91 V/6 Rodeo Surpentine belt change

According to Chilton's repair manual I can loosen the tensioner by inserting a 1/2" drive breaker bar into the square hole it the
tensioner. Problem is, there is no square hole, (with a mirror and flashlight I can see that it is star shaped). Anyone know what tool and what size is needed. I have a few star type sockets but none this large. Looks in the neighborhood of 3/8".
TIA, Frank
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