Underdrive pulley for 05+ V6, how-to, pics, and notes


Article written by Rookie from v6power.net.

Reposted with permission, and sincere gratitude!

Ok, first thing is that the mods I'm going to be doing to my 05 are aimed at maximizing fuel efficiency, handling, and keeping my daily driver very driveable. No blowers or turbos are planned, the goal is to have a refined, reasonably powerful and efficient car.

To that end, my baseline on gas mileage for any reasonably long trip so far has been 29.2 mpg as calc'd by my car computer. Weather conditions were little to no wind.

The first logical choice for improving gas mileage is the underdrive pulley. Reduces accessory drag (parasitic losses) and should help the car run more efficiently. ASP makes one for the 4.0, a 25% underdrive and it includes several bolts to help install it.

Tools required:
Breaker bar
19mm socket (for crank pulley bolt)
1/4" x 6" socket extension (you'll see why)
8mm socket (assorted bolts)
harmonic damper puller.
3/8" rachet (remove belt)
new bolt slightly longer and same size as one supplied in kit.

Step one: remove the air intake tube and top of the air filter box. 8mm socket, loosen the clamp holding it to the throttle body, and unsnap the top of the cover, then rotate it out of the way while keeping the air tube attached.


Step two: remove the coolant overflow tank and power steering reservoir. 8mm socket takes off the two bolts on the COT, one bolt holds the PSR. Both of these can be pulled out of the way.

Step three: remove the fan assembly. Two 8mm bolts hold it on, then just unplug the fan connector, lift it up and get it out. After these three things are removed, you have quite a bit of open space in the front of the vehicle to work with.


Step 4: Removing the crank bolt. Fun stuff. Fortunately I have a 5spd, so I put the car in 3rd gear, locked the e-brake down hard and put a socket on the end. ASP gives you a slip of paper with the install instructions, some are -not- correct. Number one is that it's a 19mm socket. Not an 18mm. Using the breaker bar and a piece of pipe on the end, it came off pretty easily, certainly much easier than putting on a 3.8 UDP. Remove the bolt all the way.


Step 5: Using a harmonic balancer puller, you'll need it to remove the pulley/balancer assembly, however, the way it's made, you'll need to use something to fit through the narrow opening and press against the inside of the crank. The main bolt on my harmonic puller was too big to fit through the hole in the crank pulley and reach the crank. Enter the 1/4" by 6" socket extension. This is thin enough, strong enough, and shaped properly to get through the hole in the damper so the puller can remove the damper. Put it in the now-vacant bolt hole, attach your damper puller and get cranking. You'll need to make sure you have some pretty long bolts, ASP supplied them - two 8mm x 160mm bolts, but no washers. You'll need two as they're socket head cap screws, and would have slid through my puller.


Step 6: The pulley should be off. Now, remove the oil-ring off the stock damper and install it on the ASP unit. I used two screwdrivers to pry it off, there's a ridge machined into the shaft to hold it there and it snaps on. It snaps off, just be careful not to damage it. Installing it on the new damper is easy enough if you have a 2.5" section of exhaust tubing lying around like I do.


Step 7: Grease the bore of the new ASP damper, not much is needed, and a little on the front face where the bolt is going to ride. A small dab of silicone goes on the keyway slot to help prevent oil leaks.

Step 8 : Installing the ASP damper, use the supplied bolt. This is where I wish they would have provided one just a little bit longer. I would (if I were you) go out, match up and buy one just about 5-10mm longer. Mine managed to catch one thread, and I used the bolt to draw the damper back onto the crankshaft. The guys at 5.0 Mustangs mag couldn't get theirs to catch at all, and had to make one. You don't want to do that if you don't have to, so just buy a longer bolt. You'll only use it to install the damper, so a grade 8 isn't required. After drawing it onto the crankshaft for a bit, take it back out and use the ford bolt. Torque to 80ft/lbs. Look how small it looks compared to the stock unit.


Step 9: All that's left to do now is install the belt (86.0"), put the fan back on and reconnect the fan wire plug. Re-attach the COT, and PSR, and reconnect the air intake tube. That's it.

Net results as dyno tested by 5.0 Mustang magazine was +10hp. More importantly for what I'm looking to do is shown below. Sunny day, no breeze, 11 mile trip, 9 hwy, 2 city. Previous best average was 29.2. New best average is shown below at 32.4, or an additional 3.2 mpg. We'll keep an eye on the averages and see how it works out long-term, and this is bone stock, without any tuning tricks. That's next on the list of things to do along with a slightly better air intake tube.



3.8 original member
did asp supply the new belt? i was planning on doing this to my 06 and it looks simple enough

No.. The ASP set does not come with a new belt. The new belt should cost around $40 and can be bought wherever you buy the ASP UDPs.


did asp supply the new belt? i was planning on doing this to my 06 and it looks simple enough

UDP's are fairly simple. If you decide to do it, take pictures! I would love to develop this article further and additional pictures would definitely help.

The same goes for any mods you do. If you take the pictures, I'll find a way to incorporate them in an article. :thumbup:


New Member
I know this is an old post, but I've been avoiding underdrive pulleys because I had been told it bypasses the harmonic balancer. Is it still functional with this set up or will everything shake? also does it completely bypass all accessories or just change the way their powered?


Keepin' it real
You'll be fine with an underdrive pulley ;)

I am in a hurry otherwise I would post more....I'm sure someone will help ya out soon :)


Active Member
I put one on my car about 40,000 miles ago. Yes it slows down all the accessories that the belt turns. It does not cause any shake to the engine. I`ve never had any adverse effects from it. I was in Missouri all those miles with summers getting 100+ degrees and the a/c still worked fine as well as the cooling, steering and charging. The car has always ran good and is very punchy with the race tune so i didnt really notice any great improvements. You can get a replacement belt for the UDP from cjpony. I replaced mine once already, and kept the old belt so i would have a spare on hand just in case.

My OEM pulley came apart around 60,000. There should have been a recall on the 05 pulleys cause they were coming apart on all the 4.0`s, cars and trucks and Fords knew it. The rubber ring was deteriorating and water caused rust between the rubber ring and the steel till it came apart. The part was $130, $7 bolt and people were paying a few hundred to have it replaced.

The pulley pullers, at least mine anyway, wont reach deep enough to hit the crank, and I had to come up with metric bolts for the pulley that were longer than what i had because they built the pulley with a deep cup in the center. The not reaching deep enough to hit the crank can be done real easy by removing the pulley bolt, then by using the bolt. Its a little time consuming but is easy enough. Just insert the bolt back in by hand to the crank and back it out maybe 3/8"-1/2". Use the puller on the bolt head till it runs out of reach, remove the puller and back the pulley bolt out another 1/2" and continue this pattern till its off.

Your suppose to replace that pulley bolt, its about $7 at the dealer.


New Member
hey guys about to attempt to install this with my mechanic. any tips besides the guide provided ?
do i need rtv to prevent leaks ? any pics are appreciated


Profile Violation
NONE of the pictures work... IT may be old, but a yahoo search
for a UDP led me here, and all of the pictures are out...