Engine

Engine Rebuild #11 (Cylinder Install Problem – Bent Piston Rods)

CB750 piston with bent piston rod

First major problem on the CB750.  When installing the cylinders on the motorcycle, cylinders 2 and 3 were not centered in their respected cylinders.  Both were bend inward towards the engine center.

It was such a bad seat in the cylinder that I couldn’t cycle the engine with the kickstarter.  It’s important to note that I had already had the cylinders bored to an 836cc total engine displacement from the 750cc stock engine.  Also, the cylinders and head were disassembled when I purchased the motorcycle, so I couldn’t assume the pistons were seated properly in the cylinder prior to the rebuild.  Because of these circumstances, the problem could have been several things:

  1. Piston Manufacture
  2. Piston Install
  3. Crankshaft Bend
  4. Piston Rod Bend
  5. Bad Bore to 836cc

Because the piston stayed inward through the stroke, it was unlikely that it was anything other than the piston rod.  You can also see it after I set the crankshaft with the piston rods attached on a level surface and measured the level.

'75 CB750 Bent Piston Rod

NOT LEVEL.  OOPS.

So, I ordered new rod bolts, nuts, and bearings and will be having a machinist bend the rods back to spec….actually not an expensive fix to a major problem.

I have no idea how both 2 and 3 could have been bent inwards.  My only thought is that a first 1 or 2 over bore job was done incorrectly and these bent the rods over time.  I welcome everyone’s comments and theories on how these could have gotten bent.

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