Diagram of a Crankshaft and Technical Terminology Reference

Crankshaft Diagram

Small Block Chevrolet Crankshaft Diagram

  • A: Threaded Crankshaft Bolt Hole / Harmonic Balancer Threaded Bolt Hole
  • B: Crankshaft Snout
  • C: Main Journal
  • D: Rod Journal / Pin
  • E: Balance Hole
  • F: Counterweight
  • G: Oil Hole
  • H: Crankshaft Thrust Flange
  • I: Rear Main Seal Surface
  • J: Rear Flange
  • K: Flexplate or Flywheel Bolt Hole
While understanding the technical terminology of individual crankshaft components is useful, especially when visiting your local automotive machine shop, it is important to understand how these components function. Below you will find more details regarding the diagram above.

A: The crankshaft bolt hole is designed to hold the harmonic balancer onto the crank.

B: The snout on most crankshafts have a key inserted into a keyway to accommodate the timing chain crankshaft gear. The length of the snout allows it to protrude through the timing chain cover so that a harmonic balancer may be securely installed and accessible from outside of the engine block.

C: The main journals on a crankshaft are all in line with each other. This allows the crankshaft to fit within the engine block and is secured by main caps with both upper and lower main bearings between the crankshaft and main bores.

D: The rod journals, which are also commonly referred to as pins, are where connecting rods are attached to crankshaft. As with the mains, upper and lower rod bearings are first installed into the connecting rod housing bores before assembly.

E: Balance holes are required to finish balance the rotating assembly. A balanced crankshaft, and rotating assembly, is essential to reduce vibration and to extend the life of the engine.

F: Counterweights on crankshafts are required to offset the weight of the pistons and connecting rods as the crankshaft rotates. The counterweights allow an experienced automotive machinist, with the proper equipment, to balance rotating assemblies with extreme accuracy.

G: The oil holes on a crankshaft are essential so that oil may travel from the oil pump and to the engine bearings. An oil hole which is obstructed, or not receiving any oil at all, will lead to engine bearing failure rather quickly.

H: The crankshaft thrust flange accommodates a special main bearing which greatly reduces crankshaft endplay (back and forth movement). Should a thrust bearing fail, in most cases it is the fault of a failed torque converter or improper driving habits for those with manual transmissions. Other factors, such as a failed engine pulley, may also lead to thrust bearing failure.

I: The rear main seal surface allows either a rubber or rope seal to keep engine oil within the engine block. Some seal surfaces are also diagonally notched so that oil is redirected back towards the engine block as the crankshaft is rotating.

J: The rear flange is where the flexplate or flywheel is mounted at.

K: The threaded holes in the back of a crankshaft are manufactured so that a flexplate or flywheel may be securely mounted to the crankshaft.

Please note that the diagram and information above is designed to give those with limited automotive knowledge a basic understanding of a crankshaft and to serve as a reference for them. To further expand your understanding of internal combustion engine crankshafts, please visit this page to learn more.
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