Upgrading A 1991 International 7100 From 5-Speed To 8LL

[MikePhua]

Background Of The Truck And Drivetrain
In the early 1990s, medium-duty vocational trucks like the International 7100 series were a common sight in regional delivery, municipal, and construction work. The 7100, typically paired with the DT466 inline-six diesel, became popular because it blended durability with relatively simple mechanical systems. The DT466 itself, introduced in the 1970s, earned a reputation for being rebuildable in-frame and routinely running beyond 500,000 miles in fleet service when maintained properly. Many fleets bought these trucks in large numbers for beverage delivery, local freight, and utility work, and thousands of units were sold across North America over multiple generations.
One example is a 1991 International 7100 originally built as a Coca-Cola bobtail truck in Georgia. After years of route service, it eventually migrated into construction use in Pennsylvania and later West Virginia, converted into a dump truck. It is equipped with a DT466 engine, a single-speed rear axle, and originally carried a Spicer ES65-5D 5-speed manual transmission. The truck is rated at a gross vehicle weight of 33,000 pounds, putting it right at the boundary of what many small contractors can use with a commercial driver’s license while still retaining good maneuverability.
For a small two-person excavation operation working the steep back roads of West Virginia, the limitations of a wide-ratio 5-speed become obvious. Long, hilly approaches when hauling stone or spoil make gear spacing critical. A driver can easily find themselves “between gears,” lacking a comfortable choice between over-revving the engine or lugging it. That is where the Fuller 8LL transmission enters the story: a gearbox that has become a favorite in vocational trucks because of its combination of highway gears and deep low-range ratios for slow, controlled operation.
Why Choose An 8LL Transmission
The 8LL is part of a family of heavy-duty truck transmissions designed for demanding conditions like logging, construction, and oilfield work. Unlike simple 5-or 6-speed boxes, an 8LL is effectively an 8-speed with two additional ultra-low gears (the “LL” range). The normal shift pattern covers typical on-road work, while the deep reduction gears allow controlled starts on steep grades, soft jobsites, or while maneuvering heavy loads.
For a dump truck hauling stone on narrow mountain roads, the benefits are clear:

• Closer ratio steps between gears for smoother acceleration.
  
• Lower starting gears to prevent clutch abuse when starting on hills.
  
• Better matching of engine power band to road speed, especially with a naturally aspirated or lightly turbocharged DT466.
  
• More control at low speed when backing into spots, easing onto soft ground, or creeping up rough lanes.
  

Rebuilt 8LL units are widely available on the used market because they were produced in large volumes over decades. In this case, the owner sourced a rebuilt 8LL transmission that had already been behind a DT466 and came with the correct bellhousing, paying around 2,000 dollars for the unit. Considering the capability gain, that cost is very competitive compared to buying a newer truck, especially in a small business where every dollar matters.
Planning The Swap And Evaluating Compatibility
Swapping from a Spicer ES65-5D 5-speed to an 8LL in a medium-duty truck is not just a matter of unbolting one and bolting in the other. Even for someone experienced with engine swaps in lighter vehicles, medium-duty drivetrain changes introduce additional challenges:

• Power take-off (PTO) mounting and pump clearance.
  
• Driveshaft length and angles.
  
• Clutch compatibility and linkage geometry.
  
• Speedometer/speed sensor matching.
  
• Frame and crossmember clearance.
  

The truck in question already had a PTO-driven hydraulic system for the dump body. The original PTO was mounted on the driver side of the Spicer 5-speed. The 8LL transmission, however, offered a heavy-duty bottom PTO pad and a standard passenger-side PTO opening. When mocking up the configuration, it became clear that simply relocating the existing PTO to the passenger-side opening would cause the hydraulic pump to interfere with the transmission case near the rear section.
This is a common issue in transmission swaps: the PTO that worked fine on one gearbox may not line up or clear the case on another model, even within the same general size class. The pump housing, hoses, and driveline can all interfere with transmission ribs, crossmembers, or frame brackets.
Solving The PTO Mounting Challenge
There are several theoretical options when the original PTO will not fit cleanly:

• Use a bottom plate adapter to adapt an older side-mount PTO to a bottom-mount configuration.
  
• Use an offset PTO adapter on the side opening to move the pump away from the case and clear obstructions.
  
• Convert to a remote pump drive, running a small driveline from the PTO to a pump mounted elsewhere on the chassis.
  

