Mark Houlahan
Brand Manager, Mustang Monthly
December 31, 2009
Photos By: Courtesy Of Strange Engineering

Project Generation Gap
We got off to a great start with our '68 Mustang project about six months ago, tearing into the rusty coupe and fixing structural issues, while converting the body shell into a fastback courtesy of Dynacorn's quality replacement panels and the able crew at Drake Customs. Since then parts have trickled in and we've done a lot of "behind the scenes" work on the project, while also creating a magazine every month, traveling to events, and so forth.

We've tried to keep our readers up to date via the blog on our website (blogs.mustangandfords.com). If you've visited the site, you've probably seen my posts on engine fitment, assembling the Heidt's four-link rear, and taking delivery of our Strange Engineering-built 9-inch rear and our Performance Automatic 5R55S five-speed automatic (yes, you read that correctly).

As with any full-on car project, there are parts you can't work on or test-fit until you have other parts that complement that area of the build. For example, you can't fit an engine without a trans. So while we've been a little behind getting our next story to these pages, fear not that the project is dead in the water. On the contrary, we've got the complete drivetrain, suspension, and brakes in house and fitted to the car, and as I'm writing this in September 2009, the body has been delivered to the great staff at Classic Creations of Central Florida (www.classiccreationsfl.com) to begin the bodywork, so we'll send plenty of stories your way soon.

As mentioned previously, we're using a Heidt's four-link rear. The adjustability of the system for ride height, pinion angle, and more make it a great option for a complete restomod project like this. The Heidt's system is designed to use the ever-popular 9-inch Ford axle assembly, and while our coupe originally had a 9-inch in it, the housing needed a complete rebuild and was very rusty. We factored the cost of all the work required, plus parts, to get it up to snuff and realized it wasn't that much more to build a fresh 9-inch assembly with some nice goodies for strength and power.

We called upon the 9-inch experts at Strange Engineering-guys who know how to build rearends for 2,000hp race cars-to build a nice 3.50-geared street 9-inch for us, and boy, did they deliver. Check out the build steps as we go inside Strange Engineering to check out its product line and see how the crew screws together the product. They have plenty to offer for street, strip, and beyond.

Strange 9-Inch Pricing
Below is the total cost of the 9-inch assembled in this story for our '68 Mustang project. Your cost may vary depending upon option choice.
ItemDescriptionP/NPrice
Axle HousingHousing, axle tubes, endsH1115$439.45
Center SectionS-Series nodular-iron, 3.50 gears, Strange Traction-Lok, iron Daytona pinion support, 1350 S-series yokePRF130$1,189.65
Forged UpgradeAdds forged differential case (stronger)OPRF14$27.10
S/S Axle Shaft Kit (Timken bearing)31-spline, bearings, studsP3104$396.00
Total (minus shipping)$2,052.20

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery

Now the Strange tech can measure the actual backlash via a dial indicator. He zeros the indicator on the drive side of a gear tooth and then rotates the ring gear without moving the pinion. The backlash must be within 0.008 to 0.012 inch, otherwise you will need to rotate the adjusters to get the backlash into spec. Check it at three points around the ring gear and ensure they do not vary by more than 0.004 inch. Once the backlash is set, the gear pattern must be checked with marking compound to ensure the gears will be quiet. The pattern should be centered from heel to toe and from root to top of the teeth. If the pattern is too close to the heel (outside edge of the ring gear), then you need a smaller shim between the pinion support and differential case. If it's too far, you need a thicker shim. Finally, check that the preload is between 45 to 50 in-lb by rotating the pinion nut with a dial wrench attached. If the preload is less than desired, tighten the differential adjuster nuts evenly-and vice versa for too much preload-and the install the adjuster lock tabs and tighten their-head cap screws to 15 ft-lb.

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery