Isolastic - part 2

At long last, I was finally able to get back to this and make some progress.

Oh, hello gearbox sprocket, my what big teeth you have... 21 to be exact. Hmm...

Yes, I know the sprocket has nothing to do with the Isolastic, but this was the typical "while you're at it" scenario, and I've been meaning to improve the gearing on my bike for a while now. More often than not, that would mean fitting a larger sprocket, but this time we're going for a smaller one.

First, the small retaining screw comes off, then the locking plate.

After that, the mainshaft must be prevented from turning in order to loosen the left-hand thread nut (in the photo above, on the right); I ended up doing this by placing two wooden chocks under the wheels and tying heavy duty straps from the frame to the lift, otherwise if you're working with just the gearbox on a bench you can use a length of chain locked in a vise. Andover Norton also sell a nifty tool to help you do this.

The retaining nut measures a whopping 1½″ across the flats (AF) or about 37.5mm. Because the mainshaft is in the way, a conventional socket just won't work, and even a deep well type may not be enough to reach. You need a tube wrench, but then the issue becomes finding a tommy bar that will hold up to the torque needed to undo the nut...

I ended up using the thumb and index finger of my left hand... because, shockingly, the nut was completely loose. I find this alarming and I've been wondering how this could be, so I've thought about it a bit and this is what I reckon:
1) the nut cannot come off if the locking plate is on; that, it was, and the securing screw was nice and tight.
2) I dug around my archive of photos taken during the rebuild many years ago, and found that I must have installed the sprocket, locking plate and screw, before handing the bike over to someone else who then finished the primary and other things for me.
3) What I think happened is that, at the time, I fitted the sprocket but didn't have the right wrench to properly tighten it (and the rear chain wasn't on, so I couldn't lock the wheel either), so I must have thought "I'll do this later" and then forgot.
Obviously this is a big deal and something like this should not just go overlooked. In those heady days there was a lot of activity but also long delays in getting from one stage to the next, so this must have been one of those things that fell between the cracks.
On the plus side, it has never been a problem and there is no damage to be seen anywhere, so all in all this was not as bad as it could have been.

The new sprocket was a nice snug fit on the mainshaft, I drove it in place using a large aluminum pipe. It can be tricky to lock the rear wheel in place, but you have to tighten the nut enough to align the lockring with the hole in the sprocket itself, for the securing screw.

Here's a very large spanner (thanks dad) that was perfect to get this done - remember this is a left-hand thread nut.:

Ah, one of the most reassuring sentences in the English language...

I used a new screw with just a drop of thread-lock.

One tooth may not seem like much but it was enough that I had to remove one link from the chain for a proper fit.
This is now done and I can move on to...

☆☆☆ Refitting the primary drive ☆☆☆

First things first: the crankcase has to be clean and free of any bits of old gasket or sealant:

I thought this would be an ideal opportunity to check all the various fasteners in the sub-assembly, that is to say the through-bolts that secure the engine and gearbox in their cradles. Shockingly, they were all loose, at least two of which dangerously so. I tightened everything down as it should be and I reckon that between the new Isolastic kit and this important bit of maintenance (for sure to be added to the yearly service before the riding season gets under way), this motorcycle should show a noticeable improvement in terms of handling and vibration.

I have high expectations from this upgrade, perhaps I shouldn't but I really want this to turn out well. 

At this point, if you're undertaking a complete rebuild of the bike from scratch, you ought to check that the crankshaft and mainshaft are in alignment (i.e. actually parallel to each other). In this case, I hope I'll get away with having refit everything as it was, seeing as I didn't disturb anything other than the primary chaincase during disassembly, and didn't move the gearbox at all. It looks ok, I'll find out for sure when I actually get to ride this thing.

New sealant goes on, then the new gasket and then the inner primary cover.

Alessio had noticed a big ugly hole in the inner chaincase and was wondering what the hell had happened; this was back when I had to replace an engine mounting bolt but didn't want to dismantle all of the primary chaincase, and obviously something I was only able to get away with because I run a dry belt drive with no oil. I tidied it up with a reamer and sealed it with a rubber plug; much neater now.

There are three retaining bolts with locking tabs that secure the inner chaincase to the crankcase - it's worth taking your time and doing this very carefully and being as gentle as possible: a mistake here could be bad news as you're threading directly into the crankcase.

The locking tabs come already pre-bent to help you during final fit; neat!

The front pulley, rotor and stator go back on without a hitch, I just replaced the woodruff key for the rotor with a new one, as the old one was showing some signs of wear.

Before reinstalling the belt, I cleaned it and inspected every tooth looking for tears or damage: all looks well. The clutch basket also got a good clean and I put everything back in place, with a new tab washer for the center nut.

I faffed around with the clutch pushrod way longer than I should have (I've fitted the RGM needle roller conversion, which is cool and all, but is a bit of a pain to set up) and finally refit the diaphragm spring and gigantic circlip, then, at last, the outer case. Done.

Oh except that now I have to actually dial in the vernier adjusters; so join me again next month (at this rate!) when I go over the front and rear Isolastic units, then the head steady.

In closing, a word of gratitude and appreciation for the late Bob Trigg, who passed away recently and whom every Commando owner will always thank for his brilliant contribution to this fantastic motorcycle.