I think many KZ650, KZ750, GPZ750 and Zephyr 750 owners, who overhaul engines of their bikes have questions if the primary chain needs to be replaced. This part is expensive, so no wonder that thinking about this issue may not be easy. Kawasaki KZ650,KZ750 and GPZ750 books say that the service limit for chain slack is 27mm. Chain play, according to those books has to be measured in next way:
It seems to be clear: if the service limit is achieved or exceeded, the chain has to be replaced. However, the chain itself looks to be tough and “lifetime” lasting. With high mileage it may become noisy, however, I’ve never heard about cases of broken primary chain. For example, my own Zephyr 750 has mileage close to 100K kilometers and it still runs the original primary chain. In fact there is information available about motorcycles with 160K and more kilometers that still have no trouble with the primary chain. Well, I decided to dig deeper into this matter.
Kawasaki Zephyr 750 and ZR7 engines share part number of primary chain with KZ650-750 as both are their direct descendants.
First thing, I took a look into ZR7 factory service manuals. I wasn’t surprised not finding any information on primary chains there, because the Kawasaki ZR7 engine is equipped with the tensioner.
Some time ago I received the newest of available editions of the Factory Service manual for Zephyr 750 (part number 99924-1138-03, printed in 1997). As I already mentioned, Zephyr 750 has the same part number for the primary chain as its predecessor: KZ650 and KZ750. And similarly to them Zephyr has no primary chain tensioner. So, information provided by the aforementioned factory service manual may be considered as the final word of mother Kawasaki about primary chains of 650-750 KZ-s, GPZs and Zephyrs.
Now guess what service limit Zephyr 750 newest factory manual states for primary chain?
Answer: none. There is no service limit for primary chain slack and the procedure of its measuring is completely and totally absent. The only indication of issues with the primary chain I found was in troubleshooting where it was said that if there is extensive noise, the primary chain has to be replaced.
So it looks like Kawasaki decided that the primary chain for our engines lasts as long as it isn’t too noisy for the ears of motorcycle owner. But sure, it’s up to you to make the final decision. If you are building a fancy and looking-like-new engine I’d rather recommend replacing the primary chain while you are still sorting out the engine. Doing it later may end up in additional expenses like gaskets, O-rings and so on.
P.S. There is also information that Honda CB900 Super Sport (Bol D’or) 1979-1981 primary chain (13610-438-004) fits KZ650-750, ZR750 and other Kawasaki models that share 13122-007 part number for primary chain. One may have thought like this: “Oh, if this chain is used in a 900 ccm bike, it might be tougher than the KZ chain”. Personally I have doubts about if it’s stronger than the KZ chain. I think it’s more likely Honda decided that such a chain (equipped with a tensioner, by the way) is enough for 900ccm engine. But it’s only my educated guess. As for the facts: the number of plates in every column of the CB chain is identical to those of KZ: 9-8. Thickness of the plates seems to be identical too. Therefore, the only way to get answer for question about chains strength is to lay both of them side to side and make measurements.
The Honda primary chain used to be cheaper than Kawasaki , so no wonder it was in demand among Kawasaki owners. However, nowadays there is a clear issue with the Honda CB900 Bol D’or primary chain which makes it less desirable. These days Honda genuine chain is “is no longer available” from most of the suppliers and there are too many not only non OEM but completely “noname” products on market. Some sellers state that “product is as good as OEM” or “Made in Japan”. And the fact is: a lot of low quality products were sold and still are on sale under those mottos…
The prices of CB primary chains that still have OEM packaging to prove their origin are mostly equal or sometimes higher than that of KZ. Therefore, personally I voted for the OEM Kawasaki primary chain. It lasts long enough to be worth every dollar thrown in it.