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The Clark 5-speed overdrive transmission has a notable place in the history of medium-duty and heavy trucks. Known for its robustness and durability through the mid-20th century, this transmission provides a unique combination of functionality and certain quirks, including a distinctive "howling" noise in overdrive that often draws concern and curiosity from owners and enthusiasts.
What is the Clark 5-Speed Overdrive Transmission?
Clark transmissions were widely used in medium-duty trucks during the 1960s and 1970s. Among them, a few models, notably the 280VO series, were equipped with an overdrive gear. Overdrive refers to a gear ratio that allows the output shaft to rotate faster than the input shaft, reducing engine RPM at high speeds for better fuel economy and less engine stress. The Clark 5-speed overdrive transmission is recognized for being rugged enough to handle significant torque and heavy loads common in medium-duty applications.
How the Clark Overdrive Works and Why It Howls
The "howl" sound reported in Clark 5-speed overdrive transmissions is often due to the design of the overdrive gear set, which tends to have a certain gear whine when engaged. This noise can be louder under load or at higher speeds and is partly inherent to the gear and bearing design.
Several factors can contribute to this noise:
Swapping or installing a Clark 5-speed overdrive transmission behind older engines, such as the Ford FE series V8s from the 1960s and 1970s, involves some challenges. The Clark transmissions typically use different bellhousing patterns and flywheel setups compared to classic American V8 engines.
Key considerations include:
Given the mechanical complexity and rarity of Clark 5-speed overdrive transmissions, many medium-duty truck restorers and custom vehicle builders prefer modern alternatives like Eaton Fuller transmissions. Eaton Fullers are more readily available, have superior parts support, and offer a wide range of gear ratio options with synchronized shifting.
For instance, Eaton Fuller FSO-5XXX series (five-speed synchronized overdrive) or the FSO-6406 (six-speed synchronized overdrive) models provide durable, tunable gearboxes that fit under pickup cabs with suitable mounts. They deliver reliability and drivability improvements without the sourcing and compatibility challenges of rare Clark overdrives.
Tips for Owners Facing Howling Issues with Clark Overdrives
A long-haul trucker from the 1970s once recounted how his Clark 5-speed overdrive equipped truck would emit a low-pitched howling sound on long highway cruises. Though slightly annoying, the transmission was so durable that despite the noise, it reliably powered his heavy loads across states without failure for several hundred thousand miles. This story highlights how functionality and reliability sometimes trump comfort in heavy equipment applications.
Term Clarifications
While the Clark 5-speed overdrive remains an interesting piece of transmission history and functions well in its intended environments, its noise and compatibility challenges make it less attractive for casual restorations or modern upgrades. For those dedicated to authenticity and vintage performance, investing in proper maintenance and understanding the unique design traits is essential. For others, modern transmissions with overdrive may deliver better performance, quieter operation, and easier maintenance, helping extend vehicle usability for years to come.
What is the Clark 5-Speed Overdrive Transmission?
Clark transmissions were widely used in medium-duty trucks during the 1960s and 1970s. Among them, a few models, notably the 280VO series, were equipped with an overdrive gear. Overdrive refers to a gear ratio that allows the output shaft to rotate faster than the input shaft, reducing engine RPM at high speeds for better fuel economy and less engine stress. The Clark 5-speed overdrive transmission is recognized for being rugged enough to handle significant torque and heavy loads common in medium-duty applications.
How the Clark Overdrive Works and Why It Howls
The "howl" sound reported in Clark 5-speed overdrive transmissions is often due to the design of the overdrive gear set, which tends to have a certain gear whine when engaged. This noise can be louder under load or at higher speeds and is partly inherent to the gear and bearing design.
Several factors can contribute to this noise:
- Gear Mesh Design: Overdrive gears, especially those made decades ago, might have straight or low helix angle gears, which naturally produce more whine compared to modern spiral cut gears.
- Worn Bearings or Gears: Age and wear can exacerbate noise, so a noisy overdrive could indicate the need for inspection and potential overhaul.
