Crankshaft balance factor for Dommie 99

According to P.L. Garratt in Norton Motorcycles (5th Edition) on page 100 the Balance factor for all twins 650cc and below is 70%. If your engine is an alternator model be sure to fit the rotor before you send it to be balanced. On wide (twin) cranks the side to side factor is as important as the percentage I am told.

I suspect you will have to fit the engine sprocket as well . I have heard that an extra weight to compensate for the oil in the sludge trap  can be added too. Would be worthwhile to balance the pistons against each other . Our Atlas (Commando) ones were well out to each other and vastly heavier than the orriginals. I spent ages  knife edging the bottom of the skirts and removing bits of casting flash ,but the extra thick heavy pins defeated me a bit.

No way of knowing if a rotor is balanced already,and I think you might need the sprocket to have something to tighten the rotor up to?.

Hi Tony,

   With any parallel twin, single or V-twin, the design makes it almost impossible to achieve a vibration free engine through-out the rev range. Dynamic balancing allows you to pin-point the rev range where you expect to use the bike most. You will probably only have around a 1,000 rpm window that the engine is at it's best balance so it depends on what type of roads you use. If you use motorways and dual carriageways you will probably want that window between 3,500 and 4,500 rpm. If you ride mostly on A and B roads and urban areas you will be better to aim for 2,500 to 3,500 rpm. When you take it to be balanced, tell the guys what you are looking for in the way of cruising speed and how the vibration will be either side of this. I think it will be best to balance the rotor individually so it is equally balanced and does not affect the crank. I don't think the sprocket will have enough mass to cause a problem.

Hi Tony

No need to include the rotor or sprocket as you say, complete piston and conrod assemblies plus crankshaft only when I had mine done. The balancer also did a wet crank calculation.

My Mk3 850 was balanced at 63.5% dry to match the combination of heavier Carrillo steel conrods and lighter JE forged flat top pistons. Both identical in weight complete with gudgeons, circlips and rings. Quality products.

This is close to the published Mk3 specs of 63% dry, 52% wet and works fine. Bill Molines post of 70% sounds good for your solid mount motor

Generally most people who got to know him accepted that John Hudson knew what he was talking about. The attachment is from his Notes and covers Norton Engine crankshaft balance.

It's a bit of a black art to me, but I thought the point of any balancing was to balance what you have, regardless of the weight/mass of the reciprocating and rotating components.  I thought that was why an engine needed to be re-balanced with whatever pistons, bearings, flywheel etc. you used but I did assume the factor would remain the same to suit the desired normal cruising speed.