First, why bother?
The originals are cup and cone ball bearings, which are an utter bastard to assemble. Stick the lower balls in with grease, carefully slide bottom yoke into headstock, dislodge a ball, remove yoke, try again, lose another ball, chuck the hateful bastard thing through the nearest window. When I did it back in 2008, I took the frame and yokes into my bedroom, and assembled it on the soft, carpeted floor. That way, if a ball does drop, it won't bounce off into a corner. If you must insist on the dreadful things, and take your bike inside, put an old shirt down to keep worst of the grease off the floor, and give extra cushioning.
Anyway, as well as being nicer to assemble, taper rollers make the steering much smoother. The rollers have more contact area, so don't pit and notch as readily as balls.
Perhaps most importantly, the quality of replacement cup and cones is dire. I ordered a set for over £40 (seems the going rate). The tracks were obviously turned, not ground, and one bearing hadn't even been finish machined-the track had heat treatment discolouration, and chatter in the track. It's the one on the right in the above photo, the left one is original, showing the shiny ground track where the balls run.
The original bearings are imperial sizes, both of them 1.875" OD. The top ID is 1" dia, the bottom is 1.065". You can't get imperial taper rollers in these sizes, unless you pay someone to tool up for a run of many hundreds. Instead, you can buy metric tapers for a tenner each, and do some metal cutting!
You will need:
A boring head
An angle plate
Some steel bar stock
A housemate with nowt better to do than patch you up on a saturday night.
I used two 48mm OD x 27mm ID bearings. The size of the inner diameter on the original bottom bearing is very fortunate, as it is 27,05mm in metric. You want an interference fit (i.e. tight, or needs interfering with a hammer). 0,02-0,05mm (or 1-2 thou in imperial) bigger is about right, so a 27mm ID bearing is perfect for this. Thanks, BSA! The top bearing has a sleeve fitted, to make it a 25,4mm, or 1" ID. You can't machine the stem down to 25mm unless you want to cut a new thread in the top and make a new nut.
Are you keeping up with the imperial/metric conversions? Good.
The outer diameter is more of a headache, 48mm=1.890". We need to take around 0.013" out the headstock, to give the 2 thou interference fit. However-if you do this, measure your new bearing first. It should be right, but maybe your micrometer isn't.
The bearing locations must be bored out concentric to each other, or else wonky steering will occur. A fellow machining enthusiast at work suggested this method:
Make or buy a boring head. This tool has a single cutting edge, which is wound out by a screw. You wind the cutting tip out a bit, take a cut, stop the spindle, wind out a bit more, until you've made the hole the right size. Go careful-I very nearly went oversize on the bottom bearing, as it was it needed bearing fit. If you do go too big don't panic, you can get 48,5 OD/27mm ID bearings. In fact, don't put your inner race on the stem until you've got the outer fitted, in case you do need a 48,5mm one!
Here is my home made boring head:
If you've just made, or not tried, your boring head, take a few trial cuts on some offcuts to see what speeds/feeds are best, and how much it will take off for a given fraction of a turn.
Make a bung on the lathe, to be a tight fit in the top bearing location. Bolt it to the bed of your milling machine. Again, drawings to follow. You'll want it to one end of the bed, you'll see why soon.
The bigger diameter at the bottom is where the bearing location diameter, in the frame, fits. The smaller one is a loose fit (about 0.020" smaller) in the head stock tube, which helps get the frame roughly straight. Note the cut out, to avoid a seam in the tube.
For the non machinist reading (Hi Mum!), a DTI, or clock, is used to detect small amounts of movement. Each division on this one is 0.001", or a thou, or 0,025mm. It can be used for making sure things are parallel, or concentric, or round, square, flat, etc etc. You've probably worked out that the hand shows the movement.
Put the frame on the bung upside down. Optional extra: Accidentally move the bung, undoing 10 minutes of clocking. Get cross, pull frame off bung. The frame, being a tight fit on the bung, will come loose, allowing you to smack your thumb straight into the freshly sharpened cutter you left in.
Bleed everywhere, get housemate to patch you up, go to bed pissed off.
Anyway: put frame on the bung, like so:
Note how putting the bung at one end gives the frame maximum support.
Clamp up as below
You now need to clock the bottom bearing true to the spindle. This is done by jacking and packing off the angle plate for front/back movement, and between the bed and top tube for side to side movement. Don't move the bed! Again, be as accurate as you can, around 2 thou should be close enough. You may find the original bearing location is warped, damaged or badly machined in the first place. My frame had terrible chatter marks in it, this being built at the same time honda were making CB750s! Also note, it will move when you tighten the clamps, you will probably spend a long time getting it right.
Now it can be machined out. As I say, if you're new to this sort of work go carefully. You might do well to make some plug gauges on the lathe-I'd make a 47.94mm bottom limit one, and about a 48.02 top limit. Aim for bottom limit to just go, and top not to. If the top only just goes in, you should be OK fitting bearing with loctite. If the bearing has any noticeable rattle, you've buggered it. I have some telescopic gauges, which I'm pretty confident with, so I didn't bother.
Take small, slow cuts, winding the cutting edge out barely a breath of a turn. You'll probably want to touch on the bottom of the location seat to clean up (i.e. take metal off all round). As this headstock tube is flared, I'd be wary of going deep, you might thin the wall out.
This should leave you with the bottom bearing ready to fit. Don't put it in yet, though since the bottom bearing takes all the weight, you could probably leave it at that, and have a taper roller bottom bearing and the best bits of the old ball race in the top. Though that does seem a bit half hearted.
The top bearing is more tricky, as the rest of the frame gets in the way. If you had a huge mill, you could repeat the process above with the bung raised up to avoid the downtube. Not having that luxury, I turned the lot sideways.
Set up is pretty much the same, but this time the bung goes on the angle plate. You'll need a new one too, as it locates in the hole you just bored to 48mm. Again, snug to tight fit.
When you put the frame on (after clocking up the spindle), you move it by jacking/packing off the bed to get up/down movement. For side to side, partially tighten the clamp, and knock the frame until it is true. Again, take time and be accurate. This is probably the trickiest bit to get right, so accuracy in the previous steps will lessen the effect of any errors here.
Once machined, tap the outer races firmly home. Use a soft mallet, and if you can make an alumium (or other soft metal) drift, to avoid damage.
Tap the bottom bearing inner home with a clean tube. Box spanners, long sockets and bar stock are handy, just make sure they aren't full of rust, ready to drop in the rollers.
When you grease the rollers, don't just wipe a bit on. Give it a really good rub in, making sure it gets through the cage onto the inner track. Hopefully, you'll not be seeing these again, possibly ever, so give them enough to last.