KZ200 to KZ250 cylinder jug interchangable?

Bike is 1978 kz200. Bike has poor compression (85 psi), so I tore down the engine. The cylinder is worn past the service limit according to the manual. If Im going to go to the trouble, I thought I might as well do a "big bore" upgrade to the bike, rather than getting an OS piston/ring kit. It appears that Kawasaki used the same base cylinder, and just bored out the 250 the tad more, while keeping the locating pins, oil passages, and cam chain tunnel all located in the same spot. I cant find any thread here via the search function that fleshes out whether a kz200 bottom end and kz250 cylinder jug are plug and play or not; lots of "yeah, you can probably do that" but no definitive, "yes I did it and here's what I did to make it work".

I have purchased a kz250 cylinder jug, and piston, so I guess I can report back when those arrive, but am I missing something? I cant find any information on crank throw or con rod differences between the two, which is the only thing I can think of that would throw a wrench in my plans. Looks like the passages all line up, and locating pin locations are the same for both.

A quick check at Partzilla shows the same part numbers for the cases and the base gasket. However, the crank and rod assemblies have different numbers, as do the cylinders and heads. I also noticed that the KZ 200 shares a lot of parts with the KLR 250, so that might be a possibility.

Also, the piston pins have different numbers. Maybe a difference in length only.

The factory service manual should have many, if not all, of these criticle dimensions listed.

Thanks for the reply. That makes sense. Looks like I will need to get a crank assembly and piston pin as well to do it right. I will check my service manual to see if the dimensions differ between the models.

A further web search suggests that the stroke is also different.
KZ200 - 66mm bore X 58mm stroke
KZ250 is 70 X 64

If those dimensions are correct, the 250 cylinder and sleeve would have to be longer, or the piston pin bore would have to be higher in relation to the top of the piston. Ether way, you would end up with really low compression.

TexasKZ wrote: A further web search suggests that the stroke is also different.
KZ200 - 66mm bore X 58mm stroke
KZ250 is 70 X 64

If those dimensions are correct, the 250 cylinder and sleeve would have to be longer, or the piston pin bore would have to be higher in relation to the top of the piston. Ether way, you would end up with really low compression.

Wouldn't swapping the crankshaft assembly from a kz250 fix this issue?

Yes, as long as the stroke is the only difference between the two cranks.

TexasKZ wrote: Yes, as long as the stroke is the only difference between the two cranks.

Looks like kz250 crankshaft assemblies with con rod are pretty cheap on the used market, just going to dive in and do it all the way. Will update the post when I have received all the parts and cracked the cases and seen what Im working with. Thank you TexasKZ, I know just enough to be dangerous, your input was very helpful.

Decided to get a 250 head too, all in for "new" 250 top end it was $115; head, jug, piston crankshaft and con rod out of a 250, which is cheaper than it would cost to get it bored and sleeved oversize. Fingers crossed it all works and I can report back some actual results on the 200 to 250 swap.

Looking forward to some definitive answers.

I have the engine back together and in the bike. I compression tested it: 165psi cold, which is way better than previous. Overall dimensions of the engine are the same; it bolted right back into place perfectly. I was intending to post a video of it running tonight, but I lost spark for some reason so I’ll have to chase that down in the next couple days. More updates to come, but so far so good.

Have it running now, but poorly. Turns out the intake boot for engine to carb connection is not interchangeable, even though they appear to have identical dimensions. There is a very large gap (like 0.5mm) between the head and the boot. I have ordered a new boot, but it wont run right until it get that sorted, and the boot isn't going to arrive til middle of May. Im going to say the swap was a success though. I did some measuring while I had both cranks out of the engine, and it seems like they're completely identical. I bet you could get away without splitting the cases and swapping the crank, but I'll leave that for someone else to test.

To do the swap I used KZ250 parts for: Crankshaft + Piston, Cylinder jug and cylinder head (including valves + tach gear), and intake boot. On these parts I spent ~$130, but it could be realistically more like $200. I got a head that needed a serious rebuild in the valves department, and scored a pretty good deal on a jug and piston combo. I will say that you can definitively swap the top end of a 80/81 KZ250 onto the bottom end of a 78/79 KZ200 (minus the crankshaft, which appears to differ in part number only but I havent tested that).

Good to know. Thanks for the update.