BMW engine conversion thoughts

 Briefly reviving this blog to answer a question that comes up every few months on various commie bike forums/pages. This is just so it saves me having to write the same answer every time. Other than that, I got bored of spending ages writing drivel no one seems to read...

So: in 2016-18, I rebuilt a Dnepr MT of some kind with a BMW engine, completely new electrics, and everything else reworked to iron out the terrible build quality, and literally criminal levels of incompetence of a "restorer". 4 years and 5000 miles later, I think I've ironed out most of the mistakes, "temporary" bodges and got a pretty decent outfit from it.

The spec is:

Some kind of Dnepr MT chassis

1982 BMW R80 engine, light flywheel, electronic ignition, all standard BMW.

Dnepr gearbox and final drive

Front disc brake, hydraulic morris minor drum on the side wheel, standard rear brake (inc parking brake)

12v electrics

Twin fuel tap tank

It has done the Dragon rally twice, several long motorway trips (150-200 miles each way), and probably annoyed everyone at NABD 2018 by winning best sidecar, despite not having £20ks worth of chrome and paint. It's done a bit of green laning, handles like a dream in snow, carries around a quarter ton of bricks, can sit at around 65-70mph for longer than I can not need a wee (not while carrying the bricks though), and I flog the arse off it everywhere. I'm dead pleased I made it.

In more detail:

Dnepr frames fit the BMW engine perfectly, I'm told Urals need the rails jacking apart. Frame rail does get in the way of the oil filter cover, but I've found I can pull the engine bolts out and drop the motor in the frames to get to it. Everything else stays connected, bar the rear exhaust mounts. Some people fit external filter kits of various types, others have a detachable frame rail. I can't be arsed to do either. Every bearing has been replaced, and every spacer and widget remade to closer fits, better quality and in stainless steel. Expect the fork bushes, which are brass. They should probably be some kind of bronze though. Wheels have new UK sourced rims (they're a standard size) and new spokes, again OE quality is dire.

Any BMW type 247 engine will fit. These are the R60, 75, 80, 90 and 100, from the early 70's /5 to whatever the last airhead twin was. (Note, not the R65, completely different engine) The bigger, the better, and if you even think of breaking an R90S or early /5, you want your teeth knocking in. Engines up to 1979 had a bigger clutch and flywheel, which can mate to the Dnepr box by fitting an R60/2 plate. After 79, they had smaller clutches, a chap in Germany did me a custom plate. You can mix and match BMW engine bits, but be aware that if you change clutches you'll need to reshim the engine. Go to Snowbums brilliant (yet long winded and mildly confusing) website for more info: https://bmwmotorcycletech.info/
Airbox is the Dnepr one, with air hoses to the stock BMW carbs made from garden pond hose. Exhaust is, now, stainless steel. Stock BMW headers with the ends chopped about to fit the stock BMW silencers, which in my case are the much later, shorter ones.

 Dnepr drive trains are apparently stronger than Urals. My gearbox internals look utterly horrible, the box sounds like two skeletons shagging in a dustbin, but it has given me no trouble. Again, new good quality bearings (ask for C3 grade, they have more clearance). The gearbox is adapted to fit the engine by welding up the bell mouth and remachining. Crude, but my DTI says it's not made the box any worse than it ever was. The casting is big and chunky, it didn't appear to distort. Bevel drive runs standard Dnepr solo gearing. Do not mix BMW and Dnepr gearboxes and bevel drives, or you'll have 4 reverse gears and one forward. Also, pay attention to the length of the gearbox shaft. The Dnepr clutch is two plate, if you leave it over length it will interfere with clutch operation, which is how I burnt mine out. Also, the gearbox output is through a rubber doughnut with a steel band round it. These are welded, badly, mine fell out with a tremendous clatter on a dual carriageway. Make a new ring from billet steel before yours falls out!

Stock Dnepr brakes are utterly dire. I found a disc conversion for a Chang Jiang, which mates into the drive spline in the wheel hub. I only ordered the disc, disc carrier and caliper carrier, then modified the latter to fit some calipers off a suzuki SV650 I had lying around. This is probably the most important upgrade. I'm also running a hydraulic drum on the sidecar wheel, on a separate pedal. This is great for corner assisting, and keeping the whole rig straight when braking. Brake itself is a bit weak though, I want to go disc on that too.

Electrics are completely rewired, based on the BMW wiring diagram for the whole bike. Remember you have double the lights on an outfit, so either upgrade the alternator or fit LEDs everywhere. Left handlebar switches are Dnepr, right are BMW for the electric start switch. I cut off the rh bar and had a smaller piece welded on to suit the BMW switches, think they go on a 22mm bar. Battery lives in the sidecar, you want as much weight in the chair as possible.

