Need to cut a circle in concrete block wall


I have to install an outdoor shower for a rinse off to the pool. The shower valve has a solid brass body and I have to cut a hole about 7" in diameter to recess the valve into it.
How can I cut a 7" diameter for this? The trim is not much bigger so a clean cut would be necessary. My thinking is that I draw the outline on the concrete wall, then drill holes (many holes) along the perimeter using a hammer drill. Any other ideas greatly appreciated.
MC
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On Sat, 13 Sep 2008 21:13:44 -0400, "MiamiCuse"

Do you have a router or a rotozip? You can rig that to a center pivot and cut a perfect circle with a carbide bit.
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I do have a rotary tool (a cheap imitation of Rotozip). I guess may be I can use it if I can buy the gadget that makes it go around like a compass. Will the carbide bit really be able to cut concrete I remember using it to cut drywall and remember it being a bit flimsy.
Thanks,
MC
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On Sat, 13 Sep 2008 22:44:37 -0400, "MiamiCuse"

I made a jig to cut circles out of plywood with my cheap router and I had the depth about 1/4" too deep on the last one. I ended up with a perfect circle in the concrete. You could test it on a spare block. You probably don't need to cut all the way through to get a decent edge. You are still going to have to chip out the center and chip back to your scored circle.
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MiamiCuse wrote:

Get a nice looking piece of 12 x 12 tile or something similar to mount on the wall - something that you can cut more easily. Then you won't have to be as concerned about the neatness of the hole in the block wall. Form follows function.
Build a kind of shelving system with a box to hold the faucet so you don't have to recess the valve body in the wall. You know, blend the box into some shelves to hold shampoo, soap, whatever. Form follows function.
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I thought about that. However, since it is outdoor rinse shower it really does not need any shampoo or soap, and my exterior wall is already textured so I prefer not to make the shower valve a focal point, I am going for more minimalistic in this case. I guess one option is to over cut a large square area then use small mosaic between the circle and the square. However it may stand out too much still.
Thanks!
MC
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Rent an diamond grit concrete hole saw, Home depot and others have them.
Sal

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sal wrote:

Second that. These circular concrete hole makers - available in several diameters - are used to cut holes in concrete slabs for mounting a money safe. If they can go through 6" of concrete, it won't take much to handle concrete blocks.
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Your chances of making an exact and neat round 7" hole in a concrete block is just about zero. That size cuts into both the top and bottom webs plus at least one of the block webs that goes between the sides. Might be possible with a big 'core drill' but you won't find one at your local rental outfit. Basically you are talking about removing half of a block. Drilling with a hammer drill won't get it.
Your choice is to break out half of a block and then 'neaten it up' somehow. Harry K
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wrote:

Your chances of making an exact and neat round 7" hole in a concrete block is just about zero. That size cuts into both the top and bottom webs plus at least one of the block webs that goes between the sides. Might be possible with a big 'core drill' but you won't find one at your local rental outfit. Basically you are talking about removing half of a block. Drilling with a hammer drill won't get it.
Your choice is to break out half of a block and then 'neaten it up' somehow. Harry K
I understand. I was hoping to get a perfect 7" diameter hole just for a depth of may be about 1/2", behind that it does not matter. The valve changes shapes to something different and has inlet outlets coming out of it I know I have to knock out more of the 7" inside the block and the web.
I was trying to avoid having to do any concrete work to neaten it up because the texture on the stucco is hard to match.
Thanks,
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MiamiCuse wrote:

Knock a 7 inch hole in the concrete with a hammer and chisel. Then insert a paper cylinder and mortar in all around it to make a neat hole.
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bet would be call a concrete sawing CO. If it has to be exact and neat I cant think of any other way .
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Drill a small hole for the handle and mount the valve BEHIND the wall ;)
Free men own guns - www(dot)geocities(dot)com/CapitolHill/5357/
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wrote:

That will not work in my case. I am already trying to mount it from behind. The entire valve body is wider than 7" espacially with all the inlet/outlets. The handle part, the part that the manufacturer suggested to be flushed with the finished wall, the part that will be 90% covered by the trim, is 7" in diamter. I know the cavity behind the finished wall inside the actual block has to be bigger to accomodate the piping that are coming off the sides, and top.
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Trusty angle grinder with a masonry blade.
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MiamiCuse wrote:

free-standing? Already enclosed?
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I am not sure I understand what you mean by free standing. The wall is a 8" thick concrete block wall that is an exterior wall of the house. I need to go through it to get to the plumbing because the hot and cold copper lines comes up on the inside of that wall. The inside of that concrete wall has 1/2" furring lumber and the copper lines take up that space, then will be covered by sheet rock.
The hot and cold lines need to connect to the valve.from the inside of that wall. I am not worried about the inside hole being perfect or not. I already chipped out a rectangular hole for the lines, The valve has a circular template that is 7" in diameter which is the hole I need to make. The trim is slightly larger than that.
Thanks,
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