Case 780CK Engine A336BD: Torque Specs, Rebuild Challenges, and Field Wisdom

[MikePhua]

Overview of the A336BD Engine
The Case A336BD is a naturally aspirated diesel engine commonly found in the 780CK loader backhoe. With a displacement of 336 cubic inches, it shares design lineage with the Case 301 engine but features a larger bore. Known for its mechanical simplicity and robust construction, the A336BD remains a favorite among restoration enthusiasts and field mechanics.
Key Terminology

• Main Bearing Cap: Component securing the crankshaft to the engine block.
  
• Connecting Rod Cap: Fastens the connecting rod to the crankshaft journal.
  
• Torque Spec: Manufacturer-defined tightening force for bolts, measured in foot-pounds (ft-lbs).
  
• Crank Journal: Surface on the crankshaft where bearings ride.
  
• Undersize Bearings: Bearings designed for reground crankshafts with reduced journal diameter.
  

Torque Specifications

• Head Bolts
  Tighten in three stages:
  • First pass: 70 ft-lbs
    
  • Second pass: 140 ft-lbs
    
  • Final pass: 200–210 ft-lbs
    Use a crisscross pattern starting from the center outward.
    
• Connecting Rod Bolts
  Torque range: 95–105 ft-lbs
  Ensure clean threads and light oil application before tightening.
  
• Main Bearing Caps
  Typically torqued to 120–130 ft-lbs depending on bolt grade and lubrication. Always verify with a service manual for your specific engine variant.
  

Field Anecdote: Manitoba’s Midnight Rebuild
A mechanic in Manitoba attempted a late-night rebuild of a 780CK’s A336BD engine after discovering a spun #3 rod bearing. Without grinding the crankshaft, they polished the journal with fine emery cloth and installed standard-size bearings. The engine ran for 20 minutes before knocking resumed. The lesson: polishing may restore surface finish, but it doesn’t correct out-of-round journals or undersized wear.
Historical Insight: The Rise of Torque Discipline
In the 1970s, torque specifications became standardized across diesel engine platforms. Prior to this, many field repairs relied on “feel” or rule-of-thumb tightening. The introduction of calibrated torque wrenches and multi-stage tightening sequences dramatically improved engine longevity and reduced bearing failures.
Modern Case Study: Regrind and Retrofit in Ohio
A restoration shop in Ohio tackled a similar A336BD rebuild. After measuring crank journals with micrometers, they found the rod journal diameter at 2.473" and mains at 3.475". These readings confirmed the need for undersize bearings. The crank was reground to .010" under, and matching bearings were sourced. The engine passed break-in with no abnormal wear, and oil pressure remained stable under load.
Tips for Successful Rebuilds

• Measure Everything
  Use micrometers and plastigage to verify journal roundness and clearance.
  
• Don’t Skip the Grind
  Polishing is cosmetic—grinding restores geometry and bearing fit.
  
• Use Assembly Lube
  Protect bearings during initial startup.
  
• Check Bolt Stretch
  Reuse of old bolts can lead to inaccurate torque. Replace if stretched or corroded.
  
• Document Specs
  Keep a log of torque values, bearing sizes, and journal measurements for future reference.
  

Alternative Engine Options
Some operators explore retrofitting other Case engines like the 301 block with 336 sleeves and pistons. While feasible, this requires careful matching of crankshaft, camshaft, and head components. Turbocharged variants (A336BDT) feature different pistons and compression ratios, making direct swaps more complex.
Conclusion
Rebuilding the A336BD engine demands precision, patience, and respect for torque specs. From head bolts to rod caps, each fastener plays a role in maintaining oil pressure, bearing alignment, and combustion integrity. Whether you're restoring a vintage 780CK or troubleshooting a knock after rebuild, the key lies in accurate measurements and disciplined assembly. With the right approach, this classic diesel can roar back to life and serve for decades more.