Hey Steveo. I just went through the same project you are beginning. If you have any questions send me a U2U and I will do my best to answer your questions.
Here is what I ended up doing.
'77Kz650 bottom end from the cases upwards. The 650 bottom end is bullet-proof, but given that you will have to pull the motor anyway, take off the oil pan and inspect the crank and rods. It might be a good idea to put in new crankshaft oil seals at this time as you cannot put them in when the motor is back together.
The case openings where the connecting rods come up through the crankcase have to be slightly enlarged with a die-grinder or aluminum metal cutting bit to fit the bottom of the 750/4 cylinders-jugs. You are going to have to do test fitting as you proceed through the case opening process, so keep a marker handy to show you where the metal has to be gradually removed. This is not a big job, it just takes time and patience. It is messy so wear a dust mask as aluminum dust is very dangerous to the lungs.
I found a set of 1980 Kz750/4 cylinders and had them bored for the 810 kit from Wiseco. Try to get a reputable machine shop that specialises in high-performance bike engine rebuilds as they will do the proper cross-hatch pattern with the final hone. Additionally, make sure that they chamfer the bottom of the cylinders so the pistons and rings will slide in easier when you are doing the engine assembly. Call Wiseco and get the exact bore info that is required for the machine shop as you don't want anyone experimenting on your head.
I used my '77 Kz650 head and had it milled 10 thou to clean it up on the bottom surface. I also ported and polished the intake and exhaust runners, basically cleaning them up.
I used the cams from the 750/4 and put my 650 sprockets on them. This allowed me to retain the 650's roller chain assembly. The reason I used the 650 head is that the general belief is that the slightly smaller combustion chamber will give better low end power compared to the larger 750/4 head. I was looking for bottome end and mid-range grunt more than high end power.
If you decide to use Gpz 750/4 cams which will move the power into the upper RPM range, you will need to hook up an electronic tachometer as the Gpz cams don't have a machined gear drive for the tach on the exhaust cam.
The only change I had to make to the 650 head to make it fit the 750 jugs was to slightly enlarge the two bolt openings that fasten the head to the cylinders. Remember to use lots of red Loctite on the internal bolts.
Check the Kawasaki website for the '80 750/4 head specs and you will see that the two bolts at the front and rear for the 750/4 are slightly larger than the '77 650's. This is a simple fix with a sharp drill bit. If you use a 750 head this is not required.
After a lot of research and discussion with racers who have done the 810 big-bore, I decided to go with the larger 750/4 intake valves. I left in the stock 650's exhaust valves but did a triple angle valve job to clean up the seats. I went with Gpz springs all the way around as the 810 can easily pull 11,000 rpm and I wanted the piece of mind from the stronger springs.
I ended up doing a valve job and having new intake seats put into the 650's head. This was done by a professional machine shop who also checked the guides and installed new valve seals. If I remember correclty, I had 15 thou taken off the length of the 750 intake valves.
If you stick with the 650 head and modify it like I did then you can use the stock 650 valve covers. If you go with the 750/4 head and hy-vo cam chain assembly you will need to use the 750's valve covers. I believe the 750/4 had the reed valve/smog ports which cannot be accomodated with the 650 vavle cover.
I suspect that you will be going with bigger carbs and therefore you will need to change the carb boots or rubber manifolds. I ended up getting carb boots for the Mikuni 28's I purchased from Wired George and had to open up the bolt holes on the boots to fit the 650's intake runners..
You will also have to go with a different throttle cable to fit the new carbs as the stock 650/Mikuni 24 throttle cables will be too short.
So, there you go. A step-by-step tutorial on what I did and the motor I built pulls like a locomotive. You won't be disappointed with the 810 package and combined with the nimbleness of the 650 package you will have a nice little sleeper.
Cheers-Colin Firth-Ontario Canada
1977 Kz650 Custom -810 big-bore with lottsa hidden go-fast goodies tucked inside.
-1977 Kz650 Custom bought new by brother. Now with 810 kit, GPz750 cams, intake valves, Mikuni 29 smoothbores, velocity stacks, Dyna Igntion, MAC pipe and other goodies.
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