Cummins 855 Adapter Plate Fitment for Automatic Transmission Conversions

[MikePhua]

The Cummins 855 and Its Industrial Legacy
The Cummins NTC 855 series engine is one of the most iconic heavy-duty diesel platforms ever built. Introduced in the 1960s and refined through the 1980s, the 855 cubic inch inline-six powered everything from highway trucks and mining haulers to military vehicles and stationary generators. Known for its robust block, mechanical simplicity, and high torque output, the 855 remains a favorite among rebuilders and retrofitters.
With horsepower ratings ranging from 250 to over 400 HP depending on turbocharging and fuel settings, the engine was often paired with manual transmissions like the Fuller Roadranger. However, as automatic transmissions became more prevalent in off-road and vocational applications, the need for adapter plates to mate the 855 to Allison or Clark automatic gearboxes grew significantly.
Understanding Adapter Plate Function and Geometry
An adapter plate serves as a mechanical interface between the engine bellhousing and the transmission case. It must align the crankshaft centerline with the transmission input shaft while maintaining correct bolt patterns and pilot engagement.
Key functions include:

• Matching bolt patterns between engine and transmission
  
• Providing a pilot bore for torque converter or input shaft centering
  
• Ensuring correct flywheel clearance and ring gear alignment
  
• Supporting starter motor mounting and engagement depth
  
• Maintaining axial alignment to prevent shaft wear or vibration
  

In the case of the Cummins 855, adapter plates are often custom-machined or sourced from specialty suppliers due to the variety of transmission configurations used across industries.
Common Transmission Pairings and Adapter Challenges
Popular automatic transmissions mated to the 855 include:

• Allison HT740/750 Series
  • Used in dump trucks, fire apparatus, and oilfield rigs
    
  • Requires deep bellhousing clearance and specific torque converter pilot dimensions
    
• Clark 18000 Series
  • Found in mining and logging equipment
    
  • Often uses a separate flexplate and drive coupling
    
• Twin Disc TD61/TD81
  

• Marine and industrial use with remote-mounted converters
  
• Adapter must accommodate external oil cooling and pump drives
  

Challenges in adapter plate fitment include:

• Misaligned pilot bore causing input shaft binding
  
• Incorrect flywheel depth leading to starter misengagement
  
• Bolt hole mismatch requiring re-drilling or inserts
  
• Vibration due to poor axial alignment or unsupported weight
  
• Cracked plates from improper torque sequencing or material fatigue
  

A technician in Alberta once retrofitted an HT750 to an 855 in a snowplow chassis. The adapter plate had been machined ⅛ inch too shallow, causing the starter to grind the ring gear. After correcting the depth and shimming the starter, the system ran flawlessly.
Material Selection and Machining Guidelines
Adapter plates are typically made from:

• Cast aluminum: Lightweight and corrosion-resistant, but prone to cracking under heavy torque
  
• Steel plate: Durable and weldable, ideal for high-load applications
  
• Billet aluminum: CNC-machined for precision, often used in racing or custom builds
  

Machining tips:

• Maintain concentricity between pilot bore and bolt circle within 0.002 inch
  
• Use dowel pins for repeatable alignment during installation
  
• Surface finish should be flat within 0.001 inch to prevent gasket leaks
  
• Include access holes for torque converter bolts and inspection ports
  
• Verify starter pocket depth and angle before final assembly
  

A restorer in Texas built a steel adapter plate for a Twin Disc transmission using waterjet cutting and TIG welding. After aligning with a dial indicator and test-fitting the torque converter, the plate was powder-coated and installed with zero runout.
Installation Procedure and Torque Specifications
To install an adapter plate correctly:

• Clean all mating surfaces and remove burrs or corrosion
  
• Use thread chasers to clean bolt holes and apply anti-seize
  
• Install dowel pins and verify pilot bore engagement
  
• Torque bolts in a crisscross pattern using manufacturer specs (typically 75–120 ft-lbs for ⅝-inch Grade 8 bolts)
  
• Mount starter and verify ring gear engagement with feeler gauge
  
• Rotate crankshaft manually to check for binding or misalignment
  
• Install transmission and verify input shaft depth and spline engagement
  

If using a flexplate, ensure it is balanced and compatible with the torque converter bolt pattern. Always use new fasteners and lock washers to prevent loosening under vibration.
Field Modifications and Retrofit Success Stories
In 2021, a mining crew in Nevada retrofitted a Cummins 855 into a haul truck originally powered by a Detroit 8V92. The team fabricated a steel adapter plate and used a custom flexplate to mate the engine to a Clark automatic. After 200 hours of operation, the system showed no signs of wear and improved fuel efficiency by 12%.
Another example comes from a logging operation in British Columbia, where a Twin Disc transmission was paired with an 855 for a yarder winch. The adapter plate included a PTO mount and oil pump drive, allowing full hydraulic integration. The machine ran continuously for three seasons with minimal maintenance.
Conclusion and Recommendations
Adapter plates are critical components in engine-transmission conversions, especially when mating legacy engines like the Cummins 855 to modern or specialized automatic gearboxes. Precision machining, correct material selection, and careful installation ensure long-term reliability and performance.
Recommendations include:

• Measure all dimensions before fabrication and verify against transmission specs
  
• Use dowel pins and pilot bores to maintain alignment
  
• Select materials based on torque load and operating environment
  
• Test starter engagement and flywheel clearance before final assembly
  
• Document all measurements and part numbers for future service
  

With the right approach, the Cummins 855 continues to power heavy equipment with unmatched torque and durability—adapted to modern drivetrains through engineering and craftsmanship.