If it's true that Fiat S.p.A. is offering to buy the remaining assets of TK Aluminum, then some recent assumptions are going to have to be reconsidered. (Read the spare details are here.)

TK Aluminium, you may recall, is the holding company for the Teksid Aluminum organization, which casts automotive components. Until 2002, Teksid Aluminum was part of the larger Teksid S.p.A. — wholly owned by Fiat. That year, the aluminum operations were sold to Questor Management Co. L.L.C. for $444 million, in one of the earliest and most notable examples of private-equity groups buying what they deemed to be undervalued assets in the basic manufacturing sector.

Private equity has been the driving force, or the bane, depending on your point of view, of all businesses in the past five years or so. (Over the first six months of 2007, private investors have made $582 billion worth of acquisitions, according to one analysis of capital markets.)

That's been a good thing for industrial sectors — including metalcasting — that need fresh capital in order modernize and compete on a global basis. Private-equity investors don't buy companies, so much as they buy potential. They force changes in the organizations they buy, and when their assets reach a point of maximum return on the investment, they sell.

TK Aluminum made it know last year that the Teksid Aluminum was available for sale. Nemak acquired seven of the Teksid Aluminum plants this year, in Alabama, Tennessee, Argentina, Brazil, China, Mexico, and Poland, for a reported $485 million and some stock.

Now, if Fiat is prepared to reunite what remains of Teksid Aluminum with its ferrous casting operations, it signals that at least one automaker is no longer willing to sustain the costs and uncertainties in the design and supply of essential components — risks that have to be taken if it sources parts rather than produces them. That would be a reversal of recent trends, or maybe the start of a new trend, or a return to an old one. Or something.

UPDATE: It's happening.