Bertie down the road is sick of his Bumblebee acting like a sewing machine when it’s going down a highway. He’s got an RS and an RT as well, so he knows what the revs should be at 120 on a highway. So he buys a “longer 5th” for the bumblebee. In so doing he plants a seed in my mind, that is, to do something similar with my K ( I also have 3 airheads and so also suffer the “sewing machine revs” syndrome on K ) .
So I look into the scrapped K gearbox in the corner and decide not to go “longer 5th” route. ( Much more difficult than an airhead box. ) So out of the ether rises Anton Largiaders’ home page which has bevel ratios. 2 of my airheads are 2.91 ratio bevels, the 3rd is 3.00, K is 2.82. But K has “sewing machine syndrome” because of its 1.66 ratio in 5th gear, making it worse than airheads’ 2.91 at 1.50 ratio in 5th gear.
This is where the rot stepped in : what if the 2.54 of the R1200Cs gears could fit into a K bevel housing ? ( Careful ! There is also a 2.62 ratio on R1200Cs. ) Buggering around reveals that the bearings are the same, the drawings look the same, the PCD of the 4 wheel bolts is the same, etc, lots of things seem interchangeable.
Now the rot is morphing into a quagmire because I’ve found a R1200C bevel drive from a crunched bike where the housing is hurt but the seller says the splines and insides are good. So I buy and open it........
Good splines, good teeth, I’m not unhappy. But the cage of the ball part of the pinion gear bearing was smithereened during disassembly. “Ag, it can’t be difficult to organise something.” ( Famous last words. )
In addition, the plastic roller retainer that was a press fit inside the outer race of the roller section of the the pinion gear bearing falls out. So now the rollers can slide out towards the pinion teeth, obviously very much a “no no”.
But of course I don’t want to cannibalise a perfectly good 2.82 bevel box for its casing, would keep it to be able to put it back.
So let’s find a casing. No go, only complete drives available. Of course, price is a lot more than for an empty casing, if such were available. Sith, this might turn out rather more financially disruptive than I expected.
Hurray for Ebay ! K75 gearbox, bevel drive ( disc brake, not drum ) type, shaft and swing arm available at about – 20 % of price of just the bevel drive here, so get it, swing arm, shaft and gearbox surely of value. Alarm bells ignored, they’re rather faint and distant at the moment. It all arrives. Gearbox good, shaft good but internal splines at bevel end quite worn. Doesn’t matter, use current shaft. Strip bevel box to end up with :
It was quite tight and fell when it popped off, doing the damage.
So off to eng shop to weld up, turn and generally fix. Thank goodness it’s only the groove where the O ring fits. But more $.
There’s an M10 X 1.0 thread inside one of the M8 mounting holes for a bolt to help separate the casing halves, which I found out the hard way.
Now I can compare pinion parts, only to find that the balls in the R1200C are Ø10mm while the balls in the K pinion are Ø3/8”. And its combination 4 point ball + roller bearing is different than the R1200C one. Very strange combination bearing : Ø25mm X Ø62mm X 1 INCH ( 25.4mm ) wide.
Remember “famous last words” above ? Well, can’t find cage to suit. “Ah well, just buy complete combination bearing.”
Then : “It costs HOW MUCH ? Are you nuts ?”
So I put the pinion together to determine sizes to make a cage for the Ø10mm balls.
And I make a brass cage, more $$$$$$$$$
Remember the the rollers sliding out towards the pinion teeth, a definite “no no” ? Obvious way to stop that is to put a washer in front of the rollers to keep them in place, easy.
Not so easy, the pinion is hardened, far too hard to turn. So I find a cylindrical grinder to grind a step to install a washer but, of course more $$$. Rollers moving the other way is stopped by the inner race of the balls.
So now assembly.
“Hey, this blueing business isn’t as easy as I thought.”
Leads to : “Michael, can you blue and assemble ?”
He phones : “I’ll have to make a jig to hold the casing, its doesn’t fit the airhead jig.”
“OK, go ahead.” Yet more $$$$
So now to install onto bike.
And now I discover that the number of splines on the original drive shaft are less than on the R1200C pinion.
But the drive shaft that came with the K75 gearbox, etc, fits, so we pull off the original shaft and push in the “new” shaft. I can understand a different number of splines on the driveshaft ends, to stop it being put in the wrong way around and can only assume that the “old” driveshaft with 16 splines at each end was discontinued for this very reason.
So now it’s all working and I’ve done 183 kms in the local suburbs as well as highway. At 120 it’s turning at 4000. The rev counter lags behind the speedo a lot. The vibrations are also quite a lot less, I expected that. Very satisfying.
But ( a very big but ) :
I found a post 1985 20 spline driveshaft and on with plan B...........
So this little “experiment” has cost about 2½ - 3 times what Berties’ “longer 5th” has cost and I have no hope of ever recouping the cost in fuel saved, but I have great satisfaction in getting it right. Revs are lower and the bike is definitely slower than before....