a shackle or some other type of swaged ball connection. My pendant line is braid and it comes up into the cockpit. Maybe yours comes off a winch located on the table stanchion down below.
Regardless of how it is set up, the shackle needs to connect to your centerboard so that it is in line with the pendant line pipe when the centerboard if fully retracted.
What you are going to need to do is find that point on trailing edge of your centerboard. Even if someone were to give you a measurement of their centerboard for the shackle connection, it probably would not coincide with you centerboard.
when Rudy at D&R Marine sells centerboards and rudders for these boats, they are not bored for this reason.
It's important that the shackle needs to connect to the board where the line exits the pipe of the shackle will jam. Perhaps you can remove the pipe and use a magic marker to mark this spot on the board with the board in the up position.
Is you centerboard pendant line wire cable or braid line? Does it come off a winch on your table stanchion or from the cockpit in front of the companionway like mine.
If you have wire cable, now is an excellent time to convert to braid. The boards on these boats aren't that heavy that they require cable. Braid line can last for years provided that it connects straight to the board where it exits the pipe.
I think that you're going to need to take a logical approach if finding the spot for the shackle. Then bore the hole and see if the board goes all the way up and down with no hang up or binding. Where I am not familiar with your keel centerboard set up it's hard to speculate on an exact approach to this problem but I'm sure you'll come up with an idea that will work for you.
Good luck!
Joe