In practice, heavy truck PTO suppliers like Muncie and Chelsea provide detailed application tools that match specific transmissions, PTO models, gear ratios, rotation directions, and pump speeds. Modern online selectors let a technician plug in the transmission model, the desired PTO location, and pump requirements and get a list of compatible PTO assemblies and adapters.
In this project, instead of struggling to adapt the existing PTO, the owner chose a more straightforward route: purchasing a new 8-bolt bottom-mount PTO designed for the 8LL. The new PTO cost under 1,000 dollars. Combined with the 2,000-dollar rebuilt 8LL, the total outlay for a fresh transmission and air-shift PTO came in under 3,300 dollars. For a working dump truck that hauls revenue loads, this is a relatively small investment for a major drivability upgrade.
The benefits of going with a correct, purpose-built PTO include:

• Guaranteed bolt pattern and case clearance.
  
• Correct gear mesh and pump speed.
  
• Cleaner plumbing and easier service access.
  
• Reduced risk of vibration or misalignment-induced failures.
  

Driveshaft, Flywheel, And Clutch Considerations
Physically installing the 8LL into the 1991 International 7100 required surprisingly few hard modifications. Two key changes were needed:

• The driveshaft had to be shortened slightly to accommodate the different overall length of the 8LL.
  
• The flywheel was machined to accept a larger pilot bearing required by the new transmission input shaft.
  

When changing transmissions, it is important to ensure:

• The clutch matches the new gearbox’s input shaft diameter and spline count.
  
• The clutch type is appropriate: single or double disc, and whether a clutch brake is required.
  
• The release fork travel, linkage geometry, and pedal feel remain usable.
  

An 8LL behind a DT466 in a vocational truck will often be paired with a double-disc clutch including a clutch brake. This setup provides more friction surface for frequent starts under load and allows the driver to stop input shaft rotation while stationary to engage gears more easily.
Clutch Linkage Geometry And Pedal Effort
After the 8LL was installed and the truck became driveable, one issue surfaced: the clutch pedal effort became extremely high. The same clutch and pressure plate that had been manageable with the original transmission now required a great deal of force to operate, turning normal driving into a workout.
Inspection showed that the clutch linkage had been altered so that the clutch bar was pinned higher on the arm descending from the pedal linkage. This change in pivot points reduced the mechanical advantage (leverage) the driver had at the pedal. At the same time, the frame and transmission placement limited how the linkage could be routed, so returning to the exact factory geometry was difficult.
A heavy clutch pedal is more than a comfort issue:

• It can cause driver fatigue over a full workday.
  
• It may encourage partial clutch use or bad habits.
  
• It can make feathering the clutch in tight spots less precise.
  

The eventual solution turned out to be simple but clever. The 8LL in this case had been previously installed in a Ford truck and came with a short aluminum clutch lever at the transmission. That short lever exaggerated pedal effort. By visiting a local salvage yard and digging through used parts, the owner located a longer clutch lever that fit the transmission shaft with only minor filing for proper fit. The longer lever increased mechanical advantage at the transmission side, effectively lightening the pedal.
With this revised lever:

• The linkage geometry improved.
  
• The clutch pedal returned to a reasonable effort.
  
• No exotic parts were needed, just careful selection from existing heavy-truck components.
  

This is a good reminder that in mechanical linkages, small changes in lever length, angle, or pivot position can dramatically affect feel and performance.
Speedometer And Electronic Compatibility
The International truck used an older-style, square-faced electric speedometer. Many trucks of this era transitioned from cable-driven mechanical speedometers to electronic gauges using a magnetic or Hall-effect speed sensor on the transmission tailshaft.
A reasonable concern when swapping transmissions is whether:

• The speed sensor threads and drive will fit.
  
• The output signal type (pulse rate, voltage) will match the existing cluster.
  
• The axle ratio and tire size will still produce a reasonably accurate reading.
  

In this case, the solution was pleasantly straightforward. The 8LL transmission already carried a compatible speed sensor. When that sender was wired into the existing vehicle harness, the factory speedometer read correctly without additional adapters or recalibration. This will not always be the case in mixed-brand swaps, but it illustrates how using a transmission that previously worked behind a similar engine and in a similar GVW class can simplify integration.
Performance Change And Driving Impressions
With the 8LL installed, PTO sorted, clutch linkage refined, and speedometer functioning, the International 7100 returned to work. For a small excavation contractor in hilly terrain, the difference was significant:

• The deep low gears made starts on steep grades smoother and less stressful.
  
