Re: 59AB with A head
The 239 was introduced with the Mercury in 1939 as the 99A engine. You can put 59 series heads on the 99 series of engines but you can't put the any of the early heads on the 59 series 239 unless you cut the valve reliefs in. The 81A head was the first head used on the 99A engine but they changed to a purpose made head for them somewhere in 1940. They also had the 99T head for the optional 239 CID truck engines that were available on special order.
After the war, a lot of rodders used the Big A 81A heads for the compression ratio they gave but the deck coolant ports are different. The 59 series of heads have a better gasket design for cooling the rear of the engine and the 59 series heads had different coolant ports to work with that design. The left and right heads are interchangeable. Sometimes you find an engine with a 59A head on one side and a 59AB head on the other but I would want the 59AB heads for better cooling and better compression myself.
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