KZ400 Cylinder Boring

I had a 1976 KZ400 given to me, the catch being the engine is seized. After pulling the head the cause was pretty apparent.... Thankfully I have a Bridgeport mill and I'm not afraid to tackle the boring job.

My question is, will I need a different head gasket to match the oversized pistons and rings? I'm not sure if there's such a thing as an oversized head gasket, but I'd thought I would ask before I started!

My other option is buying a decent cylinder block on ebay. Any chance a 440 cylinder and head would fit my early 400 bottom end?

I'm no expert, but this old thread here says that you can't fit a 440 head onto a 400 unless it's a 1978-1980 KZ400.

On head gaskets, I'm less experienced with motorbikes, but I'm pretty sure it applies to both cars and bikes. Unless you're going for a big bore, the standard head gasket will be fine. Head gaskets usually have a fair amount of clearance from the cylinder walls anyway. Have a look at your old gasket - if your rebore brings it out further than that, you might need a custom gasket (unless they do sell overbore head gaskets for these, overbore gaskets do exist).

Neither the later 400 (78+) nor the 440 top end will fit the early KZ400 bottom end (74-77). Not sure about the headgasket. Have you already obtained the oversize 400 pistons and rings? If not, they are pretty hard to find and if you do find them they are usually pretty expensive, but you might get lucky on Ebay.

You may end up spending much less getting another early engine to replace the existing or better yet upgrading to a later 400 or 440. But if you're set on overboring and refreshing the current one for the challenge then I'll definitely be along for the ride.

Its also maybe worth looking into a 500/550 top end. If youre going to do all the work anyway,why not go for a bit more umph!

Head gaskets are usually slightly oversize, I assume to allow for 1st and 2nd oversize up to 1mm. As long as the bore is no bigger than the gasket internal bore then you should be ok I think. Alternatively, cometic make oversize head gaskets.

You might get lucky and find a pair of NOS oversize pistons on ebay, alternatively depending upon the pin size you might be able to find oversize pistons rom something else, bearing in mind piston shape, compression and minimum cylinder wall thickness.

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