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In converting to disc brakes, I noticed a hole centered at the outermost end of the spindle. I had bearing buddies when I purchased the boat and trailer. Should I put a zerk fitting in that hole? I wasn't planning on using the B.Buddies again. If not, what is this hole used for? Thanks for any info you can share.

Yes. It should have a zerk fitting installed. You should see another small hole in the spindle where the grease will come out if you pump it into the zerk. Just like the bearing buddy, it is not a replacement for proper servicing of the bearing but it does allow you to add grease to the hub between servicings.

I tried 1/4 inch zerks ,the screw in type didn't catch. I also have same size that are press in or I guess hammer in type that I was afraid of using. Has anyone actually placed a fitting in that hole?

It may be a metric thread.