I've never made this kind of repair, but I'm wondering if it might also be done with a small piece, or pieces, of a thin double-face adhesive tape.
Apply the piece(s) of the tape to the back of the loose part and trim off any excess material with a small X-acto or craft knife. Then peel off the backing paper and press the part into place. This sounds like the best way to make sure that no adhesive gets into anyplace where you don't want it.
Scotch double-sided tape, sold at Staples, etc., could work too and it is thin and less permanent than glue or adhesive. I think the heavier double-sided tapes would be too thick, like the 3M products. I am not sure either if there is anything underneath that disc that one might need to get at in the future for some other repair or reason. A picture of it would help
This might be a good use for a few drops of hot melt glue because it is easy to remove if you need to take that label disk off for something else but should be plenty strong to hold it in place. Put three small drops evenly spaced on the upward facing surface of the switch and then immediately press the label disk in place firmly to spread the glue while it is hot and still has very low viscosity.
Any of the various brands of Cyanoacrylate (aka "super glue") will work but is more problematic if you need to remove that disk in the future without damage. If you do use this type of adhesive, give it plenty of time to set and finish out-gassing before putting your Rebel in a case or other enclosed area so that the fumes don't react badly with some other part of your camera.
I didnt think I wanted anything permenent because there seems to be a screw in the middle of the dial securing it. The hot glue seems to be a good idea. I was thinking about the gorilla glue but I'm not sure how permenent it is.
I was imagining the loose part to be more of an applique with a larger, smooth surface to be adhered to. The cap appears to be a molded plastic part, as does the mode wheel on the camera body. Considering the irregular shape of the original adhesive surfaces the use of hot-melt glue seems to make more sense, especially if you can use a glue with a lower melting point. It might allow for easier removal in the future, if necessary.
Hi, I have a T5i and last night the top of the mode selection wheel popped off. What adhesive would work best for reattaching this piece?
You're lucky. You found the plate! Most people don't! Good luck on the repair, too.
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