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Tim from Boston
Sep 3, 2017

Plugging Cast Iron


Hoping that at some point there might be a forum here for "Tips & Tricks" for beginners. One of the main reasons I love Keith's videos is that he takes a lot of time to show beginners "Here's how you do X".


Starting out, one of the biggest challenges for me is tapping holes in cast iron for small BA threads (7BA in this case). In wood and other metals you can repair a mis-tapped hole by drilling out and filling the mis-tapped hole with and identical wood/metal plug. Cast Iron is a little bit of a challenge because there doesn't seem to be a source that I can find for cast iron rod smaller than 1.25" in diameter. Turning stock that size down to plug 3 out of 4 mis-tapped 7BA size holes on a 10V cylinder seems to me to be an incredible waste (never mind all of the carbon dust that it produces).


I was wondering if anyone has any tips/tricks/recommendations on filling *small* cast iron holes. I've tried JB Weld, however its not strong or fine-grained enough to tap 7BA holes through it.

Gerald Gardiner
Sep 8, 2017

In Industry you drill out the hole larger thread in a plug weld it over and cover the outside with JB Weld.

At home I drill out the hole larger thread in and install a plug and cover it with JB Weld.


Tim from Boston
Sep 9, 2017

Hi, Gerald. The challenge I'm having with the "drill out and plug" method is that I can't locate cast iron rod in any diameter smaller than 1.25-inches. That seems an awful waste for plugging mis-tapped 7BA holes.


The internet concensus for cast iron repair seems to be welding (as you mention). Unfortunately that's not an option for me. It would be much cheaper for me to buy an extra half-dozen 10V cylinder castings until I get it right than it would be to learn welding and buy a welder.


I'm doing some experimentation with a variation of an old lutherie (stringed instrument making/repair) trick that is used for repairing hard bone nuts and bridges: Creating a filler using some original/alternate ground material with a binding agent to make a aggregate filler. In guitar repair, it's called the "Baking Soda and Superglue trick". You fill the hole with multiple layers of baking soda, soaking each layer with water-thin super glue before putting the next layer on top. When you're done, you end up with a fill as hard as the original bone that you can re-drill/re-file.

So, I'm experimenting with doing same thing with iron filings. I just got finished trying iron filings with super glue as the binder, however the super glue doesn't appear to hold up well to tapping. It breaks down under the mechanical forces and leave me with iron filings. The next round will be cast iron filings using JB Weld as the binding agent. Preliminary tests on test fixtures are very promising and I'm looking to do the first "trial run" on one of my botched 10V cylinder castings tomorrow.


The procedure goes like this (I'm filling mis-tapped 7BA holes):

  1. Drill out and tap the hole M5. This gives the aggregate filler something to "grab" onto the walls of the hole.

  2. Mix up some JB Weld and add cast iron filings to great a filler of the consistency just shy of being a "putty".

  3. Using a Q-tip, coat the walls of the M5-threaded hole with straight JB Weld, to ensure that the filler has maximum adhesion to the walls.

  4. Pack in the Cast Iron filler from (2) into the holes, packing constantly to ensure no voids.

  5. Let cure and re-drill/tap the hole 7BA.


Interestingly enough, the spark for the idea was one of Keith's recent videos where he stated that the gray component of JB Weld had some amount of "metal" in it. I'm not sure how true that is, but it caused me to remember the old "Backing Soda & Superglue" lutherie trick. "Why not mix up some JB Weld where there's a *whole lot* of metal in it?" :-)

Gerald Gardiner
Sep 9, 2017

Hi Tim,

Have you looked at McMaster-Carr

They go down to 5/8", I am surprised that there is no metal supplier in your area that carries it.

The problem with Super glue is it releases at higher temperatures, JB Weld lists 500* as max temperature.

Jed Pusczykowski
Oct 14, 2017

I had a miss drilled hole in my No9 for one of the bearing caps at the crank end. I threaded a piece of cold rolled steel and used loctites green bearing retainer to hold it in place. I then milled it flat and re-drilled the hole. You would never know its there.

Tim from Boston
Oct 14, 2017

Hi Jed,

The challenge I'm having isn't so much filling the hole as the need to re-drill/re-tap it. When you plug a hole with a dissimilar metal and then attempt to re-drill it, depending on the location of the new hole the drill bit can 'wander' towards the softer metal. So in the case of needing to re-drill/tap a mis-tapped hole, you need to ensure that the plug is much larger than the original hole to reduce the possibility of drill bit 'wander'. Unfortunately the #10 castings are quite small, necessitating the need to plug holes with the same metal. This leads me back to my original problem: either turning down/wasting large diameter cast iron rounds or finding an alternative.


As of now, the solution I'm running with is making a filler which consists of iron filings (the kind you can purchase from educational suppliers for magnetism demonstrations) and JB Weld as a binder. The filler uses a little JB Weld as possible (to ensure the filler consist of as much iron as possible) making it a thick paste that is packed into the holes being filled. For filling 7BA holes this has shown promise, although I'm drilling the original holes deeper for extra "insurance".

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