This is the gearbox top bolt adjuster assembly that was fitted to all 'upright' Norton gearbox models (i.e. rigid and plunger framed), that screwed through the top gearbox bolt (Item 0566) and allowed the gearbox to be moved backwards and forwards to adjust the primary chain.
We manufacture all the parts for this assembly in stainless steel (see more information below) and the assembly includes adjuster bolt, stainless steel lasercut securing plate and stainless 1/4" securing bolts and washers.
The second photograph shows how this assembly mounts into the top gearbox casting of both Rigid and Plunger Norton models, that used the Norton 'upright' gearbox. The thread of the bolt screwed through the large top gearbox bolt (Item 0566) and the plate is held against the back of the frame gearbox mount by the two 1/4" BSC bolts (supplied with the assembly).
Because the gearbox top mount casting lugs are often tight against the casting (of previous owners have not fully loosened the top and bottom gearbox nuts before trying to adjust the primary chain tension!), it is not unusual to find that the original Norton bolt fitted in these models has a badly worn thread, or the hex of the bolt is badly chewed. I have also seen the this bolt so badly worn, the bolt is actually twisted along its threaded length. This makes adjusting the primary chain much more difficult - and sometimes not possible at all, if the thread has fully stripped. Replacing with this re-manufactured item will resolve those issues - particularly if also replacing the bolt at the same time.
As you will see, this item is not cheap - but by looking at the accompanyning photographs - you will see the different steps we have to go through to manufacture it, and provide background of how it is made - it is a very labour intensive part to make . . . which is probably why you never seem to find good ones at autojumbles! - I assume Norton's themselves had to go through a similar process themselves to manufacture them originally.
- First of all we lasercut theframe retaining plate from heavyweight stainless steel plate (to original pattern). On the corresponding Norton top gearbox mount there is a counter bore that allows the spindle lip to feed in, but that is then held captive by this plate, once bolted in place
- We then CNC manufacture the threaded bolt assembly as two seperate items. The threaded stud is turned down from bigger diameter bar than the finished thread diameter, because there has to be the lip that feeds into the frame lug. We then machine long threaded hex 'nut's (chamfered on the innermost side and countersun on the outer face) which can be screwed onto the stud. The assembly has to be made in this way to allow the plate to be held captive
- Once a plate is fed onto the stud, up against the lip, we then feed brazing flux along the length of the shorter thread and screw on very tight the short length of hex - ready for the hex part to be brazed to the stud. Before carrying out the brazing - we centre punch the end of the threaded shaft, as an additional safeguard against the hex moving on the shaft.
- Then we braze the hex head to the shaft as shown in the 5th photograph. It is very important to get heat to the inward part of the hex area (i.e. closest to the lip of the thread assembly), so that the braze that is being fed in from the end of the head travels th full distance of the thread. By doing this, there should be no chance of the hex head coming adrift (and the main reason why this assembly is difficult to replicate). We then fill the head with braze to finish off (this is cosmetic only though).
- The brazed assembly then goes back onto the CNC machine for final profiling of the rear head (to Norton's normal curved profile). You will see if purchasing one of these bolts that being of stainless steel - the end of the hex head is a slightly different colour, where the gold coloured braze has been shaped into the stainless hex. This process makes the finished assembly very strong -as often a fair amount of pressure is required to move a tight gearbox.
- final task is to clean and deburr any flux or braze from the final assembly. Please Note - for anyone that regularly brazes stainless steel will know - the heat and brazing flux will cause the stainless steel to darken. We very carefully linish/deburr the hex head and shaft assembly of the bolt after brazing . . . but we do not try and polish it to a bright polished finish . . . it is far more important to ensure the hex is not 'rounded' by overpolishing, on this non cosmetic item, to .
However, we do make every effort to ensure the finish is good and fit for purpose. And being stainless steel, the assembly is far less likely to rust than the original dull nickel plated Norton steel item.
Adjusting The Primary Chain on Norton Models Fitted With Upright Gearboxs and Using This Type of Adjuster Assembly:
Models fitted with this type of primary chain adjustment have sometimes got bad press over the years - and have a reputation for either breaking gearbox top lug castings, breaking the frame mounting lug, or wearing out the thread on this adjuster assembly. Although sometimes a bit fiddly to adjust - there are just a few simple rules which will help:
- If freshly restoring a motorcycle with this type of frame gearbox mount - make sure you remove paint/plastic coating around the frame lug before final fitment of the gearbox - with the gearbox loosely fitted in the frame and bottom long bolt fitted (but not tightened), it should be possible to slide the gearbox smoothly backwards and forwards without noticeable tight spots. Neither should the gearbox wobble from side to side at the top or have perceptable play between the two top gearbox lugs. Having play is one of the main reasons for one of the top lugs snapping off - a common fault with this gearbox . . . and why so many gearbox shells with broken lugs are seen at autojumbles. If there is perceptable play, gently file the frame lug on either side to remove paint and make sure each side is parallel and vertical, then assemble gearbox into frame with shims on the side that is loose.
- When bike is operational, if needing to tighten/loosen primary chain, always remember to loosen the gearbox lower mount bolt, before trying to rotated this adjuster bolt - sounds simple, but easy to forget! It is this that often results in excess wear on this adjuster thread. If a road model, you will find it easier if you loosen the primary inner chaincase nut first which will allow access to the lower gearbox bolt with a good quality open ended spanner on the clutch side, while using a ring spanner or socket on the timing side.
- I also find it helps, once having loosened both top and bottom gearbox mount bolts, to then just 'finger tight' both, so that when adjusting the chain, the gearbox does not have excess 'side to side' play, which can give an erroneous view of the chain tightness. And of course, when adjusitng the primary chain, you should have first loosened the rear wheel spindle nuts and rear wheel frame adjustment bolts, so that the rear wheel is not stopping the gearbox adjusment.
- Having tightened everything back up (I do the big gearbox top nut first and lower nut second) . . . dont be 100% suprised if the primary chain seems to have moved position again! that is just sod's law, and each bike seems to do that by a differnt amount, you soon learn to account for how much change there is in the final quarter turn of tightening the nuts!
- Finally once all tightened, it is always worth a final check on top nut to see if it can be tightened any more. I have found (particularly when racing) this is one of the most common faults of this frame design - under fierce acceleration the gearbox can still move and not unusual to find one or other of the chains has gone tight again . . . another reason why both the main top gearbox bolt and the adjustment assembly listed here have a hard life and wear out threads faster than other fastenings on this bike.