Dismantling and re-assembly of the Gearbox
Dismantling and re-assembly of the Norton 4 speed gearbox is generally relatively
straight forward when following the instructions given in the Maintenance and
Instruction Manual. There is no difference between the 16H and Big4 boxes apart
from the gear wheels.
There are some point of attention. The M&I Manual does not give any figures for the
total wear allowed on the Bronze Clutch Thrust Washer. Additionally, the gearbox
exploded view of the M&I Manual shows a dished washer at the end of the main shaft,
which in many cases is missing in many actual gearboxes, and the same exploded view
does not give the Main Gear Wheel Sleeve Bearing Roller Retaining Washer, which is
a crucial part in the gearbox! (I have drawn this washer into the original gearbox
exploded view below, encircled by a red ring.)
The Clutch Thrust washer is the washer taking up the reaction forces when the Clutch
is operated. It is positioned on the main axle facing the hardened steel ring that keeps
the rollers of the Main Gear wheel in place. (Main Gear Wheel Sleeve Bearing Roller
Retainer Washer).
The Bronze washer has three grooves in one face, which to be correctly positioned
should be facing towards the Main gear wheel (Clutch). When these grooves are worn
away, the washer needs to be replaced. The Clutch Thrust washer has a thickness of
approximately 7/32 inch (5,55 mm). Maximum permissible wear of the Clutch Thrust
Washer is 1,58 mm or 1/16th inch.
Investigation of the Clutch Thrust washer is indicated when the max end float of the
clutch itself exceeds the 1,6 to 2,38 mm. This can be roughly inspected by pushing and
pulling on the clutch in axial direction.
End float of the main axle should preferably between 0,254 and 0,58 mm (0,01 - 0,02
inch). If the end float is still outside this range while the Clutch Thrust washer is
within the specified thickness, the main axle should be shimmed behind the Clutch
Worm nut.
This is where the elusive dished washer may be become handy although flat shims will
do the job as well. The exploded view drawings of the gearbox in the Maintenance and
Instrucion Manual as well as various books, show a dished steel spring washer at the
end of the main axle. I personally have never found such a washer in a gearbox, and
wonder if they were actually used on all boxes, or only the later ones. Its not given in
the spare parts list of either old or newer contracts or postwar civilian lists. The
dimensions are I.D. 5/8", O.D. 1 7/16", steel thickness 1/64" and dished height 1/16"
(as measured from an actual washer).
Main Gear Wheel Sleeve Bearing Roller Retainer Washer.
This hardened washer has the tendency to fracture. Two of the 3 gearboxes I have
showed this to be the case. The washer is given in the spare parts lists (all) under spare
part no 3598 (Main Gear Wheel Sleeve Bearing Roller Retainer Washer). I have
however not been able to find original replacements for them, and had them made
from a cold work tool steel (AISI D2 or UNS T30402) and hardened to 59 Rc. Remains
of the original were measured to be 60 Rc so I think I am close enough.
The manuals for Norton gearboxes all ignore the basic problems of making these type
gearboxes change perfectly at racing speed. The standard gearbox with a lever about a foot
long is very forgiving as this encourages slow gear changing. The basic problem is the lever
at the top end of the link-rod is following an arc in line with the bike, and the lever at the
lower end of the link-rod follows an arc at right angles to the bike. This works OK by having
enough clearance in the clevises (the very early bikes have a superior double clevis in the
top joint), but worse is the fact the four gear positions of the cam plate are equally spaced,
as are the four positions of the positive stop mechanism, but the lever and rod system
connecting them is essentially a 'toggle'. This means the pedal movements are equal, but
the cam plate movements are certainly not! In typical Norton manner they make this still
work well! This is because the stops in the foot change mechanism only actually work
correctly in 2nd and 3rd gear, as the limit to movement in 1 st and 4th gears are the gear
selector pins hitting the end of the cam plate grooves, not the foot change stops, so the
over changing caused by the toggle effect does not matter. Because of this, the link rod
should be adjusted by putting both the upper and lower levers in 2nd gear position and
adjusting the rod length so the pins slide in freely. Then move the pedal to 3rd gear position.
Remove the lower pin and make sure the cam plate and the foot change detent plungers
have 'clicked' home. If you are lucky the holes will still line up and the pin will push in and
out easily. If the movement is slightly too little, this is OK, and the rod should be adjusted so
the gearbox slightly under changes in both directions. This is much better than over
changing, as the inertia will carry the cam plate into the correct position. If you cannot get
this to work as suggested, you may need to restrict the throw of the positive stop
mechanism. As standard this is done by the ratchet plate in the positive stop hitting the
pillars that hold the spring plate. If the gear pedal centralizing spring is removed, the
plunger in the back of the positive stop will hold the ratchet plate in each gear position. The
travel of the ratchet can be changed by building up the rather flimsy tips of the fan shaped
ratchet plate with bronze weld, and then grinding them back until the mechanism slightly
under-changes in the 2nd to 3rt1 and the 3rd to 2nd changes. Also check the pillars for wear
on the 'stop' faces. It is also essential to make sure that the ratchet is centralizing correctly.
