How do underdrive pulleys work?. From 1994-1998, if you wanted to put an underdrive pulley on your V6 Mustang, all you had to do is replace the crankshaft pulley.   It became more complicated in 1999 when Ford combined the harmonic balancer (aka harmonic damper) and the crankshaft pulley into one unit.   Ford also changed the design of the harmonic balancer late in the 2000 model year.  Shown above are the two harmonic balancer/pulley combinations that ASP sells to match these changes.   Some 2000 Mustangs produced late in the model year may need the newer style.   If you have a 2000 Mustang, you should check the stock harmonic balancer on your your car before placing an order.   Look for a raised portion on your harmonic balancer like the one indicated with an arrow above.   If it's there, you need the 99-00 model. Above is a comparison of the stock balancer/pulley unit off of my 2000 V6 (left) and the ASP820117 balancer/pulley unit that will replace it. (right)   The raised portions are shown again with red arrows. What is the toothed wheel on the back of the harmonic balancer?   That is the trigger wheel for the crankshaft position sensor.   It has 35 teeth spaced 10 degrees apart and one empty space for a missing tooth.   A magnetic transducer on the engine block monitors this trigger wheel and sends crankshaft position and speed information to the car's computer.   The computer uses this information to calculate spark timing.   I didn't much like the look of the trigger wheel that ASP provided, so I replaced it with the stock trigger wheel.   They both bolt on the same way. Here are some tools you will need for this installation.   I bought a harmonic balancer puller set at Pep Boys for $9.99.   You can do without the installer tool (center) if you buy another mounting bolt a bit longer than the stock one shown on the right.   Get an M14 (metric) hex-head bolt with 1.50 pitch (fine) thread, about 2.5" long, preferably threaded its full length.   I found one for $3 at Ace Hardware. Installation instructions   1. Raise the front of the car (ramps work fine), put the transmission in 5th gear, set the emergency brake, disconnect the battery's ground wire.   2. Remove the radiator overflow water bottle (9/32" socket).  Unclip the fan wiring harness and remove the two bolts holding the fan (5/16" socket).   Remove the fan unit.   3. Remove the harmonic balancer mounting bolt (crank bolt) with a 13/16"(21mm) socket and breaker bar.   If your car has an automatic transmission you may need an impact wrench to break this bolt free.   4. Put a 3/4" (18mm) socket on the tensioner pulley and turn counter clockwise to release and remove the serpentine belt.   5. Use a harmonic balancer puller to remove the stock pulley/balancer unit.   There are three 8mm threaded holes in the pulley - hard to see, but you can find them by feel.   6. When you get the pulley/balancer off, you will see that there is a key in the crankshaft.   Note its location and make sure it stays in place.   7. Apply a dab of high temperature silicone sealer (gasket maker) to the keyway slot on the new harmonic balancer.   8. Place a light coating of grease on the crankshaft and on the inside surfaces of the new harmonic balancer.   9. Line up the keyway and slide the harmonic balancer onto the crankshaft.   The grease you put on the new balancer should allow you get it started about 1/4" or so.   That's not enough to engage the threads of the stock mounting bolt.   You will need to tighten it down using either a balancer installer or the longer bolt you purchased.   Don't tighten the bolt all the way.   Before the pulley is almost completely on, remove the longer bolt.   Apply Thread Locker to the threads on the stock mounting bolt.   Install the stock mounting bolt and washer and torque to 115 ft./lbs. 10. Apply Thread Locker to the threads of the 8mm bolts that will hold on new crank pulley, and bolt the pulley to the harmonic balancer.   Torque to 45 ft./lbs. 11. Reinstall the fan, the fan wiring harness, the water bottle and a new serpentine belt (Gates K060966 or equivalent, 96.6" - 96.8" long).   Ford will charge you ~$40 for a new belt.   I found one at Checker Auto Parts (their house brand) for $20. 12. Inspect the belt to make sure it fits properly in all the pulley grooves. 13. Reconnect the battery ground wire. 14. Start the engine and check to make sure the new pulley does not wobble. That's it.