• Gear spacing allowed the DT466 to stay closer to its torque peak under load instead of falling out of the power band.
  
• The truck felt more capable and less strained when hauling stone to job sites on back roads.
  

Drivers who have worked in multiple configurations often compare multi-speed transmissions in colorful ways. Tales of over-geared highway tractors reaching extreme speeds due to tall rear differentials and powerful engines illustrate how gearing dramatically changes vehicle behavior. In vocational trucks, the goal is not high top speed but controlled torque and flexibility. An 8LL behind a midrange diesel like the DT466 is a well-proven combination in construction fleets because it strikes exactly that balance.
For the owner of this particular truck, the verdict was clear: the swap was worth the time, cost, and learning curve. With a rebuilt gearbox and new PTO for under 3,300 dollars, the improvement in daily usability and the reduction in strain on the drivetrain turned an aging Coca-Cola bobtail-turned-dump into a much more effective tool.
International 7100 And DT466 In Context
International’s medium-duty lines, including the 4000 and 7000 families, built their reputation primarily on the DT series engines. Across several decades, these engines powered hundreds of thousands of school buses, delivery trucks, and vocational chassis worldwide. The DT466 in particular became known as a “forever engine” in fleet circles because it could often be rebuilt multiple times within the same chassis.
The International 7100 series targeted vocational and municipal users needing:

• Higher GVW ratings than light/medium 4000-series trucks.
  
• Configurations suitable for dumps, plows, tankers, and cranes.
  
• Good low-speed capability with engines like the DT466 and suitable manual or automatic transmissions.
  

Many of these trucks remain in service in smaller fleets because:

• Their mechanical systems are relatively straightforward.
  
• Parts availability for DT466 engines and common transmissions like the 8LL remains strong.
  
• They can be economically refurbished and repurposed, as in this example.
  

The ability to upgrade major components like the transmission extends the practical life of these trucks and provides small contractors with a cost-effective alternative to buying new equipment at modern prices.
Lessons Learned And Practical Advice For Similar Swaps
For anyone considering a similar drivetrain upgrade in a medium-duty vocational truck, a few practical lessons emerge from this project:

• Plan around the PTO early
  • Determine whether you will reuse the existing PTO or replace it.
    
  • Verify pump clearance and rotation direction.
    
  • Use manufacturer selection tools for Muncie or similar brands when specifying a new PTO.
    
• Expect minor hard-part modifications
  • Budget for driveshaft shortening or lengthening.
    
  • Be prepared to machine the flywheel for a different pilot bearing if needed.
    
  • Confirm clutch compatibility, including disc count and clutch brake requirements.
    
• Do not underestimate linkage geometry
  • Small changes in clutch lever length or pivot location can radically alter pedal feel.
    
  • Junkyard parts can provide inexpensive solutions when you know what dimensions you need.
    
  • Test pedal effort and engagement before finalizing the install.
    
• Check electronic interfaces but don’t fear them
  • Electronic speedometers often use standard pulse senders that can be reused.
    
  • Start by wiring the new transmission’s sender to the existing harness and test before buying adapters.
    
  • Inspect the instrument cluster for age-related issues like cracked solder joints, as older clusters can fail independently of the swap.
    
• Keep cost vs. value in perspective
  • A few thousand dollars in drivetrain upgrades can dramatically increase the earning potential and practicality of an older truck.
    
  • For a small business, a well-executed transmission swap can be more financially sound than taking on debt for a new vehicle.
    
• Document the changes
  • Record transmission model, PTO model, clutch part number, and any custom changes.
    
  • This information will be invaluable for future service, clutch replacement, or troubleshooting.
    

Conclusion
Transforming a 1991 International 7100 dump truck from a 5-speed Spicer ES65-5D setup to an 8LL-equipped workhorse shows how thoughtful mechanical upgrades can extend the life and usefulness of older vocational trucks. By recognizing the limitations of the original gearing on steep West Virginia back roads and investing in a proven transmission, the owner turned a solid but constrained machine into a far more capable and efficient tool.
With careful attention to PTO selection, drivetrain dimensions, clutch linkage geometry, and electronic interfaces, the project remained manageable and cost-effective. The result is a truck that not only continues earning its keep but does so with improved drivability, better hill-climbing performance, and renewed purpose in a demanding work environment.