Tech / Tech Articles

Swap Feat: Upgrading a Ford 9-Inch Rear Axle with Larger Gears

 

You’ve been working on lowering your elapsed times all season—bumping up horsepower here, tweaking the suspension there, and practicing your tail off at test and tune night—when you hit that proverbial brick wall. No matter what you do, your ETs won’t budge. What do you do?

Try a rear axle gear swap. Changing the gear ratio changes the effective drive ratio at the rear wheels, which ultimately determines how fast your car accelerates. And after all, ET is simply a measure of acceleration.

We decided to go the gear swap route on our Chevy S-10 race truck. With a 383 cubic inch small block, Turbo 350 tranny with 6,200 rpm stall converter, ladder bar suspension, a Ford 9-inch rear axle with a 4.33 gear and a Strange spool, and 29 1/2 inch Mickey Thompson ET Drag slicks, the S-10 was running 11.30 second ETs. We figured a switch to a Summit Racing ring and pinion set with a 4.56 gear ratio would put the truck in the 11-teens.

The slide show covers the ring and pinion installation. But while we had the rear axle apart, we also upgraded the carrier assembly with a Strange aluminum pinion bearing support, and added a Moroso filler cap to the axle housing to make filling chores easier.

Follow along and see what it takes to upgrade a Ford 9-inch for track duty–or any kind of duty, for that matter.

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Pinion Gear Assembly 5
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Strange Support 8
Pinion Yoke 9
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In the quest for better elapsed times from our S-10 race truck, we decided to swap the 4.33 ratio ring and pinion in the truck's 9-inch Ford axle for a set of 4.56 gears. The beauty of the 9-inch is the removable gear carrier. It allows you to do gear swaps on the work bench instead of under the vehicle--much easier on your back. Here are the parts we reused from the existing carrier, clockwise from left: Strange steel spool, iron carrier housing, bearing caps, backlash adjusters and set screws, and bearing races. Depending on the condition of your carrier, you might have to replace some of these items.

This is the 4.56-ratio Summit Racing ring and pinion set. According to the math, the new gear ratio should reduce the elapsed times by about two-tenths of a second compared to the 4.33 gearing.

To complete the gear installation, we also procured a Strange Engineering bearing installation kit and a Summit Racing ring and pinion installation kit (shown). Between the two kits, we had all of the bearings, gaskets, and hardware required to do the swap properly.

Unlike OEM gears, the Summit Racing gear set is not marked with a pinion depth marking/checking distance. That meant we had to determine pinion depth ourselves. We first measured pinion head depth as shown in the photo. It measured 3.360 inches. Subtracting this measurement from the master gear case dimension of 6.375 inches determined the proper pinion depth setting of 1.015 inches.

This illustration shows how the pinion gear assembly goes together, up to the point of installing the pinion support body. From right are the pinion gear, Torrington washer, adapter sleeve, rear pinion bearing, shim spacer, shim pack, and shim carrier. These parts are included in the Strange pinion installation kit (less the gear itself).

Compare the 4.56-ratio pinion assembly on the left with the smaller 4.33-ratio on the right. The 4.56's bigger bearing will displace the load better, allowing the pinion to live longer under racing conditions. Since a hydraulic press is required to press on the bearing, you'll probably have to take yours to a machine shop.

The larger pinion gear bearing requires more support than the stock Ford pinion support can provide. This CNC-machined, forged aluminum support from Strange will do the job nicely. It even comes with the bearing cups already installed.

This illustration shows how the Strange support and the front pinion bearing are assembled. A hydraulic press is required for this step, too. The pinion support we're using is for drag racing only. For street axles, Strange offers a nodular iron "Daytona" support for 28-spline applications.

