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Adding German Overdrive to American Steel

Posted in How To: Transmission Drivetrain on July 18, 2017
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Since the 2013 Cheap Truck Challenge we have upgraded nearly every component on our 1977 F-150. The tired 351 made way for a 460 from LA Speed, and the factory axles were replaced with Super Duty axles filled with Sierra gears and Detroit Lockers. One component that remained, though, was the original NP435 transmission. These transmissions are popular for rockcrawling because they offer a very low First gear (6.68:1) and are strong, compact, and relatively inexpensive. They are also terrible for a street-driven truck.

The low First gear essentially turned our four-speed into a three-speed on the pavement, requiring us to slip the clutch every time we left from a stop. The gaps between gears are huge, and without an overdrive our 460 was wound out when we were doing 65 mph on the freeway. In other words, the NP435 transmission didn’t make our truck very much fun to drive. Oh, and did we mention that we had plans to drive the truck 2,500 miles to chase the NORRA Mexican 1000 across the Baja peninsula? We needed a solution, and fortunately Axleline had one.

Axleline has a raft full of ZF cores sitting on the shelves. Each transmission is built to the customer’s specific needs. Note how the transmission housing has PTO covers on each side, allowing you to run winches or other PTO implements.

Aaron Lechner of Axleline recommended that we consider a ZF five-speed transmission, since it would bolt up to our 460 engine and our existing transfer cases with no need for aftermarket bellhousings or swapping output shafts. The German-built ZF S5-42 was used from 1987 to 1994 in Ford F-250s and F-350s behind small-blocks, big-blocks, and diesel engines. A slightly stronger S5-47 (47 refers to a 470 lb-ft input torque rating) was used from 1995 to 1997. Various gear ratios were used over the years, though. The gas S5-42s and S5-47s got a 5.72:1 First gear (aka wide ratio), which isn’t as low as our NP435 but is still well into granny gear territory. The diesel S5-42s had a 4.14:1 First gear (aka close-ratio transmission) and the later S5-47s had a 5.08:1 First gear (also considered close ratio).

Considering that we have 5.13 gears in our axles and an Offroad Design Doubler providing 4:1 gear reduction, Lechner recommended using the close-ratio transmission. His reasoning was that the close-ratio transmission would make the truck much more streetable due to the reduced drop in rpm between gears. In fact, Lechner’s personal ride is a 1990 F-350 with a fuel-injected 460 engine, ZF transmission, and 37-inch tires. The bad news is that using the close-ratio transmission isn’t as easy as it sounds because these transmissions were only found in diesel trucks and would not mate up to our 460 engine.

Axleline’s solution is to swap the internals from a diesel transmission into a gas case. The effort was worth it, as the new transmission has indeed made our old F-150 much more fun to drive to and from the trail. We saw our rpm drop between Second and Third gear go from a 2,000-rpm drop with the NP435 to a 1,200-rpm drop with the ZF, keeping our engine in the powerband and making it much easier to drive on the street. Our only issue now is wearing out 42-inch tires on the pavement.

