I finished up fitting the Newby clutch and belt on the Comet, and thought I would document it for reference.
The verdict is that it fits in the Comet chaincase, but just barely, and it requires quite a bit of adjustment. it is a matter of 1/8ths of an inch or less. I got what Bob recommended for ratio for a Comet:
"Most Comets we have done use a 36t x 68t, 1.88:1 ratio. The 30mm wide belt we normally use gives 208mm ctr's with these pulley sizes. The smallest we can make the 36t engine pulley is 95mm dia across the guide flanges."
With this setup the belt runs very close to the outer periphery and metal of the outer primary case and interferes in some areas, requiring some removal of casting material. As a matter of fact, even the clutch pulley itself hits the outer cover in the area above the clutch. This is with the '71 Norton Commando gearbox I used, butted up against the boss on the Comet inner cover with no adjustment of lateral position by removing material on the cover or gearbox. I also have the clutch pulley as far in on the transmission shaft as possible and use the spline runout for position. This gets the clutch pulley in the closest location to the inner cover as possible, and with my setup it is just about right. I did have to lower the circular ring on the inner chaincase around the transmission hole just a bit to keep the clutch back from rubbing. I am not sure if Bob offers different ratios with smaller pulleys, but this recommended ratio will work.
I found that the front engine pulley hit the outer cover with the .470 thick pulley spacer Bob sent. I faced this off to .320, which gives plenty of clearance on the back side and also gives the bonus of moving the belt to the inner edge of the clutch pulley for more clearance on the outer cover. As it turns out this thickness is just about the same as the E216 mainshaft spacer available for the Comet, and that has the benefit of being hardened, so you could use two of those. The outer flange on the engine pulley could have been removed, but with this thickness of spacer it appears to be unnecessary, and the flanges keep the belt in line where you want it.
By the way, I built in a belt adjuster, though I know some use the simpler method of fixed distance. With the flanges on the engine pulley the only way to install or remove a belt with a fixed distance is to remove both pulleys at once. With my setup at least, (TP crankshaft) the engine pulley is very tight on the mainshaft and requires use of the convenient threaded holes for pushing it off. This would be hard to do and means that every removal of belt or clutch will require that the engine pulley be removed. I built in enough adjustment to remove the belt without doing this. I installed a mainshaft oil seal, and the bolts used to push off the engine pulley would have marked up or damaged the seal housing. I slipped in two thin pieces of brass behind the pulley to prevent this, but that is pretty awkward to do.
I proceeded to do metal removal with my Gorton P1-2 industrial engraving pantograph. I was going to use the mill, but the pantograph uses a small cutter and it allows a bit of manual finess, since we are talking about areas where too much removal could break through the cover. Then it was up to the dremel and a burr to smooth and relieve the vertical edges and cuts. I still have some work to do with a small sanding disk to smooth things up, but I got the interferences taken care of and still left at least an 1/8" of metal everywhere. It took a bit of difficult measuring to ensure I did not get too deep anywhere. (I still have a bit of work to do on the inner cover just above the clutch pulley where things get a bit close)
One area I had not anticipated was the casting material around the chaincase drain plug. I was congratulating myself on the success I had with shroud and interfering material removal around the clutch pulley when I looked through the inspection port and "Oh S---", the belt is hitting the drain plug boss. Not wanting to use that area for a belt tensioner it was off with the cover and detemine what needs to be done. My comments yesterday that I would have moved the rear gearbox mount down from the center of the rear casting hole in the outer cover were wrong, since if I had done that this interference would have been worse and maybe not solvable. This area was particularly difficult since the drain plug hole on the outside has a slightly recessed ring around it and I was going to need to get very close to this on the inside. Again cuts with the pantograph and smoothing of the surfaces away from the ege of the belt run worked out fine, and the closest point of the belt to metal is 1/8" and only on the very outer edge of the belt, as the aluminum slopes away everywhere else. By the way, a thick Cometic gasket is a real boon for this installation, and even two of them stacked up might be a good idea or making a thin metal spacer is also a way to do it if you have the skill to cut one out.
So there it is and I am not sure how keen anyone else would be on using the stock primary covers after reading this, but I like the way it looks now, nothing valuable was totally destroyed, and my nerves are OK.