Here's something important to note about using a Strange pinion support with a stock Ford pinion yoke. Exactly 5/16-inch must be removed from the yoke face where it mates to the pinion support (indicated by pointer). This is done to insure there is enough thread to completely engage the pinion nut. The Strange support instructions do not say anything about this. A call to Strange gave us the answer after we tried to install the yoke and found virtually no thread to engage the pinion nut.

That 5/16-inch cut worked out just right--the pinion nut fit perfectly. The nut was torqued to 200 foot-pounds.

A small amount of oil is applied to the pinion gear assembly, and the pinion is turned to check for bearing preload. The assembly should spin with little effort (Strange says about 20 inch-pounds of torque). If the assembly is hard to turn, more shims must be added to the shim pack. If it spins too easily, shims must be removed.

Since our pinion assembly was a bit hard to spin, we added .045 inches of shims to the shim pack. The assembly was then blown apart install the O-ring and end seal on the pinion support, then reassembled and bolted to the iron carrier. The five bolts are torqued to 45 foot-pounds.

The ring gear gets bolted to the Strange spool. The bolts should be tightened in steps to pull the gear up to the spool evenly and prevent it from cocking. Final torque is 70 foot-pounds.

Pinion depth is measured with this appropriately named pinon depth setting tool. It measures the distance from the centerline of the bearing saddles to the top of the pinion gear.

Once pinion depth is established, ring gear backlash must be set. The ring gear, new bearings and races, adjusters, and bearing cones are installed. Using a dial indicator, backlash is measured at several points on each side of the ring gear (note the white mark) as the gear is rocked back and forth. Backlash should be between .008- and .010-inch; it can be adjusted by turning the adjuster on the appropriate side of the ring gear. We set our backlash at .008-inch.

Gear tooth contact between the ring gear and pinion gear is checked next. With the ring gear coated with white marking compound as shown, the pinion gear is turned until the ring gear makes a complete revolution The contact pattern should be evenly centered on the ring gear teeth. If it isn't, pinion depth will need to be readjusted by adding or removing shims between the pinion support and the carrier. The shims should be adjusted in .003-inch increments. Our contact pattern was close enough to avoid playing with shims, so we went ahead and torqued the bearing caps to 60 foot-pounds.

This illustration shows you common ring gear/pinion gear tooth contact patterns you might incounter during a gear swap, and how to fix them. The correct pattern is shown at the top.

The pointer shows the area on the gear case where the pinion gear seats. This area needs to be ground down on stock Ford iron cases to make room for the ring gear on 4.10 and numerically higher ratio gear sets. Most aftermarket iron and aluminum gear cases have this clearance designed in.

With the gears set up, the carrier is reunited with axle housing (with a new gasket, of course). The nuts are tightened in sequence to seat the carrier to the housing evenly without gapping.

The axle shafts are reinstalled and the housing is filled with three quarts of Red Line 75W90 synthetic gear oil. The case should be filled until oil just starts to dribble out of the carrier's sight hole. Overfilling will cause churning of excess oil by the gears. This will raise the temperature inside the gear carrier, eventually causing gear failure due to overheating.

The driveshaft is reconnected to the pinion yoke. Nine inch axles with 28-spline pinions have U-bolts to hold the driveshaft U-joint to the pinion yoke. Axles with 35-spline pinions use beefier steel caps.

One re-geared 9-inch rear axle. The math says the new 4.56-ratio gears will get our S-10 into the 11s, but a trip to the track is the only way to find out for sure.

Parts List

SUM-G7827 Summit Racing Ring and Pinion Set, 4.56 Ratio, Ford 9-Inch
SUM-G7814 Summit Racing Ring and Pinion Installation Kit, Ford 9-Inch
STR-N1923 Strange 28-Spline Pinion Bearing Installation Kit, Ford 9-Inch
STR-N1922 Strange Aluminum Pinion Support, Ford 9-Inch
MOR-85280 Moroso Universal Filler Cap Kit, Steel Bung
RED-57904 Red Line 74W90 Synthetic Gear Lube, Quart (3 Used)

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