Hex Wrench

We are rebuilding the engine for a 94 Mitsubishi 3000GT?

Can anybody tell me what the size hex head or allen wrench is used for the headbolt?

I think 10 mm sockets in 1/2 inch and 3/8 are available at Sears. Mine are Snap-On and 4 inches long to get past the camshafts on some cars.
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