Re: Square Bolt behind flywheel
NO! Those holes were drilled by the previous owner in an effort to get less backpressure and gain power. They will blow out exhaust and make the engine more noisy, not really add any power, and serve almost no useful function.
Above the prop there should be a small plate held by a bolt. This plate is bent and runs down the front of the exhaust snout. It has multiple small holes in it. This is the water pick-up and it was supposed to use water pressure off the prop to help ease water to the pump. A brass tube runs at an angle from inside this plate up to the water pump. It was a crappy system and you can not use muffs with it. You must immerse the lower unit in a bucket to have cooling water.
I believe that by 1970 they changed the water pick-up to a screened opening just above the gearcase torpedo and changed the water pump to a four bolt pump. Your engine has three flat head screws holding the water pump in place on the lower unit.
