NEW YORK — The month of June showed the first clear sign of weakness in the class 8 truck building market since the pre-buy boom erupted a year ago.

The truck equipment group of New York market analysts Bear Stearns reports that heavy-duty truck builds weakened 11.5 percent from the same period last year — versus a year-to-year boost of 15.6 percent in May 2006.

Navistar International slowed the most — posting a 33.5 percent drop y-y. Following was DaimlerChrysler’s trucking outfits with a 16.9 percent decline. Paccar was less affected at 12.3 percent.

The firm blames a shortage of aluminum wheels and disc brakes for the slump. But Bear Stearns doesn’t expect new orders to bounce production back up during the typical ordering season of September and October.

Even though production is generally strong from a historic standpoint, the firm foresees a much different market than the one industry has experienced over the last six months.

“Class 8 monthly net new orders will go lower and stay down longer, than what’s reflected in general market expectations,” the firm stated in its monthly report. “We think the market will be even more disappointed when class 8 orders don’t rebound later this year.”

Bear Stearns expects orders to average between 10,000 to 15,000 a month for the next year and half — a far cry from the 30,000 average over the first half of this year.

Industry — specifically investors — greatly underestimated the magnitude of last pre-buy, states the report. The average age of the (public) TL fleet declined roughly 28 months in ’02 to 17 months in ’05. Conservatively, the firm projects a further decline to 14 months over the balance of 2006.

There’s brighter news for the medium-duty market, however. After posting soft order trends through 2006, class 5-7 seems to have firmed up. Part of the reason for the stability is pre-buying, says the report, although the medium duty market is not has affected by the pre-buy trend as class 8 is.