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I was changing the cylinder head of my kz650, that I learned has kz750 cylinders and now new (old) cylinder head. After installing the new head and fitting the camshafts, I spun the engine a few revolutions with a 17mm wrench to see that everything spun freely, which of course it did. But I did notice a rather worrying "thump" at a certain point when spinning the engine. I pulled the cover of the left side to where the "generator" or what it is called in english is positioned and found that I could actually pull and push the crankshaft from side to side and re-produce the exact sound that I heard. I am pretty sure that it is not supposed to be like that?

I can say that the engine ran and did not really make any seriously bad noises. First time I started it when I got it home it sounded a little harsh and "clunked" more than I would have liked, but it was like after it had been run and started a few times, then this "clunking" disappeared. I am now pretty sure that what I heard was this lateral movement of the crankshaft.

But any thoughts on why/how that can happen? It seems pretty certain that I would need to pull everything apart and have a look this winter. But I am so far in re-assembly with the new cylinder head that I will finish it and then start it and re-evaluate.

There will always need to be some tolerance there for oil film. you might be surprised by how much lateral movement is tolerable and still considered in spec. Unfortunately I think you need to dissemble the cases to measure this. I have my 750 twin cases split right now and measured this because there is some wear on the thrust surface. I'm debating getting new cases because of this damage. my cases are over the standard range but still in spec. snip from 1976 KZ650 manual
. you night be able to measure through the oil pan.

DoctoRot wrote: There will always need to be some tolerance there for oil film. you might be surprised by how much lateral movement is tolerable and still considered in spec. Unfortunately I think you need to dissemble the cases to measure this. I have my 750 twin cases split right now and measured this because there is some wear on the thrust surface. I'm debating getting new cases because of this damage. my cases are over the standard range but still in spec. snip from 1976 KZ650 manual
. you night be able to measure through the oil pan.

Thanks a lot! I am pretty sure that it is more than that, but I will have to find some way to measure it. Your idea with the oil pan is quite good because that would probably be the easiest way in. It is a bit sad to see that the only way to do anything about it is to change the casings though.

A dial indicator on the end of the crank will work. You can get one for about $30 at Harbor Freight if you don't already have one.

Hey! Once the motor is back together and running an easy indicator of Crank float and this goes for everyone with an old zed is listening for a sound change when the bike is on its side stand, a lovely old chap I knew who died last April was a kawasaki mechanic from 71 to 2010 he said the 900 and 1000s are quite whirry engines and a noisy whirring while the bike is on its side stand is perfectly fine provided that a second or so after the bike is upright it runs a lot quieter, if not then yep too much lateral movement! Incidentally he told me NEVER leave the bike running on its sidestand, always use the Centre stand unless you've got rid of it I guess! Oh and apparently the Suzuki gs1000 is even more noisey running on its sidestand I'm not sure why!!

I wonder if that applies to the kz650 / 750 because the z1/kz1000 have roller-bearing cranks and are pressed together. does the roller breaing crank thrust on the case like the plain bearing cranks of the smaller engines?

Here are the thrust surfaces on my KZ750twin. as you can see there is galling and heat discoloration apparent. This thread reminded me I needed to do something about it so I just made the decision to replace. New cases were only $85 shipped thankfully, hopefully they are good. I just checked the manual for the KZ1000. it has a thrust bearing pressed on the crank so the cases will not wear.

Been there done that. On the needle bearing crank on the J engine, I discovered the thrust bearing being loose in the case causing excessive end play. The cause was the case no-longer gripping the bearing's outer race when torqued down. The reason, I believe, was a twisted crank that expanded the case. Replaced the case, and crank, and bearing is now tight.

Thanks a lot for all your input. I have to try the trick with having it running on the sidestand vs. centerstand. I re-assembled the remaining things yesterday and had it running with its new cylinder head. But there weren't any overtly worrying noises coming from the crank or con rods. I still need to change the oil and make some final adjustments one of the following weekends. But I think I might be able to run it at least for a couple of months if I keep it somewhat away from the redline. Then I can consider doing a thorough rebuild in winter.

Sounds like a plan it's always a worry when you've had a motor in bits then re assembled and you have images of sticks of dynamite in your head and with the air cooled kawasakis there's so much extra mechanical noise to hear vs a water cooled bike, but by the sound of it you've been very careful, patient and methodical and I'm sure the bike will be fine