The clean transmission in the foreground is a 4WD model, while the dirty tranny in the background is a 2WD unit with a drum parking brake. The 2WD and 4WD transmission tailhousings are different except for the 2WD F-450 Super Duty models that came with this parking brake. These transmissions use all the 4WD components, but with the brake bolted on the back instead of a transfer case.
The close-ratio/diesel gear cluster (left) can be interchanged with the wide-ratio/gas ratio gear cluster. This allows the ability to put lower transmission gears into a diesel truck used for heavy towing or with huge tires. Or in our case, we put the close-ratio gears in our 460-powered truck that is already equipped with 5.13 gears.
While swapping gear clusters Axleline went through our transmission. After thoroughly cleaning the transmission it inspected all of the components. Any bearings or synchros that were questionable were replaced prior to assembly.
Nate’s Precision built a high-clearance crossmember for our truck when it installed the ORD Doubler. The crossmember is a combination of tube and plate that ties into the frame in four places and bolts to the transmission tailhousing and the Doubler plate.
The bolt pattern on the back of the transmissions is identical, which allowed our ORD Doubler to bolt right up to the new ZF tranny. We did not need to make any modifications to our drivelines or exhaust system with the new transmission either.
Our measurements revealed that the new ZF transmission is only 0.6 inch longer than the NP435. The biggest difference between the two is the tailhousing on the back of the NP435 is considerably smaller than the ZF’s.
Our crossmember was built to wrap around the NP435 tailhousing, but was incompatible with the larger diameter of the ZF five-speed. We had to cut one of the mounting legs off the crossmember in order to reinstall it. This was just a temporary fix until we could get back to Nate’s Precision to have a new crossmember built.
ZF transmissions came with dual-mass flywheels in diesel trucks and single-mass flywheels in gas applications. We could have used our old flywheel from the NP435, but it was heat checked from slipping the transmission each time we started in Second gear, so we resurfaced the ZF flywheel prior to installing it.
We sourced a new Luk clutch and pressure plate from Axeline. This clutch has good holding power without a heavy pedal or being too aggressive and grabby for technical off-road use where you may have to slip the clutch.
We used an empty ZF case to mock up our crossmember and check for fitment with the exhaust. It was much easier to address these issues with an empty case than with a heavy, fully assembled transmission.
The transmission mount on the bottom of the ZF case was even the same as the pedestal on the bottom of the NP435 tailhousing. This allowed us to reuse our Energy Suspension polyurethane transmission mount.
We were able to bolt the ORD twin-stick shifters for our NP205 to one of the threaded bosses in the side of the ZF case. The positioning differed from our NP435; however, the shifters use threaded rods with rod ends that are easy to adjust.
One component that was not able to reuse was our gear reduction starter from Summit Racing Equipment. Note how the ZF starter (right) has an offset gear while the Summit starter is centered.
The ZF transmission uses a hydraulic slave and master cylinder instead of a mechanical linkage, which can bind when the body flexes relative to the frame. We took the entire hydraulic assembly from a donor truck.
Some transmissions use an internal slave cylinder that requires you to drop the transmission to replace them. Not the ZF. It clips the slave cylinder to the side of the transmission case, where it actuates a traditional clutch fork.
Mounting the clutch master cylinder was a larger task than we had anticipated. Aaron Lechner mixed and matched components from our pedal assembly and a later-model pedal assembly. Note how the later-model assembly on the right has more surface area to reinforce the firewall and prevent any flexing.
The mechanical clutch was actuated via a lever that went down through the floor, but the new master cylinder sits on the firewall. We had to modify the clutch pedal assembly in order to have the appropriate amount of travel with the nonadjustable hydraulic linkage.
The shifter location for the ZF is farther back than the old NP435. This is good because there is no interference with the dash, but bad because we had to hack a larger hold into the tranny tunnel cover. For now the hole is covered with duct tape, but we have plans to add a proper shift boot soon.
After installing the transmission it was straight to Mexico for a 2,500-mile torture test. With the exception of a hanging starter that didn’t want to disengage, the trip was trouble-free. We can’t imagine driving all of these miles with the old NP435.

Why ZF Instead of NV4500?

The most common five-speed manual swapped behind V-8 engines is the NV4500, and for good reason. These transmissions are strong and have a low First gear ratio for off-road use, and a variety of bellhousings are available to mate them to most popular engines. Both the NV4500 and ZF S5 use a hydraulic clutch linkage, but the ZF has an aluminum housing with an integrated bellhousing. For our truck this was a nonissue because the ZF not only bolted right up to our 460 but also had the correct output shaft and tailhousing pattern to accept our Offroad Design Doubler with a Ford NP203 and NP205. That made the whole swap much easier and cost effective than an NV4500 for our specific application.




Energy Suspension
San Clemente, 92673
Offroad Design
Nate’s Precision

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