Alternate plan for the keel bolt

So I’ve come up with another plan. Correct me if this is a bad idea, but what I hope to do is to drill out the hole completely and then re-thread it. I’m going to get a bit the size of the replacement eye bolt, drill it out completely (threads and all) and then re-tap with a bigger square bottom tap. Then I’ll buy a replacement eyebolt rated it for some ridiculous high weight and use it instead of the eyebolt I purchased.

Note that you can find a tap chart anywhere on the web by searching the key phrase ” tap size chart”.  I use this one from  Here is an excerpt:


Here are the specifics:

My new keel eyebolt is 1/2 inch. It is a 13 thread.  This is highlighted in yellow on the chart.   The orange highlight of 27/64 is the matching drill that you can buy with it from the well-known Catalina parts supply company .  Am I reading this wrong, or since this is an iron core, should the drill size sold be 29/64, not 27/64?

It doesn’t matter for my keel.  Given that the existing hole is 1/2 inch size my new drill size for iron should be slightly bigger.  I picked 33/64 , which is highlighted in pink. I will also need a tap sized for a 33/64 hole with a 9/16-12 thread.

A tools/parts supply website, mcMaster-Carr sells taps and has a great website to help you find out which one you need.  The problem is, taps don’t come in the sizes I just calculated.  I can’t get a 9/16-12 sized for 33/64, but I can get a 9/16-18 with a 33/64 hole.   The chart says the hole should be 17/32 for iron.  Does making the hole 1/64 less matter?  Seems like they wouldn’t put it in the chart if it didn’t matter.
tap specs


Assuming that will work, now I need a 9/16 eye bolt with a 18 thread.  Let’s head back to McMaster-carr, they also sell eyebolts in all sizes.

Working my way through the different options I found a 9/16 – 18 Zinc plated steel eyebolt (with shoulder) rated for up to 5,000 lbs.  That should handle my little 550 pound keel.  (Note that it can still lift 1,000 lbs even when the lift angle is 45 degress)  Note also that the shank length is 1 5/8.  The eyebolt that I bought has a shank length of only 1″.

eye bolt specs


So now all I have left is to drill a new hole and tap new threads.

One other thing. The site says “All taps are for use on most metals and plastics. Also known as hand taps, use them with a tap wrench”.  I don’t have a tap wrench.  They sell a tap wrench for a tap this size (over 1/2″) for $44.  All this for a one time use.  Can these be used with a socket or wrench?  The welder used a metric socket on his tap, but the socket stripped out before he finished, so that’s a strike against it.  Any alternatives?  

On more other thing.  Once I get the drill bit , is it going to be possible to drill into the iron?  Do I need a special drill?  How hard is this going to be?  I didn’t post the drill bit search result, but they have those too ($22 each).  Should I get two?  Will a single one do the job?

What do you think?



12 thoughts on “Alternate plan for the keel bolt

  1. I had the same scenario as you. What I did was drill out the hole till the eye bolt would sit flush (no threads showing). I then put JB Weld on the threads of the bolt (generously) and pressed it into the hole. The next day or so I used a tungsten bit and drilled through the side of the keel, through the eye bolt and out the other side and pressed a pin through this drilled hole. If the JB Weld wasn’t enough, the pin will hold it. Finally I slathered JB Weld in to hole with the pin and when the JB weld started coming out the other side I then used tape to cover the area to prevent it from running out and seal the pin in. It is still holding to date. The only other option I have seen was a tang which had a bolt running through the keel and the cable attached to the tang. This isn’t very streamlined, but it is just another option.

      • I used a tap at first, but didn’t get even as far as you did before having a problem. I started to tap the drilled hole and after a few threads I removed the tap and test the bolt and somehow ended up messing up the threads on the bolt. I think it cross threaded which ruined the threads of the bolt. At this point I just drilled out the hole and did the steps listed above. I did test the JB Weld before drilling the hole for the cross pin using the engine hoist and it held. I just felt if the JB Weld were to give out someday, the cross pin would only help secure it.

      • Thanks for the clarification. I’ll try the “tap a bigger hole” method first. It’s good to know I have a fall back plan. Sometimes the scariest part is trying something I don’t know how to do and being worried I’ll screw it up so bad it will wind up costing more than if I had a pro do it in the first place.

  2. If you keep the bit oiled while you drill, I think you should be ok. You might check with your local Home Depot, Autozone, O’reillys, etc. to see if any of them have a tap wrench you can rent or borrow. The O’reillys Auto here loans tools for a deposit and then you get your deposit back when you return it.

    • Thanks for the suggestion. I was looking at Home Depot, but hadn’t thought about an auto parts store. Apparently the issue is that going over 1/2″ means it is less common and less available. Sure drives the price up.

      Do you think the approach is sound?

      • As long as there’s enough meat left around the hole when you drill it out it should work fine. Was the original eyebolt stainless? You might want to paint or powder coat the new zinc plated one just as a precaution against corrosion.

    • Thanks for the comment. That might work, but I don’t believe that there is room for that in the keel slot in the bottom of the hull. Not much clearance there.

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