For those who don't know, kawasaki made their small triples in 250 (S1 and KH250), 350 (S2) and 400 (S3, KH400) sizes. They are all basically the same bike, the main differences being barrels, pistons heads and carburettors. You can drop a 350 top end straight on a 250 engine. In fact, very early S1s had cylinder liners 3 feet thick because the same castings were used on the 350s. Of course, spotty youths riding 250s on L plates soon figured that, and so had them bored out with no outward indication of what they'd done.
Problem is, the 350s are rare, peaky and thirsty. Getting pistons isn't the problem it used to be, but finding a useable set of barrels is quite a search. The 400 cylinders are more plentiful, but won't fit.
As you can see, the cylinder liner (i.e. sticky up bit) that projects into the crankcase is a bigger diameter on the 400 (left) than the 250 (right). So you need to bore the crankcase mouths out.
This is a mod well worth doing if you ever have your 250 (and probably 350 too) engine in bits. Even if you don't intend actually fitting another top end, the fit between liner and crankcase is a huge clearance. On the 250 it has about 5mm! (see below)
So you can safely bore out to take a 400 top end, then pop your 250 one back on with no ill effects. Should you do it later, you don't need to do a full rebuild. You could also pop a 250 set on your 400 too, if you're hard up.
There's a few other bits that need replacing too. Obviously you need 400 heads to match the barrels, and you'll need bigger carburettors. Stock 250 has 22mm carbs, 400 has 26mm, though some people fit 28mm or flatslides.
The clutch is different too. The 400 has a deeper basket to take an extra set of plates. To fit that in the primary gears are narrower too, though they are the same size as 250 parts. You'll need the matching primary gear on the crank too. I've not got that far yet, I'm hoping to unearth a clutch that will be easier to find.
Finally, the 400s have a slightly different cylinder stud arrangement, using shorter studs and sleeve nuts. This makes getting the heads on easier, as the fins are bigger. Just cut the 250 studs down and rethread.
Assuming you do the work as part of a rebuild that your 250 would need anyway, then the extra cost isn't actually that much. When I've got it all finished I'll put up a breakdown, the main cost is a set of carbs.
Anyway, on to the metal cutting.
Dismantle your engine, including removing all the studs from the top half. These are very tight, they may need heat.
Clean up any burrs or damage on the crankcase joint face, and the
case/cylinder joint faces.
There is one complication. You need to pick up the centres of the mouths to bore them, but they have big cut outs for the transfer ports. In addition, on 250 cases the mouths are cast finish, not machined.
For the latter, give the mouths a light sanding to remove the worst of any raised lumps and bumps. To be able to clock them up, you need to make a ring to fit in the mouths.
Note the shoulder, so it sits square on the case/barrel joint face
Make it so it just goes in the tightest of the three holes. If it is slack in any of the others, wrap a sheet of paper round it. The bore and the diameter in the case mouth must be concentric, as you clock the bore.
You'll need some clamps, and a decent boring head. I bought one off ebay for about £60 and was amazed how good it was. Your mill will need to have a quill feed, and be able to angle the head in at least one direction. Ideally you want enough travel to be able to pick up all three centres without having to move the case.
Clamp the top case on the mill, loose enough to still tap it true. You need it positioned so you can angle the mill head parallel to the mouths. Clock across the face of the crankcase mouths like so (between my finger and thumb in the pic), to get the case square to the travel of the bed.
You'll now need to tip the head over. Set at roughly 20 degrees, then clock it true. Using the clock mounted in the spindle, spin it through 180 degrees, using the slip as before.
You now need to find the centre of the first hole. Put your bung in, and clock round the hole to get centred. Lock the X and Y (front/back and side/side) slides, and zero the Z/knee dial.
Remember, if you alter the up/down position, it will move the side/side position due to the angle. As long as you wind back to the same position you'll be OK, but make sure you have enough room to get your boring head on. Which you're now ready to do, so get on with it.
Touch on and machine in the usual manner. I took 10 thou cuts to be gentle. You'll notice that the cutting is intermittent at first, due to the cast finish in the mouths. It should cut fairly evenly, but if you're touching really heavily in one area only you might want to move position a bit. Also remember that with it being cast, the mouths are tapered a bit, so you'll cut heavier as you go down.
I highly recommend that you measure your own barrels for fit. However, if you don't have them yet my measurements are:
Barrel diameter 2.518"/63.96mm. Schurminator on the KH triples board measured his 400 as having a 0.4mm (15 thou) clearance, so bore your cases out to 64.4mm/2.533".
Spigot length: 25mm. Whilst the base gasket does reduce this a bit, don't rely on it. Make sure the barrels seat firmly on the face with no gasket, dremeling out the lip if needed. If you do have trouble once the bottom end is assembled, you should be OK putting a bigger chamfer on the barrel spigot to clear. The length I got after machining to touch on the ports was just, just enough.
Before you move on to the next mouth, put your barrel on and check that all the studs go in. If you've been careful they should do, but if you cocked up the position it could be too off centre to get them in. If that happens don't worry, just machine a bit more out. The studs seem to be the only thing keeping it centred.
So, thats the machining out the way. Apologies if it seems a bit Janet and John and detail heavy, but the whole point of putting it up is that anyone else having a go can avoid my cock ups. The rest of the conversion will be posted later, when I've actually done it.