TOPIC:

Decided to go ahead and start my 73 Z1 build thread since technically I've already started. I'm starting with a 73 powder coated frame and swingarm and a 73 motor that came from a different bike. A NOS OEM Kawasaki gas tank I was told was manufactured in 83 and a 73 rear wheel assembly missing the sprocket hub. I've got the cases split on the engine and so far have only had use two or three outer gear bearings from a '76 motor I've torn down to replace rusty ones on the 73 that was missing the inner and outer sprocket covers. All clearances in the crank and the gearbox that I could measure are all within normal limits. I'm in case prep mode now and will hopefully get it reassembled in about a week.

keep us updated
 

Will do 

One thing I'm concerned with is that this bike has been bored out considerably to 69.86 mm. Should I worry about engine balancing?

I don't see why "balancing" should be an issue, others that know these motors inside out will be able to advise around "safe" overbore limits in regard to liner thickness however a "J" has a stock bore of 69.4mm for 998cc's so I don't think it's excessive (only my opinion) but the likes of Zed1015 will better be able to advise

Very common modification on Z1 engine was to overbore with KZ1000 pistons (70mm) to achieve 1015cc. Stock Kawasaki cast pistons are usually marked as "ART" on the insides, if not they may be aftermarket forged usually at a higher compression. No issues with that engine size similar to all stock Kawasaki 1000's.

 

Interesting build. I thought about a “mutt” build as I have about 80% of what would be needed. Your 70mm is considered 1015 which is the standard Kz1000 bore size so you’re good. No worries. 

No markings on the inside. The top of the Pistons are stamped COOK6 and a bunch of numbers. Glad to hear about the balancing.

They are not OEM pistons. They are aftermarket higher compression (brand unknown currently). Most common CR on aftermarket is 10.25/1. I have this set at home and can look at the markings, later tonight. 

Those pistons look like 10.25 -1 Wisecos but the Cook6 code may mean they are Cosworth.
As for the engine size, you can go as big as you like , they are self balancing as long as the crank is true..