Long ago some would put petcocks in the blocks for ease of draining, many still does today.
On our Fs we have small threaded steel plugs that have been in for over 40 years and most become one with the block. So much can go wrong removing them for the small benefit. If you could easily get to them they are still hard to remove from rounding off the top to pulling some threads from the plug or the block. Many times with the block out of the car Ive had to resort to welding a lug nut on the steel plug to get it out with a little heat and even then Sunday morning after church might be the best time to try.
They sit very low in the wet area of the block and most are covered with scale and casting materials so if you did get them out for the first timer it might not be the best to be poking a coat hanger or worse yet a small sharp item on the scaled areas of the cylinders in the water jacket.
If you must try, I hope it works for you but its really not needed to flush your system. If its overheating now pre flush you may or may not gain much if the core is semi plugged.
But if you must.... here is a V8, you never said V8 or /6
Right behind the motor mounts.
So in this pic youd be looking up and the pan would be on, see the pain rail and the motor mount bosses then right behind in the circle is where the plug goes.
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