- Lubrication Issues: Inadequate or old lubrication can increase metal-to-metal contact noise.
- Mounting and Supports: If mounting rubber or supports deteriorate, vibrations and noise transmission can increase.
Swapping or installing a Clark 5-speed overdrive transmission behind older engines, such as the Ford FE series V8s from the 1960s and 1970s, involves some challenges. The Clark transmissions typically use different bellhousing patterns and flywheel setups compared to classic American V8 engines.
Key considerations include:
- Bellhousing and Mounting: Clark transmissions designed for medium-duty trucks have distinct bellhousing bolt patterns, incompatible with many lighter-duty truck engines without custom adapters.
- Flywheel Balance: Different engine families have various flywheel balance types (external vs. internal) and mounting specifics, making direct swaps complex.
- Clutch Components: Pressure plates, throw-out levers, and pilot bearings may require tailored solutions when mixing Clark transmissions with non-original engines.
- Driveline Modifications: Crossmember placement and driveshaft length might need adjustment due to differences in transmission length and mounting height.
Given the mechanical complexity and rarity of Clark 5-speed overdrive transmissions, many medium-duty truck restorers and custom vehicle builders prefer modern alternatives like Eaton Fuller transmissions. Eaton Fullers are more readily available, have superior parts support, and offer a wide range of gear ratio options with synchronized shifting.
For instance, Eaton Fuller FSO-5XXX series (five-speed synchronized overdrive) or the FSO-6406 (six-speed synchronized overdrive) models provide durable, tunable gearboxes that fit under pickup cabs with suitable mounts. They deliver reliability and drivability improvements without the sourcing and compatibility challenges of rare Clark overdrives.
Tips for Owners Facing Howling Issues with Clark Overdrives
- Diagnosis: Confirm that the noise is indeed gear whine from overdrive gears and not from worn bearings or insufficient lubrication.
- Maintenance: Use proper transmission fluid with correct viscosity and additives recommended for the transmission.
- Overhaul: If bearings or gears are worn, consider rebuild kits or professional transmission services specializing in vintage Clark gearboxes.
- Mount Replacement: Ensure rubber mounts and supports are in good condition to reduce noise transmission to the chassis.
- Lubrication Upgrades: Some operators have reported noise reduction by switching to higher-quality or synthetic lubricants suitable for gear wear protection.
A long-haul trucker from the 1970s once recounted how his Clark 5-speed overdrive equipped truck would emit a low-pitched howling sound on long highway cruises. Though slightly annoying, the transmission was so durable that despite the noise, it reliably powered his heavy loads across states without failure for several hundred thousand miles. This story highlights how functionality and reliability sometimes trump comfort in heavy equipment applications.
Term Clarifications
- Overdrive (O/D): A gear that spins the output shaft faster than the input shaft, reducing engine RPM at cruising speeds.
- Bellhousing: The protective casing that encloses the clutch and connects the transmission to the engine.
- Flywheel Balance: The method used to balance the engine's rotating assembly, which varies between engine families.
- Throw-out Lever/Bearing: Part of the clutch mechanism that disengages the clutch when the pedal is pressed.
- Synchromesh: Mechanism in transmissions that synchronizes gear speeds for smooth shifting.
- Clark 280VO Overdrive Ratio: approximately 0.80 (i.e., output shaft turns 1.25 times for each input turn)
- Common Years of Use: 1960s-1970s
- Typical Applications: Medium-duty trucks, commercial vehicles
- Comparisons: Often considered less user-friendly than Eaton Fuller transmissions
- Known Issue: Characteristic overdrive “howl” gear noise during operation
While the Clark 5-speed overdrive remains an interesting piece of transmission history and functions well in its intended environments, its noise and compatibility challenges make it less attractive for casual restorations or modern upgrades. For those dedicated to authenticity and vintage performance, investing in proper maintenance and understanding the unique design traits is essential. For others, modern transmissions with overdrive may deliver better performance, quieter operation, and easier maintenance, helping extend vehicle usability for years to come.
