Industrial filtration may not sound exciting to many of us, but to Donaldson (NYSE:DCI) investors, it's sweet music. Last night's earnings report showed a strong quarter, with 10.7% sales growth and a 16.2% bottom-line boost, both year over year. But even better, management reiterated strong forward guidance at a time when you might expect business to drop off over the next few quarters.

Let me explain. Donaldson's two main foci are filters for the engines of heavy-duty equipment such as diesel trucks and heavy construction machinery, and industrial filtration. The latter segment should remain as strong as the economy, selling to customers like Boeing (NYSE:BA), Seagate (NYSE:STX), and IBM (NYSE:IBM). So far, so good. But it's the other half of the business that had investors worried until now.

The engine filtration business accounted for 58% of sales and 72% of operating profits in fiscal 2006, making it the mainstay of the company. New North American emissions standards for diesel engines are taking effect at year-end, and many end customers have accelerated their truck and construction machinery buys to avoid the higher price tags of new engines with stricter emissions limits. That puts a crimp in 2007 outlooks for companies like Paccar (NASDAQ:PCAR), Volvo (NASDAQ:VOLV), and Caterpillar (NYSE:CAT) in exchange for boosted sales in 2006. These are the guys Donaldson is serving.

But of course, new trucks aren't the only ones needing engine filters. Donaldson points out that its aftermarket sales are picking up, as the ever-expanding American trucking fleet needs a steadily growing stream of air and oil filters to keep going. Overall, management is looking for 2007 EPS to track between 8.5% and 15% higher than 2006, based on the just-completed Q1 and this market outlook.

Investors like the news, and the stock opened about 3% higher this morning. That's a 21% one-year price gain, making these filters look squeaky clean to me.

For further Foolishness:

Paccar is a Motley Fool Stock Advisor recommendation. See what else the Gardner brothers have been cooking up with a free 30-day trial to the Fool's flagship service.

Fool contributor Anders Bylund is a Volvo shareholder but holds no other position in any of the companies discussed here. You can check out Anders' holdings if you like, and Foolish disclosure is always good, clean fun.