Abstract: A blowout preventer for use on wellheads for sealing around tubing strings, polished rods, and wirelines, including a body having a bore therethrough and connectable on a wellhead, integral laterally opposed side chambers in the body, a closure cap over each of the side chambers, an operating screw through each of the closure caps, a seal assembly in each of the side chambers adapted to come together in the bore of the body to seal off flow through the body, each of the seal assemblies including a ram plate having only an upper backup plate and side retainer walls, and an elastic seal supported on each ram plate below the backup plate inside the retainer walls, adapted to compress together within the ram body bore for expanding and extruding sufficiently to fully seal off the bore through the body.