Like a penny in size and domed ,not flat in shape .In the block it will have a grove around the hole it fits in to .Make sure its nice and clean in the slot or the new pkug wont seat .After your happy with the grove it sits in possition the new plug with the dome on the outside .With a piece of round wood knock it in the midle it will spring outwards to site in the grove in the block. This is the old type you may have plugs that have shoulder on ,i think these just push in with a little sealer. I am sure others can help with the answer further.