Installing a motion detector light to the outside wall


I apologize if I don't use the correct electical terms in the post.
I recently purchased a Heath/Zenith motion detector (model SL-5511-BZ-B if that helps)light. The base of the light fixture that connects the the wall is circular and is about 4 inches in diameter.
My junction box in the wall I guess is 4" in diameter as well. It has 2 tabs on it bent inward to connect, I guess, a backplate to attach the fixture and ground wire.
The light fixture's base has 4 indentations of where a hole would most likely go, top left/right bottom left/right, but there are no holes.
No backplate was supplied with the fixture. So I am using the existing backplate. And it's not the right one. The tabs of the junction box are at the top left/bottom right of the box, like the \ in an X. The backplate is attached to those tabs. There's another bar, about 1/2" in diameter that can swing around.
My problem is that I drilled out two of those holes to match the \ position on the tabs. If I connect the top left hole to the tab, I am way out of position for screwing in the 2nd screw. Can't be done.
What do I need to do to attach the lamp fixture to the junction box/wall? Do I need a special backplate? What do I do about the fixture's base that has no holes (but does now because I drilled them out).
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Is this what you have?
http://ecx.images-amazon.com/images/I/51eFgenSVpL.jpg
If so, don't worry about screwing into the tabs on the box.
Drill your holes on the fixture through the junction box even at an angle and use some long screws (I would use some 2"-3" drywall screws) and screw it into the box. Might want to use a piece of "socket" foam if it is exposed to rain. If it's under a overhang, no big deal.
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Ron wrote:

Yes.
Looking at the picture, on the left hand side of the base you can see indents where I'd normally expect a screw to go thru. But there are no holes. So I guess one has to drill them out someplace. On the middle of the base in the picture I can make a large circle. Would that be where you drilled the holes, on either side, to connect to the junction box?
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If you drilled out the four indents would you be able to screw four screws into the back of the box? Or are those indents to the outside of the box? If outside of the box can the fixture be screwed into the side of the house? If not just drill a hole in each of those "lager circles". As long as you can screw it to the back of the box it really doesn't matter where the screws are.
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wrote:

No hole in the center either?
There are 2 threaded holes in the junction box? Can you measure the distance, then mark two places on the fixture base the same distance apart?

Generally not used afaik, at least not at my house, but a closed foam gasket came with my flood light fixtures to keep the rain out, and I'm guessing maybe bugs if there were an uneven surface.

Hopefully you learned a lesson. My father said the most reliable way to learn is the hard way. This was the hard way.

It didn't have holes becuase they didn't know where you would want the holes, I think.
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