I found the antifreeze and even the Dawn Power Dissolver clean the outside nicely, but did not really do much inside the motor. I still used scotchbrite, and acetone on the piston / cyl. I would guess that the contra piston would be similar to that. Once it is freed up it should be fine, as has been mentioned. A good soaking in kerosene should be enough for the head? and then whichever method you choose for the rest. I am too lazy to use the crock pot now, and just use the Dawn PD. I have even left the motor on the plane. I have heard rumours that 'Totally Awesome' cleaner works well when used straight with no heat. I bought some to try, but have been busy, and have no dirty motors left.