I'm hoping someone can help me identify what type of bolt(if it is even a bolt) this is, what it is used for and most importantly how to remove it.

This is used to attached a boat engine to a transom, there are three other normal bolts (one in the bottom of one of the pictures).

Front of bolt

back of bolt

  • 2
    could be some type of a security device, similar to the kind on expensive automobile wheels
    – jsotola
    Feb 14, 2019 at 7:33
  • @jsotola you are correct, it is to prevent the bolt from being removed without a key, I will add an answer and some pictures later.
    – AidanO
    Feb 15, 2019 at 14:51

1 Answer 1


My guess would be a carriage bolt, but it's impossible to tell for sure. Take the nut off the inside, hit the bolt with a mallet and see what happens.

Edit based on addition of interior photo...

I wonder if that big black cylinder is some type of damper intended absorb shock and vibration. Maybe the one on the other side failed and they replaced it with a regular bolt. You might try a channellock pliers or such to take it off (if that's your goal). Or post this to a boat forum and see if anybody has a better answer.

  • I think so too. It looks like a common carriage bolt with a proprietary retainer underneath it. The rusty galvanized bit hooks over the transom to prevent the bolt from rotating when the nut is tightened.
    – isherwood
    Feb 12, 2019 at 17:54
  • I dont see any nut on the inside, I've updated the inside picture link
    – AidanO
    Feb 12, 2019 at 23:38

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