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* S. Korean private equity firm, tyre maker buy business
* Major client Hyundai seen concerned over sale -reports
SEOUL, Dec 18 (Reuters) - U.S. auto parts maker Visteon Corp has agreed to sell its majority-owned South Korean unit for $3.6 billion, cashing in on a quadrupling of the air conditioning firm's share price in the last five years as Visteon refocuses on in-car electronics.
Visteon said in a statement on Wednesday it will sell all of its nearly 70 percent stake in Halla Visteon Climate Control Corp to Hahn & Co, a Seoul-based private equity firm, and Hankook Tire, South Korea's biggest tyre maker.
Hedge fund investors had long pressed Visteon to streamline its businesses, looking to take profits after rescuing the U.S. parts maker from bankruptcy following the global financial crisis in 2008.
Hankook Tire said on Thursday it agreed to buy a 19.49 percent stake in Halla Visteon for 1.08 trillion Korean won ($984 million) as part of a strategy to diversify its business portfolio. Hahn & Co will hold a remaining 50.5 percent stake in Halla Visteon.
Shares of and Hankook Tire and Visteon Climate Control rose as much as 3 percent before trimming earlier gains in Seoul trading on Thursday.
Visteon, which relies on Hyundai Motor and Ford Motor as major customers, will receive 52,000 won per Halla Visteon share, compared with the Wednesday's closing price of 47,400 won.
The South Korean unit, which makes heating and cooling systems for vehicles, was founded as a joint venture between Ford and Korea's Mando Corp in 1986. Mando sold its stake to Visteon in 1999 in the wake of the Asian financial crisis.
While the deal sits well with Visteon strategy, South Korean media reports said Hyundai Motor had expressed concerns to Visteon that Hahn & Co would focus on short-term profits rather than longer-term investments.
A Hyundai Motor spokesman declined to comment on the reports.
"The deal is likely to trigger Hyundai to further reduce its reliance on Halla Visteon," said Yim Eun-young, an auto analyst at Samsung Securities.
(Reporting by Hyunjoo Jin; Editing by Kenneth Maxwell)