Small sized knockouts in wall box


Old 02-22-05, 02:16 PM
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Question Small sized knockouts in wall box

Hi I just moved into a 1930's home and I am redoing some of the knob and tube. The house also has plaster lath walls.

I tried to remove the wall box and put in a new plastic one but it was very difficult. I changed my mind and am trying to use the old boxes in a safe way.

The problem is that these boxes have very small knockouts already knocked out. They have 4 actual size 5/8" diameters and one 1/2" trade size knockout.

First question: I cannot knock the 1/2" trade size knockout out. I am pounding with hammer and screwdriver while the plaster feels like it is going to crack away. I just cannot hit it because I fear everything cracking. Does anyone have any ideas of how to remove the knockout without such a banging force? Is there a way to do this with a threaded vise or clamptype tool that would add a lot of force but more slowly? Or should I drill it out?

Second Q: In some places I cannot use the big knockout. Is there a knockout clamp that will work for actual size 5/8 and 12-2 romex? (I need a push in version not a screw type because I cannot get to all of them.) Is this ok with NEC? I would guess that this would be a trade size about 3/8 but I cannot find it at my local home store. Does anyone have a webstore with wide solection?

3rd Q: Are there knockout covers for actual size 5/8?

Thanks so much for the advice.

Chris in Sunnyvale CA
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Old 02-22-05, 03:33 PM
Join Date: May 2001
Location: Dry Side of Washington State
Posts: 738
Did you try tapping on the left or right side of the KO? Don't tap on it at the center. The knockout is attaches to the box at its front and back.

Last edited by John Nelson; 02-22-05 at 06:15 PM. Reason: remove redundant quote
Old 02-22-05, 06:07 PM
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Is this a knockout with a straight slot in the middle of it? If so, it's probably meant to come IN and not be knocked OUT. Put a screwdriver in and give it a gentle pry or even a twist. If it's what I'm thinking of, it was stamped from the other side and is actually almost bigger than the hole it came out of because of the stamping process and it will never go back through. If it's tough to get to, try a right-angle screwdriver (flat tip).

I have the same sort of thing in my house. I think those boxes should have clamps that don't actually engage in the holes but sit just inside them? Maybe you can salvage clamps from other boxes around the house? Maybe some of them are unused. Maybe you can use a plastic gromet for the holes instead of a clamp. Best of luck.
Old 02-22-05, 06:56 PM
Join Date: Sep 2004
Location: NW Ohio
Posts: 188

Working with old lathe and plaster can be tuff and discouraging. If the other suggestions just don't seem to want to pan out you may want to consider this:

What I have done in the past is to remove the box as carefully as possible but do expect some patch work after-just manage damage control. Then if you can't fasten the new box to the wall stud and face it out flush with the wall, you can adjust the plaster ears on a new (metal) box to face the box flush with the surface and screw the ears to the lathe. Drill small pilot holes for your screws so you don't split the lathe. Get your wires into the box before you secure it, and make sure your box screws are tight so the box itself stays secure.

Then take pieces of lathe and insert on the inside of the wall around the box as necessary and run a screw through the plaster and lathe into your insert and draw it up snug on the inside. This will act as support and a small frame to secure new drywall into. Then square the opening around the box by cutting the old plaster away with a utility knife. Then it's measure your drywall patch and tape/mud as necessary. You may be suprised at how well you can make it look.

Now you will have to be the judge as to what will work the best, especially if you have a lot of these to do. But I can assure you that as you progress, you will get better, quicker, and neater as you go.

Let us know.
Old 02-23-05, 10:37 AM
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Thanks for all of your help.

Last night I tried to knock the large one out with a nail set because I was hoping that it would concentrate the force in one small place. It did not help. The knockout looks to have one place as the anchor (=)
/ \
See this (weak) ascii art. The knockout looks like a standard one but I think you are right. It seems to stick out a bit to me. On the first box I am messing with, I can get behind the wall. I will try from the other side. However, it does not help me for most of them that are not accessable. I am thinking of buying a nice hole saw to do it. I found the remains of a hole saw ring in one of the other boxes that was previously "upgraded" by someone else. (don't get me started - they left a hot dead leg wire from a different circuit in this box w/only electrical tape around it)

The current clamps work but they are seem unsafe - they are a weird sheet metal shape that wedged in to hold the wire. It is really sharp. I can't see that this is safe or to code.

The box flanges are really wider than I thought. The one side is anchored to the lathe face of the 2x4 and the other side has a 1" flange too. I think I would have to take out about 8" wide x one box high. I don't want to do it. Thanks for the advice. If there is an easier one, I will try it.

thanks, chris
Old 02-23-05, 11:25 AM
Henry V
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Remove Old Boxes

I recently completed a similar project, and when I started I thought about keeping the old boxes in place in order to minimize plaster damage. However, I quickly learned that this wouldn't work. For one thing, the old boxes tend to be smaller, which in many cases creates box fill problems. Also, the old boxes generally don't have grounding screws, so you have to work with clips or some other alternative. It was the fill limits that required me to change several boxes, and once I learned to do that, it became so easy that I went ahead and replaced all boxes.

Which brings me to how I avoided significant plaster damage--buy a Rotozip or similar tool. Start with a template for the replacement box. I used Carlon old work at first, but then switched to Slater because I like the fasteners better. Trace the template onto the wall and then, using a masonry/grout bit, cut away the plaster along the sides of the metal box, down to the lathe. I wouldn't make the cut all the way to your line on the first try, just to be sure you don't cut away too much.

If your boxes are like mine, there will be fasteners on the top and bottom screwed into the lathe. I cut through those using a small, round metal cutting bit (it looks like a small circular piece of sandpaper). It creates a few sparks that may alarm you, but proceed with caution and wear protective gear and you should be alright. You will then need to work the old box out, which will require a little of patience and finesse. My biggest problem was with a tab on each side of the box where screws fastened the whole thing together. They weren't big, but I yanked my first box out and the tabs took a little plaster off that required some minor patching. I later learned to maneuver the box out without hurting the plaster.

Next, you need to cut the plaster to fit the new box. I would take this slowly on the first one and continually hold the box up to the hole to measure your progress. You are allowed a 1/8" gap on each side of the new box and obviously want to limit or altogether eliminate your patch work. After that, you need to cut the lathe to size using a special wood cutting bit. I found that these bits took the most abuse and wore out rather quickly, but fortunately they were not too expensive.

Hopfully this helps. After the first two boxes, I found that I could remove and replace a box inside of 20 minutes and leave myself with little or no patch work. Pros could do better, but that was good enough for me.

Good luck.

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