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:
- Piston Manufacture
- Piston Install
- Crankshaft Bend
- Piston Rod Bend
- 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.
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.