This is the job of the pedal spring. The gear pedal spring is often the cause of gear
changing problems and a new one often doesn't go straight in and work correctly. The
common faults are when you push the pedal and the lever doesn't centralize. so that when
you push it again the pedal just goes down, without changing gear. Also the pedal has a
dead spot' in the middle. Both these faults are fixed by bending the spring legs where the
legs push against the stop plate. If the plate is worn where the spring seats, a repair with
bronze welding works well. Because the spring must touch the retaining plate in two places,
and touch the two stop pins on the ratchet plate at the same time to avoid a 'dead spot' it
may be that one leg needs bending, but at the same time you need to check the ratchet
claws are equally spaced (central) on the ratchet plate. This is also fixed by bending the
spring legs. Do this by holding half the straight part of the leg in a vice using "soft-jaws" and
tap the overhanging part with a small hammer. It will bend quite easily. Don't turn the
spring over after bending the legs, as this will ruin all your good work by offsetting
everything the other way! I mark one leg and one pillar with paint. If the spring is going to
break in service, it will always do so where the straight legs meet the coils. Beware of any
rust pitting in this area. The dolls head spring has one complete coil, the lay-down type two.
THE NORTON CLUTCH
By DEREK WYBORN
In my previous article on the Norton gearbox I stated that the efficiency of
the clutch had a marked affect on the operation of the gearbox. I propose therefore to go into
the matter more fully in the hope that some of the information given maybe found to increase
reliability. It is my considered opinion that 75% of drivers use their clutch once only during a
race, i.e., to
leave the starting line. After that a perfunctory stab at the pedal with their left foot, usually
sufficient only to take up the free play in the cable, a pile driving thrust with the right arm,
and"jolly good luck we've found the next gear!"Maybe!
If only the thunder of the "double knockers" or J.A.Ps. could be silenced on these occasions,
the loudly protesting voice of the transmission would assuredly be heard. Try making a
clutchless change on your normal road car, you won't have to listen very hard for the bonks
emanating from the back axle.
Another piece of butchery in common practice is the slipping of clutches to"keep onthe
mega." This is quite unnecessary except in exceptional circumstances and a tenth of a second
gained by this malpractice can quite easily burn out the clutch and lose the race. The heat
generated by a slipping clutch is tremendous and will cause the rubber cushions to melt and
the necessary lubrication to dry up.
Norton clutches vary slightly according to year and model but the basic
principles of operation and construction are almost identical. The main
difference being in the type and number of plates used. In diagram 1 I have
taken the ex-W.D. heavy-duty five-plate clutch as an example, as this is
probably the most widely used among half-litre competitors. The clutch itself is
comprised of some 45 separate pieces, not counting rollers and inserts, whilst
the operating mechanism (diagram 2) consists of six items, plus cable and
adjusters. For those unfamiliar with this type of clutch much can be learned by
completely dismantling and inspecting each item before reassembling. The
following hints may be of assistance in this operation.
Once the end plate cups, springs and mushroom have been removed, the main
body of the clutch can be separated from the gearbox mainshaft by unscrewing
the locking nut and washer and withdrawing with a special puller. This puller
can be obtained from most Norton dealers and is well worth its few shillings
cost, as the use of tyre levers or hammer and drift can cause far more expensive
damage to both gearbox and clutch, not to mention frayed tempers and barked
knuckles.
Removal of the remaining three screws from the front plate will enable the rest
of the clutch to be completely dismantled. When this has been carried out, the
inspection of each component should be proceeded with on the following lines.
Check the back plate for truth and excessive wear on the spider shaft. Two types
of back plate are in general use, one being considerably thicker than the other. It
is recommended that this plate be used as it will give much longer service and is
not prone to buckling.
A very careful inspection of the clutch sprocket should be carried out, especially
around its inner edge where it comes in contact with the rollers. This edge is case-hardened
and consequently quite brittle. A hair crack undetected here may lead to a complete fracture
of the sprocket. The segments in this sprocket need
periodic replacement. Wear can be detected by the presence of side play when
the clutch is assembled.
Inspect the cage and rollers for fractures or cracks. A split roller can cause serious damage. In
replacing the cage care should be taken that the open end faces away from the back plate. The
rollers should be lightly greased before reassembly.
The centre spider wears very little, but the rubber cushions need replacement at
least twice per season, or after the clutch has been overheated. Assembly of these rubbers is
simplified if they are lightly smeared with grease. When both back and front plates have been
smeared they should be locked by centre-punching the respective screws and nuts. If on
inspection the clutch
caps are found to have a small ridge around the outside, caused by repeated
contacts with the clutch centre, these must be filed away or the caps replaced because they
can cause a clutch to stick open. It is essential that these caps
move freely in their socket s otherwise an undue strain may be put on the cable. A little
grease will help in this direction. Springs also get tired and need replacement at least once
every season. Do not over tighten the special studs which retain the springs as the heads
break off very easily. As far as the actual linings are concerned some people prefer the metal
discs with segmented inserts, whereas others, like myself, prefer the metal bonded solid disc.
The solid disc has the advantage of greater frictional area, but also has the disadvantage that
the locating ears may tend to break. The operating mechanism is very simple (diagram 2) and
consists of a worm assembly which operates a pushrod passing through the hollow main shaft
of the gearbox. This pushrod operates a mushroom which in turn bears against the end-plate
of the clutch. When the clutch pedal is operated, the end-plate is forced outwards which
relieves pressure between the frictional plates and allows one to rotate against the other.
It has been found that if the clutch is kept open, with the car in gear and the
engine running for any length of time, the pushrod is liable to weld itself to the
hardened button at the end of the worm. To eliminate this fault a simple
modification is necessary to the end plate. This takes the form of a thrust race
against which the mushroom rotates more easily. The race is located by a housing welded or
riveted to the end-plate. It is also advisable to place a collar
on the main shaft immediately behind the clutch. This will keep the bush in the constant mesh
pin in place, as it may sometimes move along the shaft. It is important that this collar is quite
free to move when the clutch is fully home on its shaft. About 1/8in. to 3/16in. free play
should be allowed on the cable at the gearbox end when adjusting the clutch. If new plates
have been fitted, rather more clearance should be allowed as free play will be taken up as the
plates bed down.