SOHC Cambox rubber pad set, cut/planed from hard wearing oil resistent rubber (Nitrile), to the same width and thickness as originals, using a special jig for this purpose. These are fitted both above and below the rocker arm central boss in all SOHC cambox’s.
Along with the cork side washers, it is these being worn that normally results in these cambox’s leaking badly.
We use an original pre-war 'New Old Stock' Norton cambox rubber as the pattern to re-create these (bought many years ago from that great Norton exponent Phil Heath) to ensure the correct width. Although they look very simple, they are not particularly easy to make . . . we buy slabs of the special oil resistent Nitrile rubber, cut them to approximate size blanks - but then have a special hardened jig (photos attached) to linish mill them to the correct dimensions (remembering that they may need further trimming to fit your own cambox).
Four rubbers are supplied, two for each rocker arm - one fits above (and is non adjustable) and the other fits below, and has some adjustment for wear - as it has a steel pad underneath it and an adjuster (both of which are manufactured by us in our Cambox section). It would be nice if the top rubber had also been designed to be adjustable - but this would probably have meant the adjusters would foul the top tube of the frame.
The rubbers are slightly longer than required, which can be carefully trimmed to your required depth. They should be trimmed so they protrude just proud of the cambox slot, so the rocker arm has to be forced – carefully, into the cambox, as the originals were. Take care not split or nick the rubber as you are pushing the rocker arms in place. The smooth end should be abutting the rocker boss and your trimmed end opposite.
Cambox Oil Tightness - Assembly Notes:
It is worth noting, it is never particularly easy to fit rockers after new rubbers and corks have been fitted - it is easy to think you are having to overly force everything together. However, if everything is not a tight fit on first assembly - the cambox will quickly start to leak again once the engine has been ran a few miles.
Therefore try and trim and assemble the rubbers so as much pressure as possible is being applied by the rubbers to the rocker surfaces. As an indication - once the cambox is first assembled, the rocker arms should be very difficult to move by hand, feeling almost 'locked' up.
The rocker arms should initially feel very stiff when first assembled, but they will soon loosen up in operation - but being like this should help ensure the cambox staying oil tight as long as possible. This also applies to the rocker arm side washers (0062/0062b/0062c) - try and fit washers thick enough that some effort is required to fully tighten cambox rear cover (without damaging threads of course). These will also compress a few thou in use - we sell different thicknesses for this purpose.
Final point on SOHC Cambox's leaking oil - after a period of time the top and bottom rubbing surfaces of the rocker arm centre hub (i.e. the area where the nitrile rubbers press against) get worn and sometimes scratched through dirt getting caught. As long as smooth and reasonably evenly worn, the rubbers should still work. However if scoring marks get excessive, even fitting new rubbers will only provide limited improvement. If this is the case with your rocker arms, it may be necessary to have the rubbing areas of the rocker arms lightly milled.
There are some specialist Norton engine restorers who will sometimes perform this task, Email us at firstname.lastname@example.org if you think want more information on this.
I was asked for more information recently on fitting these by a customer . . . as it is indeed a fiddly and time consuming operation to trim them to fit your own cambox . . . and each time you try and push the rocker arms in . . . it requires some effort (unless you have over trimmed them . .. in which case it is quite easy to assemble!!) - so took some pictures of one of my own (magnesium) cambox shells and then overwrote the general sequence of trimming. I know it is not particularly easy to read my writing - but look closely, follow the arrows and hope this helps. It is important to note that there is 3 'planes' of the rubber - the Width, the Depth and the Breadth.