Difficulty: Easy-Moderate
Time: 1-2 hours
Yes, you have to drill holes into the underside of the bumper (12x).
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*More pics attached at bottom
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Tools required:

• Power drill
• 1/8 drill bit
• Hex wrench (don’t know the size, for BMW screws part no. 07 11 9 902 267)
• Painter’s tape
• Q-tip
• Some way to get the front of the car up so you can see under it and drill holes (ramps, 2x4, lift...). You MUST be able to get under the bumper.

1: PREP
Clean the bumper. This will make being under it a more pleasant experience and more importantly ensure that no dirt, grass, etc is sandwiched between the spoiler and the bumper.
-->Also cleaning will reduce the risk of scratching the paint when you tape.
-->Ask in the detailing forum if you need help. I used a waterless wash (Optimum No Rinse).

2: FIT + TAPE
Test fit the spoiler underneath. It will fit perfect on both sides. Once you have it in place, TAPE it there with the painter's tape as shown (PICS @ BOTTOM), this will hold it so you can drill unassisted.
-->Don't use duct tape, scotch tape, etc. Painter's tape will not hurt your paint and is very easy to remove with no residue.
-->Make sure when taped the spoiler does not move easily.

3: DRILL/SCREWS
The bumper is of course plastic and easy to drill through. Do one of the leftmost screws and put a screw in and then one of the rightmost, then do a center.
-->Now the spoiler is anchored down and hole locations of the other spots will not change.

While drilling, you'll want to make sure the plastic bits drilled out are not in between the spoiler and the bumper. Use the Q-tip to push them out of the way.
-->I suggest drilling three or more holes sequentially (after drilling/screwing one on each side and a center) without screwing them in; this way you can actually get the Q-tip in there, otherwise you won't be able to.
-->Drill CAREFULLY and SLOWLY, if you hit the CF you can break/splinter/crack it.
-->Recommendation: set the drill on REVERSE and try drilling first, this will tell whether the bit is contacting the edge of the spoiler. It's not pleasant to hit the CF ...

Screws installed:
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Untape + done!

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2011 BMW 335i - 6MT, M Sport, Le Mans blue, Logic7