kz650 big bore to 830cc

With a max. 25 Celcius you don´t need any oil cooler but it does´n´t hurt if you go that way.
Last summer we got 40+ Celcius here in Germany, riding a bike was like a skandinavian Sauna.

A thick liner doesn't dissipate heat better, but it does hold it's shape better.

There are lots of guys running 810 kits with a 750 block but personally I've wondered about long term seal from the thinned out liner. I wish there was a 2mm overbore kit instead of 3. Maybe I'm just paranoid though.

MTC back in the day offered 831 cc kit

There are lots of guys running 810 kits with a 750 block but personally I've wondered about long term seal from the thinned out liner. I wish there was a 2mm overbore kit instead of 3. Maybe I'm just paranoid though.

I bought my 1991 Zephyr with 11000 km on clocks. On 22000 due to shitty fuel I've got an issue with hole burnt through the piston in first cylinder. Since then I rode Wiseco 810 kit. Now bike has around 98000 km and I still have no issues with oil level which means that 810 kit with 3mm overbore worls as good as standard pistons with sleeve of original thickness.

Interesting stuff.

gazzz wrote:

There are lots of guys running 810 kits with a 750 block but personally I've wondered about long term seal from the thinned out liner. I wish there was a 2mm overbore kit instead of 3. Maybe I'm just paranoid though.

I bought my 1991 Zephyr with 11000 km on clocks. On 22000 due to shitty fuel I've got an issue with hole burnt through the piston in first cylinder. Since then I rode Wiseco 810 kit. Now bike has around 98000 km and I still have no issues with oil level which means that 810 kit with 3mm overbore worls as good as standard pistons with sleeve of original thickness.

+1

I had a 720 kit in mine with about 30,000 hard miles on it . When I swapped it for the 810 kit the cylinders and pistons looked great. I honed the cylinders, installed new rings and put it on another 650 I had. Still running great.

Maybe search Worlds biggest KZ650 on here for some info and Goggle "The GPZ750 modification FAQ" ... remember that MTC sold liners that were thicker for the 650 way back but a 750 block would be the way to go. They sold a 761cc and a 781cc kit also. Have a old MTC catalog around somewhere and I had a list for everything with costs in it...... pretty cheap but then so was everything compared to now.....can't believe I let the years slip on by and never got around to it....... Wiseco had a 740cc kit for a while too. Imagine how thin that was.......... I agree on a oil cooler only depending on where you live..... Arizona yes but Ilinois and never riding in anything over 85 like I do and maybe not.

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Some people claim a big bore engine will run hot but I don't understand why; if a bike is going down the road at xx speed the amount of power needed will be the same regardless of the engines displacement. So if, for example, 15 hp is needed to push the bike at 70mph why would an 810 engine make more heat to produce 15hp than a 750 or 650 engine would? I understand that an 810 has more power capability and because of that there is the possibility of more heat, but that doesn't mean the engine will run hotter all the time.

My 831cc street bike runs real hot without a oil cooler, on a +80F day when I pull up to a stoplight the heat radiates up on my thighs. I try to avoid stop and go traffic or idling it from extended lengths, airflow is the key to keeping to cool.

I don't run a oil cooler because I want the bike to look stock, sleeper mode.

I figured out that my compression ratio is a actual 10:1 from cc'ing the chamber, I am running 400 lift webcams set at 110LC, and my cylinder head is heavily ported and running valves bigger than the GPZ750 head.

I am not sure if any of my motor mods would make it run any hotter, maybe some extra compression or advancing the ignition would.

I know of a guy with a similar Z1 street bike, his is a 1200cc 11:1 in a 1015 block, he reports the same issues with heat (wild head/cams etc.)

It may be running hot because there is no material left around the liners for the fins to wick the heat away? Not sure... Maybe that 1 extra mm is enough to make it just on the edge for street riding.

I would have not mentioned the oil cooler if I did not have this issue with my bike, I know its basically a drag bike motor stuffed in a street bike so I expect there are limitations.

As far as a motor generating heat. I think a higher compression ratio would be the main reason for more temperature being generated than the increase in displacement.

I run a compression ratio right around 12.4 to 1 in my Kz1000 Superbike. When I rode it on the street you would not believe how fast it would start throwing off massive waves of heat if I got stuck in slow traffic!!!