The stock Commando cam is of the same form as the Dominator SS cam of 1961. This cam was good enough to win the Thruxton 500 mile race in that and the following year, and also the 1000 kilometres race at Silverstone. So, the general notion that a stock cam cannot be any good rather depends on what you want it to do. This cam is fine for street use as it has a nice flat torque curve, however the hardness of the metal often leaves something to be desired and a lot of them could do with some heat treatment to reduce the wear. It is rumoured that the wrong treatment (nitriding?) was originally used on some of them, which actually made them softer.

It is worth checking the timing of this cam because at Woolwich it seems they had some trouble with the old cam grinder which had come down from Birmingham, and while the cam lobes came out fairly accurately in relation to each other, they were not at all accurate in relation to the driving keyway. The timing is checked at 0.13″ valve lift: set the clearance to 0.016″, then the opening/closing position is when a 3 thou. feeler gauge can just be pulled out from between the valve stem and the rocker adjusting screw.

The crucial figure with these cams seems to be the 8^o lead of inlet lobe over exhaust lobe. To achieve this lead, the the timing can be advanced or retarded in 5^o stages as follows. One tooth on the camshaft sprocket represents 40^o on the crank, and one tooth on the intermediate gear in relation to the half time pinion equals 15^o on the crank. Therefore, to retard the timing by 5^o, rotate the cam sprocket one tooth anticlockwise, which advances the cam 40^o; then rotate the intermediate gear three teeth clockwise, retarding the cam 45^o, giving a net retard of 5^o. To advance by 5^o, rotate the cam sprocket one tooth clockwise and turn the intermediate gear three teeth anticlockwise.

Because the timing mark on the intermediate gear is a marked space between teeth, it is obscured when the timing chain is in position, so it is an advantage to mark the tooth on either side of the dot with white paint. At the same time, mark the teeth three spaces away on one side of the dot with red paint, and the ones on the other side with yellow; then it is much easier to see what you are doing when assembling chains and sprockets.

Combat

Just to cause confusion, another SS cam, henceforth known as the 2S, was introduced for the 750cc Combat engine. This cam had more lift and duration and virtually equal overlap, i.e. no lead. The result was more power at the top end at the expense of bottom end torque. The 2S was also recommended for 850cc Stage 1 tuning, i.e. fast road work. There was another grind known as the 3S which had the same timing as the 2S but more acceleration, and 0.016″ ramps instead of 0.010″. The 2S and 3S cams may be advanced up to 3^o to restore some lead and presumably result in a less peaky power curve. Advance by 5^o as above, then file a tiny step on the camshaft key to turn the sprocket 1^o anticlockwise. The net advance is then 3^o.

Other Sports Cams

The 4S has advanced inlet timing compared with the 2S, while the exhaust timing remains the same. This yields more midrange power than the 2S. It was recommended for 850cc Stage 2 tuning, i.e. road racing, but it might be better than the 2S for sports road use as well. Other cams you might come across from the olden days are the Dunstalls. The Dunstall race cam had a pressure oil feed to the lobes and usually ran in needle rollers.

Modern Developments

The PW3 was a Peter Williams design developed from the works John Player cams of 1973/74. Mick Hemmings says he won the race on his first outing with this cam and it is the best he has used. There is also a cam known as the 7S, but we have no information on this one.

American Cams

The Axtell #3 is the legendary grind developed for flat track racing in the U.S.A. The particular torque characteristics make it very suitable for road use as well. C.R. Axtell stopped making them years ago and apparently refuses now to make Norton cams. However, we have it on good authority that the Norris SS/Megacycle 560-NSS grind is equivalent. This cam has less lift, longer duration, and no lead compared with the traditional British performance grinds. It is best in 850cc and larger motors and gives power throughout the rev range. Other Norris grinds now available from Megacycle include the R (all round performance for a 750, with strong low and midrange power) and the D+ and 480 (top end power for road racing and drag strip use). Megacycle’s own cams include the 560-00 which is good all round, but needs radiused tappets.

The standard Commando rocker ratio is 113%.

(1)  Factory cams measured at 0.013″ valve lift (see text)
(2)  Timing with tappets set to standard clearances
(3)  Megacycle
(4)  Needs 3″ radius tappets