Updated from 7:07 a.m. EDT

Most industrial stocks have gotten killed lately, as investors fear a global slowdown will affect the sector's forward earnings power.

But the time to buy industrial stocks is when their forward P/Es seem sky-high and their sales seem to be failing off a cliff. Conversely, the time to sell industrial stocks is when their forward P/Es seem extremely low and revenue is running at 110% of capacity.

Some industrial stocks, such as Gardner Denver ( GDI), have gotten too cheap to ignore.

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