Fuel tank has two new bosses, one each side, threaded to take the excellent quality BMW fuel taps. You need this, the engine drinks like Boris Yeltsin. Before this, I really noticed fuel starvation problems at any decent speed. This was particularly bad at below 1/3 tank, which was hilarious because the original tank leaked if it was more than 3/4 full. This is now fixed.

There are changes I either will make, or would make if I did another. I would be interested to hear from anyone who has done any of these:

• 1000cc, heavy flywheel engine. I'd be curious to fit the softer R80 cam, and lower the compression ratio from stock R100 levels (you'll certainly need to do this on an RS engine, they don't like modern fuel). I think it would give a fat, lazy engine, lovely for sidecar work, but don't know for sure.
• Lower first gear ratio. I think there were some variations among dneprs, but I've no idea what they were other than some military bikes had lower first gears. That said, a bigger engine and chunkier clutch as above might negate the need for this.
• Fit disc to the sidecar. I've got some bits, but there's not much room, and I gave up and did other things instead
• If money were no object, have completely new gearbox internals made to better quality.
• Leading link forks sound interesting.

I might, one day, if I can be arsed, do even more detailed write ups on each bit of the bike, but don't hold your breath.

BMW into Russia-part 1, intro

So, in 2016 I bought myself a Dnepr 650:

The Dnepr when I first got it

It was sat on the Cossack Owners club stand at the October Stafford show, looking immaculate, and with a for sale sign smiling coquettishly at me. Gave it a slightly excited look over, clearly a fresh restoration. Some phone calls later and a deal was made. A week or so later I collected it.

I'd previously bought an MZ, despite everyone being an expert in how crap they are, and was pleasantly surprised at what a great winter bike it is-3years later and I've barely touched it mechanically. So, I was a bit sceptical as to how bad bikes from the Iron Curtain really were. I was in for a bit of education...

A few problems made themselves known immediately. The brakes were purely decorative, the wheels a bit wonky, and it wouldn't do more than 50mph. Within two weeks the engine had crapped out, and I'd found signs that the person who rebuilt it was a bit clueless. So, it got dumped under a tarpaulin for 6 months until the pain of shelling out 3 grand for a large garden ornament had subsided. Then I got a plan together. Everyone knows you can drop a BMW engine into these...

Why???

Most obvious question, why not put a chair on a nice BMW? Adding a sidecar to a bike is a very involved job, requiring all sorts of lugs and brackets adding to the frame. Also, the /5 and later BMWs aren't designed for sidecars at all, meaning lots of bracing is needed. Whereas the Urals and Dneprs are built as outfits from the factory, so will be easily up to the job.

Let me be clear though, Russian bikes are absolutely horrific quality. Everything is done to the lowest possible standard, presumably because only a decadent capitalist would take a bit of pride in his work. Also, they were built under one of the most dreadful regimes of modern times, so if the boss raised production targets by 500% you gave him whatever crap you'd hacked out. I will describe how to fix some of this later.

Here's a preview of the gearbox-the teeth were covered in burrs, circled, and the finish of the machining was dreadful

The biggest concentration of woe is the engine. The stock 650 lumps are pretty gutless, have poor lubrication and absolutely useless alternators. The later bikes suffer from too much power, meaning crankshafts tend to let go. Even then, they struggle to beat 55mph. Later Urals had 750 engines, which to begin with weren't much better. As well as the alternator, the rest of the electrics are pretty terrible too. Replacing engine and (by necessity) wiring removes a lot of problems.

Despite the quality, remember that the underlying design of the bike is German. They are well thought out, and easy to work on, once you address said build issues. They look like a much older bike and you get a reverse gear! Watching peoples amazement as the funny old bike suddenly goes backwards doesn't get old, and it is very practical. Some of the bikes come with a driven sidecar wheel, which apparently handles much better, and is great fun off road. This is only available on right hand chairs though-I'll come to them and their legal standing later. Sadly, mine is only 1WD, and it turns out you need an entire new sidecar chassis to fit 2WD. Still, it could be done.

Bottom line is, you get a fun, reasonably practical vehicle, with classic looks yet reasonable performance. It is not a job for the beginner though, and if you don't have a tame machinist, forget it.

At time of writing, I've had my outfit on the road for 3 days, and I love it. It handles well, and the engine has the perfect amount of power. I've not just thrown an engine in, I have reworked pretty much every part of the bike.

