KZ400 head onto KZ440 case?

I know there is plenty of talk about putting a 440 head onto a 400 case, but what about the other way around? Specifically, the 400 lower cylinder sleeves would be smaller than the holes in the top of the 440 crankcase, would there by any negative effect with having a [larger] gap there? You would still have a gasket at the mating surfaces, but is a tight fit absolutely necessary between the outer cylinder sleeve and upper crank case?

The reason I ask is I have a seized '81 440 (left in the rain with no carbs for who knows how long, rings rusted into cylinder walls and pistons), but the low end is most likely serviceable (yet to be determined, could be a lost cause), and I have a '78 400 with a good upper end that I was thinking about putting on, just because I have the parts leftover from my other 3 projects and it would save me from having to buy yet another parts bike/engine or bore out the seized 440 cylinders and get oversize pistons and rings (which would lead to carb rejetting, most likely going 2-1 setup and new carb, pretty much a cash snowball which I want to avoid).

It looks like I might be able to side step that question and put the upper crankcase half from the 400 onto the lower 440 half, possibly? It looks like they are the same part except for the hole diameter in the upper case. I could probably answer that myself by going out to the garage and ripping into the 440 engine but it's freezing cold this week and I haven't freed the rings yet.

Otherwise, I might be able to just rebuild the 400 whole using needed salvageable parts from the 440. The previous owner of the 400 took the engine down to every last bolt and clip and when I went to put it back together I noticed a lot of missing things that got lost in the move. I know I can get a lot of stuff from partzilla if available, just not sure what my best option is as far as using what I have instead of buying a lot more parts.

I'm in no hurry, I have two bikes I just finished (three actually but I just sold one) and I'm just wanting to see if I can do anything with the leftovers

Only difference is the bore dia., cam chain and kick start.
You DO NOT swap case haves. They are machined as a unit.


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Well, I kinda answered my own question, but first, about the 400/440 top end...

Started honing the cylinders on my spare kz440 sylinders to see if they would be usable. Typical measurements after a tad bit of honing are at 2.658" across the board, which is 67.5132mm, spec calls out 67.6mm max and no more than a .05mm variation. At the pitting where the stuck piston rusted from sitting so long frozen, the measurement is 2.662" (67.6148mm). That's .116mm off, to one side, meaning to smooth that out I would need to increase the bore .232mm. Standard piston to cylinder clearance is .035-.062mm. Basically I would need to overbore to re-use the cylinders because honing out that pitting would throw it way out of spec. Now I get to decide if I want to ebay another 440 jugs and rings or just use the spare 400 jugs, pistons and I already have from another parts kz engine (only have to buy new rings). I'll probably go with the stuff I already have as this is only a spare engine I'm building for when one of the others go out...no need to buy even more parts (which is what led me to having 6 bikes in the first place).


(those big looking gouges above are just streaks in the oil. Disregard the dog hair too?)


SO, to answer the original question on if it would be an issue going from 440 to 400, instead of the other way around, I looked at the 440 cylinder sleeves in the top of the crankcase and it's definitely not a snug fit, plenty of room. SO, going to the 400 cylinders shouldn't be an issue, except for the lower power output. Conversely, I've heard that to fit the 440 cylinders to a late model kz400 case, you have to ream the top holes out to fit the larger diameter 440 sleeves. What I discovered is, the reaming out doesn't have to be precisely machined, just make the hole large enough, it's not an exact snug fit.