While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Joy Global (NYSE:JOY) gained 1% in pre-market trading Friday after Bank of America upgraded the mining equipment manufacturer from neutral to buy.

So what: Along with the upgrade, analyst Ross Gilardi planted a price target of $70 on the stock, representing about 13% worth of upside to yesterday's close. So while contrarian traders might be turned off by Joy Global's earnings-related pop yesterday, Gilardi's call could reflect a sense on Wall Street that the company's growth prospects still aren't fully baked into the valuation.

Now what: According to BofA, Joy Global's risk-to-reward trade-off is rather attractive at this point. "We upgrade Joy Global to Buy on the back of a 7% FQ2 EPS beat and FY14 guidance reiteration despite a brutal coal operating environment," said Gilardi. "Joy's aftermarket business (70% of sales) is putting a floor under earnings, and aftermarket orders were up 10% for the second quarter in a row." When you couple Joy Global's still-hefty debt load with its highly volatile and suddenly hot shares, however, I'd hold out for a wider margin of safety before betting on that trend to continue. 

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Brian Pacampara owns shares of Bank of America. The Motley Fool recommends Bank of America. The Motley Fool owns shares of Bank of America. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.