Typically the seal that leaks is the front main seal (front passenger side). To do this properly you need to put a jack underneath the oil pan (support with a board), use a 2nd jack to lift the front passenger wheel off the ground, remove the front tire and then with an impact wrench you can remove the main pulley from the crank. Next remove the upper passenger side motor mount, remove the timing belt cover, and with an 8mm allen wrench loosent the timing belt tensioner and stick an 1/8in drill bit in the hole of the tensioner to keep the tensioner from spinging back. With the timing belt remove also remove the timing belt guide/pulley from the crank. It just slides off. behind that there is a plastic washer. Remove that as well and the front main seal is right there. Best way to get it out is to CAREFULL screw about a 1 inch drywall screw into it and then with a pliers grab the screw head and yank it out. Clean the surfaces and install your new seal. It must be tapped in evenly. Ideally you'll want a the ford seal install tool but i usually just use a 1/4 inch socket extension and light tap around the edges until it's flush. As long as you didn't rotate any of the pulleys etc after removing the timing belt you won't need to worry about the timing. re-assemble and you should be good to go.
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