Re: 140 HP 920cc Norton on Nitros
jseng1 said:
He has a heavy duty 5 speed 1980s English box with thicker shafts than quaife or TT Ind - it may no longer be available. Not sure about the cases. Remember that TC used to get away with stock cases - for a short time. I'm involved in supplying the longer bushless JS/Carrillo rods and lightweight 920cc pistons. The pistons are ceramic coated for this application. He told me that it runs "so much smoother" than his previous 880cc pistons which created so much vibration that it was "scary". His CR is 11.5:1 and he's using a JS 2 cam with the lightweight lifters so over-revving into valve float shouldn't be a problem.
He has it geared for 7200RPM at top speed and he has to hold it there for a minute or two so stress is a big concern and with all that power you just want it to stay together.
He uses gapless rings and a copper head gasket plus copper rings that dig into the head gasket to prevent leakage under the high combustion pressure. The head is a trick original Norton factory (R7 I think) racing head that comes with different valve locations & angles. The Huge valves required re-cutting the valve pockets in the pistons even though the pistons were already set up for stage 3 big valve heads.
The two into one exhaust was designed and developed on the dyno by Jim Mosher who also built the motor and set it up for Nitrous.
Jim M uses a computer to dial in the Nitros mix. The motor has spent a hour on the dyno so far and that's a whole lot of stress in itself to subject it to. But if its going to blow you'd rather do it before you travel and tuck in for a 160mph blast.
I can add a bit to that, Jim. Unless Fred has changed the gearbox since I sold him the bike, it is a Quaife. It is a complete heavy duty Quaife 5-speed, not the earlier gearset kit that fits in the stock AMC shell, but the later unit with all Quaife parts. It is a bit beefier than the earlier gearsets, with needle bearings in place of some of the bronze bushes, and a different clutch release and shifter pawl mechanisms. The output shaft accepts the Commando clutch, but the drive sprocket is a finer spline (21 splines) than the stock Commando.
The cylinder head is a factory short stroke 750 head (RH7) that had the combustion chamber welded up by Jim Messler, ex factory tuner, to a bathtub shape to allow high compression with flat top pistons. Jim also fitted even larger intake and exhaust valves, in titanium. Jim Mosher, Fred's tuner, has probably modified the head somewhat, but I don't have those details.
The cases are stock 850 Commando. I replaced them with a new set in 1987, when I rebuilt the engine for the original owner, Martin Adams, after a blowup at Laguna Seca. The crankshaft is a Dave Nourish one-piece with stock 89 mm stroke. The engine originally had Crower titanium rods and Omega pistons, but those have been replaced with your lightweight kit. It originally had an Axtell cam, which has been replaced with your cam and kit. The cylinders were originally stock 850 items bored out to 79 mm, but have been replaced with another set of stock 850 cylinders sleeved out to 81 mm and grooved for the copper o-rings.
Primary drive is a Norman White belt drive.
Original carbs were Amals on long manifolds welded to the head. Jim Mosher removed the long manifolds and fitted new flat slide carbs.
Ignition was a custom Lucas unit with a Hall effect crankshaft trigger, and I think they are still using it.
That's most of what I can recall at the moment.
Ken
