Some Norton 'history' that some owners may not be aware of is as follows
When Mr Phil Heath and others working at Norton in the early 1930s designed the rather clever pressed steel oil bath chain case (OBCC from here on!)they did NOT design a wet clutch that would work correctly with oil on the friction interfaces but continued to employ a dry clutch. To ensure oil might possibly lubricate the primary chain increasing its life and efficiency but did NOT enter the clutch which would result in slip and drag due to oil problems they did two things. One was to position the oil level plug very low down so that oil at the correct level would just touch the bottom of the lower run of the chain at its lowest point creating an oil mist in service and the other was to place around the basket what is described in the parts books as an OIL EXCLUDING BAND. Mr Heath told me that the correct oil to employ was a straight engine oil of SAE10 or 20. I still have an unopened gallon can of SAE10 and 20... It is because the clutch was designed to be run DRY that for example my 'Maintenance Manual and Instruction Book for Models 50/ES2/88/99/650/Sports Special /750 Atlas and 750 Scrambler' (publication P106P) states that a smear of grease should be applied to the clutch rollers (page 36) and on the problems page (page 89) for clutch slip it states and I quote 'Oil on plates(usually caused by overfilling bath)'. The cure given being 'Dismantle clutch plates and wash in petrol'. How reassuring it is to know that owners are to blame and not Norton for shoving a dry clutch within an oil bath chain case in the first place!!
As for the correct OBCC oil level for Commandos ......which are, being polite, rather prone to clutch slip and drag problrems. In yet another effort to reduce the slip problem Mr John Nelson(Service Manager?) cut up a chaincase outer inserting a lump of perspex to see exactly what was happening. His advice was that the oil level which the manuals stated was 'till oil begins to seep out of the oil level plug'...was wrong. This led to the instruction being added to section K8 in the manual...'Under no circumstances allow more than 7 fl oz (200cc) of oil in primary case'... which of course some really clever soul simply added to the original instruction which told owners to fill till oil seeped out of the level plug!! A friends Commando filled to the level plug contained approx 290cc of oil.
To prove a point a few very simple back of fag packet original 750 Commando clutch calculations..like what i do when playing at belt system clutch designing.....
1. Required clutch torque capacity. MAX crank torque 45ft lb. Assuming (incorrectly) no power loss in chain and std 26-57 tooth sprockets MAX torque at clutch = 45 x 57/26 = 99 ft lb. Applying rule of thumb clutch service/safety factor of x 2 then the required clutch torque capacity = 99 ft lb x 2 = 198 ft lb
2. In theory clutch torque capacity. 4 friction plates giving 8 friction interfaces. |Effective radius of original friction plates approx 0.205 ft. Original friction material shown on Norton clutch plate drawing is Ferodo MS6. Ferodo MS6 data sheet lists Coefficient of Friction values for design purposes as Dry 0.34. Oil mist 0.1 - 0.12.
In oil 0.09. Clamp load applied by ORIGINAL diaphragm spring (0.075 +/- 0.0015 inch thick) IF correctly set up approx 380 lbf. (According to the original Norton test results and testing the Mech. Eng. Dept. of a certain UK university did for me using two original 750 diaphragm springs). Thus the in theory clutch torque capacity is...
DRY.........8 x 0.205 x 0.34 x 380 = 212 ft lb
OIL MIST....8 x 0.205 x 0.1 / 0.12 x 380 = 62 / 75 ft lb.
OIL..........8 x 0.205 x -.09 x 380 = 56 ft lb.
With a required clutch torque capacity of 210 ft lb......Q E D?
As for the use of ATF oil within the OBCC........Does not ATF contain high levels of friction inhibitors? Not the sort of oil one wants on the clutch friction dependant interfaces of a dry clutch surely ?? Is that why Mr Heath recommended I use a straight SAE10 or 20 engine oil? I believe a straight SAE20 engine oil is available in one litre or pint or quart cans from Silkolene or Morris oil stockists. In my young days when Barry Sheen was riding his Fathers Bultaco race bikes friends used very thin REDEX in their chain cases because the chains ran at such high linear speeds anything thicker did not make it into the chain to lubricate the great many plain bearings and on the long circuit at Brands Hatch the chains would often fail ..usually on the last lap when in the lead? Ever read the Renold recommended max chain speeds for various forms of industrial chain use lubrication? For oil bath it is 'up to 2,000 ft / min'. Ideally we should be using sump and pump spraying oil onto the inner edge of the side plates just before the chain flies around the clutch at up to and above at times 6,000 ft / min we often use it at!! Guess why I use belts which run dry happily at any belt speed I will ever use.
Incidentally for those bronze plate clutch owners who believe sintered bronze to be 'high torque' might I suggest you do a few simple calculations... Max crank torque = 55 ft lb. There are 10 friction interfaces with an effective radius of approx 0.21 ft. Rule of thumb Coefficient of Frictions for sintered bronze are DRY 0.3 and IN OIL 0.06 - 0.08. With the later clutch the spring by then in use (0.084 +/- 0.0015 inch thick) gave a clamp load of approx 550 lbf along with heavy clutch lever action!!. So go play and determine for yourself if that clutch was a designed to be run wet or dry clutch..Then do it again using the 380lbf of the original 750 spring. I bet the guy 'designing' was unaware that there were 4 different springs fitted during Commando production..and another different one fitted to the rotary Norton clutches but as the only easily noticeable difference is in spring thickness and its only varies by a few thousandths of an inch.......
No spell or grammer checks done as I am very old and lazy ..as my better half will confirm!!