Over the next few entries, I shall describe how I did it. It won't be a definitive guide to bastardising all Russian bikes, but it will be as detailed as I can remember for this particular mix of parts. Some of it will be useful even if you're not doing a swap.
There are a lot of myths, contradictions, and the odd bit of utter rubbish out there. I'd like to pretend I could clear it up, but I'll only end up adding to the confusion I'm sure.

The rebuilt bike as it is now.

Quick Kwak update

Just realised I've not posted owt about the KH's pistons for a while.

After a lot of help and advice from the KH register triples board (thanks all), I still had no real idea what happened. So I bought another set of cruzin image pistons, and this time made sure every single clearance was right-not just ring end gap, but side gap and piston/bore clearance. Put a drop of oil in the petrol too for protection while the oil pump primed on start up.

So far, it has done 30 miles very happily. I'm still not entirely sure what happened, my best guess is I over revved it maybe? Anyway, it would appear cruzin image pistons are OK, which is a relief for the economy minded.

Next up, assembling a BMW boxer...

Piston disaster

Here is a run down on my problems with a set of cruzin image pistons for a KH400, 1mm o/size.
I'd done a fair bit of asking round, and having heard no comments worse than "They're basic but do the job", decided to buy them. Being as I'm using a set of barrels on max oversize, which has had some use but no serious wear, I didn't really see the point in getting top of the range wossners.

On assembly, everything was meticulously clean and well oiled. I checked the ring end gaps in the bore, and they felt to be a good, free fit in the grooves. For some reason I don't think I checked the actual groove clearance though. Still, on assembly they fitted fine, and showed no sign whatsoever of sticking. In fact, I had to take two barrels off to double check something, and the rings sprung out as soon as they came out the bore, as they should do. Obviously the ring gaps were matched with the pegs too. The rings were fitted the right way up, and in the right grooves.

The bike was kicked over several times as I fitted the carbs and exhausts, all the time it felt free, smooth and with excellent compression.

When assembled, I started the bike and ran it for about 40 seconds-1 minute, just to get a bit of heat in it and start priming the pump. Again, it felt and sounded fine.

I returned to it about 10 minutes later, and tried the kickstart again. It was immediately obvious that a lot of compression had gone, and it felt just a tiny bit gritty. Taking the head off revealed the internals to be covered in an oily grey liquid, which when checked with a magnet proved to have iron filings in. It has a very fine grittiness to it, more like toothpaste or chrome cleaner than grinding paste. Taking the barrels off revealed that all 3 sets of rings are now jammed solid in the grooves, and the crank cases have a little of the grey sludge coating the internals.

I've not been able to remove the pistons yet, they will need warming with a heat gun and I'd like to let the petrol fumes evaporate first. The pistons are scored fairly deeply, the barrels are too but I think a hone with emery should just about reclaim them. When I get the pistons off I can see if it got into the bearings, and also try and see why the rings are now stuck.

Below are some photos

I'm hoping I'll be able to rinse out the cases with plenty of carb cleaner, without having to do a full strip. If that is OK, I'll get the barrels honed, cleaned and new pistons in. Other than that, I'm pretty fed up. Still, pistons aside, the 250 to 400 conversion seems to work pretty well.

Improved boyer fitting for small kawasaki triples

As part of my KH2bitsa engine rebuild, I bought a boyer. I subsequently discovered they have a dodgy reputation in the club for being a bit wank, but having got the thing I thought I'd persevere.
The main thing I can see wrong is the awful crimped connectors on an otherwise fairly pricey bit of kit, and the use of the original, 35+ year old wiring loom. Talking to a mate who races a triumph trident running a boyer, he said that they don't do well with bad connections. In addition, I'm convinced that part of the reason the bike constantly fouled the LH plug was the crappy, corroded connectors.
To solve that, I've made my own wiring loom for the ignition. I'll doubtless rewire the whole bike eventually, so this is a handy start. Here is a step by step guide how, including all wire lengths and additional info if you want to keep points, but rewire the ignition anyway. All connectors are crimped the soldered on, which prevents corrosion getting in the joint and gives much better conduction. Obviously I should warn you that you do this at your own risk, etc etc. Also I've not actually ran it at time of writing, so let me know if you spot any major clangers! (edit-done 70 miles now, seems fine)

First, here's a shopping list. Order yourself  3 of these

http://www.ebay.co.uk/itm/170575512135

One of these

http://www.ebay.co.uk/itm/190759584648

And a packet of these

http://www.ebay.co.uk/itm/170711512131

These plugs and bullets all fit with original kwak plugs, which will be handy.

