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Third edit. While I concur with Kawasakian yeah it is a nagging mystery. These air cooled Kaw big fours were extensively raced and modified for decades, there has to be a guru corner or machine shop that still operates that can help identify the issue and give a concrete answer on what is the cause and should it be addressed and cost involved. That said I looked on the web but came up empty, maybe someone else can have better results.

 

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Looks like they still are raced.

 

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Post it here

Engine - KZRider Forum - KZRider, KZ, Z1 & Z Motorcycle Enthusiast's Forum

Camshaft, cylinder head, valves, cylinder block, pistons, exhaust, crankshaft, clutch, transmission, engine lubrication system, etc....

kzrider.com

 

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Since the motorcycle piston pin is free floating in the con rod small end the rods are sufficiently free laterally to allow crankshaft movement of a few mm.
Pic from

1978 Kawasaki KZ1000 motor build

step by step of the assembly of my 1978 Kawasaki KZ1000 motor plus the fabrication and general construction of a sweet-ass bobber from a 1978 Kawasaki police bike.

sites.google.com

shows the total gap between piston boss and rod.

 

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I think you're on the right track. Judging from this pic in the build link referenced earlier it appears the outer counterweight limits float to that side.

Have you drained the oil lately and examined the filter for metal shavings? An oil analysis can be done for a reasonable fee through Blackstone Labs. If the engine isn't eating itself I would just ride it. Locally.

Blackstone Laboratories

Learn what's going on in your engine with oil analysis. Our free kits are easy to use and we explain the results in plain English!

www.blackstone-labs.com

 

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I've not seen split keepers in bike engines, only automotive. The mystery bearing is the answer to the issue, methinks, but all the other journals are so tight. This is above my pay grade. It will be interesting to see if someone comments, or you end up taking the lower end apart. Those engines are good for many other kinds of vehicles, such as trikes. I would like to see what's going on as I want to make a tadpole type trike using a Kawasaki 1100 engine with shaft drive, so I can hook it up to a solid axle with a differential.

This style, with a Kawasaki engine though, and a front brake, different looking tank, and a little wooden pick up box in back for carrying groceries. This is Ian Roussel's trike. He used a VW carb and made a custom intake out of exhaust pipe, on a Goldwing 1100 engine!! :

That had to be a late 70s to 80's build, I haven't seen Pro Trac bias ply 50s since then.

 

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I believe there's a specialized thread locker of some kind designed to secure captured bearing shells, perhaps Kawasakian can elaborate. I would think about this as insurance after a teardown and rebuild