9-Inch Nails is the first upgrade of the second path for the Monkey Engineer. It allows it to pop up to eight bloons at once, including frozen bloons. It acts much like a Spike-O-Pult or an Intense Magic Monkey Apprentice and therefore, this is best used on the end of a long line. This upgrade is comparable to a boosted version of the Dart Monkey path 1 upgrades. Unfortunately, it only affects the engineer's nail gun, not the sentries.