600ss 99 power

If you intend to increase the performance you should consider  fitting some new stronger conrods.  Carrillo or Thunder.  No new 88/99  OE rods exist.  You could have the case  cut and welded and fit a 650 crank  with the new rods that are availiable. The old rods are not considered reliable in this longstroke motor .Make sure your existing motor is well set up. I am amazed how well my pretty std 99 is performing ,I seem to have somehow found  the answer  (maybe the lighter than std pistons) ?.Or the Siamese pipes and  freeflo "silencer" Ha Ha., extra oil to the head and better ignition.  If the points gap changes I really notice the difference ,consider getting your carbs set up on a dyno and the air /fuel ratio checked at all carb phases.

Try to find a 650ss cylinder head, as it will fit straight on. There was also a book published back in the 1970's that was written specifically to up grade the 500, 600, and 650 Dominator engines. This would be for fast road or racing. There is also Paul Dunstall's tuning book. You can gas flow the cylinder head, fit a better camshaft, or may be the 650ss or Atlas one, with the corresponding valve springs and pushrods. You can polish and lighten the rocker arms, and try to save some weight on the valve gear. Norris in the USA made Titanium valve spring collars at one. There was also produced, hollow tappet adjusters with alloy lock nuts. RGM  can supply mushroom tappet adjusters. There are also camshafts being produced with the PW3, and 2S profiles to fit the Dominator engines. You may also want to upgrade the oil flow to a better oil pump, or a six start set of gears, the later pressure feed to the rockers, and a larger oil feed and return holes in the crankcases. You can also drill holes in the con-rods as per the later 650, and 750 engine up grade to oil the cylinder walls. The problem with the 600 engine was the big end size, and in North America when they raced them the crankshaft could be suspect, hence the suggestion for a later 650ss crank and con-rods. I did most of the above on my own 600cc café racer, but that has an American Norris R camshaft in it.

Hello Ross - Your title of your query "600ss 99 Power" is confusing to me.  If your bike really is a 600cc 99SS model then it is pretty well at the maximum performance that Norton ever achieved from a 600cc motor.  Your description also defines a pair of Mikuni carbs and pipes.  Can you specify the throat size and type of the carbs and I assume that you mean the bike has twin pipes in place of the usual siamese pipes fitted to a 99SS ?   Other contributors have pointed out that the crankshaft/con-rod designs for the 99 do not really cope well with much more power (or revs).  Your engine should already have been fitted higher compression ratio pistons and with the same SS camshaft,  followers and pushrods as the bigger twins and it should already have the big-valve ported head with the same valve sizes as the 650.  In my experience the flow-rate with your head varies little from the later semi-downdraught head of the 650.  What does matter however is a smooth flow in the gasses from the carbs through the manifold and into the head.  How has this been achieved with your set-up ?  I should also check your valve seating and valve springs.  In 1961 a 99SS ridden by Phil Read and Brian Setchell was winning the Thruxton 500 mile race only for the petrol tank to split on lap 131 causing their retirement.  Brian Setchell did win the Production Machine race on it at Snetterton that same year  and a similar bike was road tested at 111mph by Motor Cycle.  There is adequate power for a 600cc parallel twin.   If you need more power then you will need a well set-up larger capacity engine.  Good luck, Howard