Now go to Maplins, and get some reels of yellow, red, white, black, blue and green wire in 0.2mm size. You also need some 5.3mm eye connectors (to go on the coils) and some heatshrink. I forget which sizes I got, but you want one that'll take 1 wire and one that'll take up to 6 wires. You'll ideally want one 6mm eye connector too, though if you don't fancy buying a whole bag you might be able to file/drill out a 5.3 one.

Here's what you get in your kit: A stator plate with 3 coils, rotor with 3 magnets, blanking plate, a magic black box and 3 diodes (wires with lumps in). You also get some shitty suitcase connectors, a couple of cable ties, a sticker and some instructions. The instructions look pretty clear, it's a shame what they tell you is a terrible lash up.

There is one major thing to remember with this system: unlike the points, the boyer fires all 3 cylinders every 120 degrees of crank rotation, thus giving two wasted sparks on each cylinder. Therefore the blanking plate covers all but the centre firing mark (used for strobing), and the 3 magnets and coils give 3 signals to fire at once, the box somehow taking an average of the three.

Remove all the alternator covers, and the points cam. Keep the centre bolt, but the cam is now retired.

Get your alternator, and remove all the points and condensers. These join the points cam in the retirement home for old triple parts. You’ll need to snip the 3 points wires.

Also, unless the screws holding the points on were immaculate and you have the correct JAS screwdriver, bin the bloody things and get some new bolts from screwfix. You can even go stainless steel!

You'll need to check the fit of the parts. The mask plate fits on the alternator thus, with the little hole lined up to the keyway and the big hole over the Centre cylinder firing mark

The new rotor has a roll pin that fits through the plate into the alternator rotor keyway. On mine, it needed some doctoring before it fitted, so check it now.

Note that the boyer rotor goes on last, don't out it on tight yet.

Finally, check the fit of the stator on the alternator casing. Mine needed the centre hole and outside diameter gently sanding out til it fitted. Don’t force it, the plate is brittle. You want to be able to rotate it freely, as you do when adjusting timing.

We'll start with wiring up the points side. For the boyer you only need two wires-green and blue. Cut a length of each to 590mm. If you’re keeping points but just want a new loom, you need a third black wire too.

Tin the wire ends, then crimp then solder appropriate bullet terminals to each of the wires. This will allow you to easily remove the stator if you need to. If you’re doing this on points, still put bullet terminals in, it makes condensor replacement much easier.

Don’t omit the rubber insulating sleeves on both male and female bullets, these go over the wire before putting the connector on. On two wires running together I usually put one as male and one female, so you can’t cock up the connections.

Put about 120mm of heatshrink on the two loose wires, and heatshrink the stator wires too. It neatens things up a bit. Give the bullet ends plenty of wire sticking out the heatshrink though, you need the flexibility for setting up.

Poke the heatshrunk ignition stator wires through the same gap that the original loom wires went through. Note that Kawasaki put them between the alternator winding wires, so that they’d be held clear of the rotor.

Since I'm in there, I put new wires on the alternator too. First job is to make new wires for the coils. Cut yourself a 620mm length each of yellow, and pink (or red if you can’t get pink) wire, and 640mm of white. Strip about 8mm of insulation and tin up one end on each

Note the alternator wires are 3 different colours. You might have to pull the outer sheaf back as shown to expose clean wire. Kawasaki have obviously taken great pains to ID these wires and ensure they only go into the regulator one way, so we’ll go with it. It’s probably important.

Do this stage one wire at a time, so you don't muddle the colours. Cut one on the coloured side, not the copper wire side, of the joint. Slide the insulation sleeve off and you’ll find this handy connector (green arrows in first pic). It's a bit blurry, because I'm complete crap at photography

As you can see, I've already removed the old wire. Heat it with a soldering iron whilst pulling on the snipped end, and you’ll be able to reuse it. Give it a good clean with electrical cleaner and a brass wire brush

Solder the correct coloured wire on of the 3 you cut before, then cover the joint in heat shrink, like this:

Repeat for the other two wires, note that the white wire doubles back on itself in the heatshrink. This is why you need the extra 20mm length, as I discovered after cutting mine too short.

The original wires were bundled together to keep them off the rotor. Use another bit of heatshrink over all three like this, it’ll give another layer of insulation too.

Finish the job here by putting about 30mm of heatshrink on the red, white and yellow wires, as shown above. (left of photo)

Cut a 440mm length of wire for the neutral switch. Kawasaki used a light green/turquoise colour, I used black because it’s what I had. If you’re still on points you already have a black wire, so use something else.

