Converting a KH250 to a 400, part one: Machining.

For anyone stumbling across this, I have a Kawasaki KH250. Go through old posts to see it's restoration, hopefully you'll learn a few things not to do if you have one.

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.

 Use any accurately ground parallel or slip gauge to clock, if you hold it on the face then clock it you avoid all the holes, gaps and any surface damage pits. Once it is as close as you can get, clamp down.

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.

Adjust the head angle to get both sides as close as you can to each other. This sets the spindle vertical to the holes you need to enlarge.

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.

Note that you won't get right to the bottom of the bore. The red arrow points at a double lip-one is from my machining, the other is the extent of the original bore. You can't get all the way down because you hit the transfer ports, as indicated by the green arrow. I learnt the hard way so you don't have to... If you go until you just touch on the ports, you should find the extra lip won't matter. Check with your barrels anyway, if needed you could dremel it out.

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.

Bantam improvements-the conclusion (for now)

I realise I forgot to finish off the bantam posts on here. Still not done the drawings, they'll come later. But for the wandering bantam botherer who is curious how well all that machining and that went, here's a quick summary.

Fantastic.

Admittedly I'd not rode the thing for nearly 2 years before rebuild, but my memory was of a bike only a mother could love. Sloppy handling, slow, painful position...I've now cured the first one. The steering is a dream, smooth and stable, quite safe to take your hands off the bars to scratch your arse or whatever. I'd recommend the head bearings, the sub-frame dowel and the tighter fork bushes to anyone reading. The slack swing arm should, of course, be done correctly in the first place!

Electrically, I rewired it (again!). This time I used nice chunky cable (think it was 0.25Sq mm), plenty of sleeving and heat shrink and japanese style bullet connectors, crimped AND soldered. The latter was the only slight let down. On the Enfield I used brass british style bullets, where you solder male ends on all the wires then join them up with either 2 hole or 4 hole connectors. The japanese style ones have male and one or two hole females, which is much less versatile. However the sleeves are better, obviously designed to actually keep rain out. Also fitted an upgraded stator, which you can buy from Hitchcock motorcycles as a RE 250 Crusader part. Again, well worth it. Whilst I'm on the subject, if you want an ignition key buy one of their Lucas repro ones. I tried using a small maplins key switch, which fell apart embarrassingly on the A52. The lucas one seems to be designed for turning on power stations, so should last.
Picture below shows the relocated electric in the seat hump, and the crappy piddly switch. Note also the neat little fuse box, available from most car shops. One fuse each for the battery and generator output, then two spaces for spare fuses.

And under the seat, showing the 12v reg/rect mounted in some air flow. I used big multi connectors for major junctions in the loom (seat, headlamp and bar switches), this works really well. Only slight flaw is I was over generous with the wire lengths, so have saggy bits of loom sticking out. Still, too short is far worse!

Had some carb problems once I got it going, after many weeks, jets, slides and rages I realised that the oval bore kawasaki KX80 carb was far too big at full throttle-about 36mm! Bought a 26mm brand new mikuni off Rex Caunt, which runs fine. Turns out japanese carbs are deliberately made with funny size slides and needle jets for different bikes, thus ensuring only stock bits fit. Buy a new aftermarket carb and you can get whatever you want. Lesson learnt!

Thundersprint was great on the saturday, despite being on such a tiny bike. Only downside was the lucas style coil crapping out on sunday, followed by torrential rain all day. So I didn't get a go, but wasn't all that bothered! Definately going next year, just have to decide between the bantam or the KH250(and a bit). Massive thanks to mates who turned up, and to Pat who took me in his van.

Here's a few gratuitous bike shots, taken by my Mum:

This bloke said the Gorgeous Biker Chick sent him. You either know who she is or are utterly baffled.

This turned out to be a good pic to get, No. 71 was ridden by Heather Barraclough, who won outright! She has owned the bike from new and rides it pretty much every day. This would explain how she was confident enough to thrash her road going, rainproof tyres whilst high end, uber-trick machines on semi slicks wobbled round the flooded track in great panic! Good on her.