By Valerie Insinna

(Reuters) -Spirit Aerosystems has reached a tentative four-year contract agreement with the 6,000-member machinists union employed at its Wichita, Kansas plant, the union said Friday in a statement.

The agreement heads off a costly strike that could have rippled through the aircraft supply chain, as Spirit is a key manufacturer for aerospace giant Boeing.

The pact is set for a vote on June 21, just a few days before Spirit's current agreement with this branch of the International Association of Machinists and Aerospace Workers union expires on June 24.

The new agreement includes additional health insurance and retirement benefits, makes overtime on Sunday voluntary, and "up to a compounded 34% average pay increase," the union said in a statement.

Spirit shares were up 0.7% in Friday trading.

(Reporting By Valerie Insinna, editing by Deepa Babington)