Cut two more bits of heatshrink sleeve, 90mm to go over the blue and green points wires (and black if you’re still running points), and the neutral switch wire of whatever colour you chose. Also cut a 70mm length, to go over the three alternator wires. Slide these over the other ends of the wires, those not attached to anything yet. The sleeves need to be 40mm in from the free ends of the points wires and 160 from the free ends of the alternator wires. Pull the wires tight and straight as you shrink the sleeve to avoid kinking later.

(note when I took this picture, I stuffed up the sleeve position on the alternator wires, hence they don't look like 160mm from the free ends)

Cut a 360mm length of heat shrink. Cut a small notch 140mm from one end, for the neutral wire. Slide this over all the wires, with the notch closest to the alternator/stator. Remember to pull the neutral wire through the notch:

See how neatly the two bits of heatshrink on the free end fit in the main sleeve. The other end will look just as neat.

On your old harness are two D shape rubber grommets, remove these and fit to your new loom once you’ve shrunk the sleeve.

Take the three alternator wires, and cut the insulation about 110mm from the end. Strip about 23mm off the end, then pull the remaining piece until you get this:

Twist the middle together, then fold in half and tin up, then solder on a male tab from the 4 pin plug sets.
I forgot to photograph that bit, but here's what it should look like, minus solder:

On the remaining end, solder the male spades from the 3 pin inline plug. Repeat for the other two alternator wires. Picture of the end result is further down.

Using the old loom as reference, fit the various tabs into the relevant plugs. Note the orientation of the plastic bodies, they only fit together one way.

 For the boyer wires (green and blue), you can either use bullets crimped and soldered as before, or another multi plug like I have. The neutral wire is the only thing left. Originally it is on the same multi plug as the points wires. Because I am going to plug the boyer box directly into the new plug (more later), I had to cut the neutral wire on the main loom out, and solder on a bullet connector like so:

 Eventually, you should get something like this.

Which will all plug together into the old loom to make this:

You can now bolt the engine back together. The blanking plate goes on the generator rotor first, with the small hole lined up to the keyway as shown at the start. The alternator goes on next with the stator plate for the boyer. Note that the first rubber grommet goes here:

And the second here, when you fit the sprocket cover

The boyer stator doesn't need to be bolted tight, as you set the timing by strobe once it is running. The last bit to put on is the boyer rotor, with the peg in the key slot as shown before.

Now for the magic boyer box. In the kit there’s 3 green wires with funny lumps in, these are diodes. Each coil needs one between the box and its negative connection. These must be connected the right way round, green wires go to the green wires on the black box, black wires go to the coils on the negative side. Boyer managed to crimp the wrong connectors to the wrong ends of the diodes on my kit, so if you're keeping them make sure they're right.
As standard, kawasaki have a small link wire that attaches to the coil terminals with an eye, and a bullet at the other. Boyer tell you to plug the bullets they fit into the bullets on those wires. For the two coils further back, I put an eye terminal straight on the diode, thus doing away with a connection. For the third coil mounted at the front, I made a new link wire.

Depending how you mount your box, you'll be best making your wire to suit. Note I've replaced the boyer crimped connectors with another multi plug, between the diodes and black box. Remember too that all the coils are now fired together, so you don't need to worry about which diode goes to which coil, or indeed which lead goes to which plug.

Talking of multi plugs, you'll need to put suitable connectors on the blue and green leads that go to the ignition stator. If you mount the black box with the wires facing backwards, you can plug the box straight into the new bit of loom without needing a link piece in between. Put a suitable piece of heatshrink on them first though.

 

 

The last two wires to connect are the power wires for the black box. The black earth wire wants an eye connector soldering on, big enough for an M6 bolt. If you look on the left hand side of your bike you'll see a load of earth wires go to one bolt, put it in here:

For the yellow wire, pick any of the yellow power leads into the coils and cut it's female connector off. Solder on a male connector, then make a new link wire with an eye on one end and a double female connector on the other. Solder a male connector on the black box wire, then connect them all up like so:

The last bit of fitting is to mount the black box. I used the cable ties as instructed, but with it facing backwards (see above):

That's about it. Timing is set using a strobe at idle, plug your strobe into any of the cylinders and rotate the back plate until the C mark lines up with the static mark. If you don't know how to do that, your local friendly bike shop should be able to help.
Hopefully someone will read this and use it, it took me longer to write up than to actually do it!

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.