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Gearbox questions

I hope this doesn't tread on any toes, but I have a couple of questions about the Norton gearbox fitted to my Dad's special.  I don't know much about gearboxes, but i'm relatively handy with a spanner, and I'm trying to get this bike running for summer.

I have no idea where the gearbox came from, or what 'model' it may be, so here's a photo of the main serial number in case that helps!

GBox number

The questions I have are:

What oil should I be using, and how much?  As you can see, the level looks pretty low! I'm going to drain what's in there first obviously.  I'm assuming that the small bolt on the rear of the box, at the lowest point is a drain??


Also - does anyone know where I can find an exploded diagram of this type of 'box? The threads on the small holes that hold the 'filler-cap' on have stripped, so I'm going to have to strip off at least that outer casing of the box in order to re-tap the threads (or make some other fix), and I'd like to know what's in there before it all falls on the floor in an oily puddle.... Has anyone else had those small threads strip?  What was your fix for it?


Many thanks for all replies!


... which is a good item - generally robust with a slick gearchange. It does have its weak points - particluarly the layshaft bearing in the inner casing. You can find exploded diagrams all over - try here: from Andover Norton.

I can't remember what the threads on those screws are but helicoils will fix them. Please use the correct thread for the age of the box ie BSF / Whit / BA if it's pre-Commando.

Oil? Straight 40 is a good starting point.


Thanks for that info, I've no idea how old the box is sadly, can that be found out from the serial number? A further question if I may, regarding the helicoils: Will a helicoil work in such a thin piece of metal?  It appears that the threaded holes are no deeper than the thickness of the case itself, which is why I was considering tapping them a mite larger.

To be honest, I've a feeling the holes may have been tapped out once already, the bolts in there are not screw-caps (which appear to be the original fitment looking at the exploded diagram), but allen heads (I haven't tried to measure the thread yet).

Can anyone recommend a good source for Imperial taps/dies, and/or just bags of Imperial nuts and bolts.  All the stuff I currently have is metric. (This is the first bike i've owned that is predominantly Imperial and I currently don't have any Imperial 'odds&sods')


The box  is probably a  late 1950's Norton box  and  could have come from one of several types of bike . I would get the inspection cover threads welded up and drill/retap back to std .The small plug is the level . drain is underneath. SAE 40 is a good option. Change the oil.


I discovered that RGM Norton do a 2BA helicoil kit - so will try that first. 


As already said, inspection cover 2BA. Which I on less precious bikes (BSA) has retapped to M5 (BA2 dia 4.7x0.81mm vs M5=5x0.80). On the RGM website, in section gearbox, they kindly tells dimensions and threads of fittings. Sadly I didn't make notes on threads on latest AMC box I rebuilt. Except that the drain plug is 3/8BSF and the sprocket lock washer screw is 2BAx3/8. About oil. Factory spec was same as engine oil, but gearbox oil EP90 has about same viscosity as 50 engine oil. I use 75W90 gearbox oil in my Nortons.


No idea, just remove the oil level screw and fill until oil starts dripping out. Of course with bike vertical. I guess with both wheels on level ground.


I put the bike on the centre stand and fill to the required level ( of ST 90 or Classic EP90 )  by looking at the level plug. I leave the plug tight, so not leak oil, same for topping up. I understand that EP 90 is about the same viscosity As SEA 50, good for Singles engines but the other two I mentioned earlier, better for gear boxes IMO.

Philip H was very helpful with identifying my last project G box (Not matching) It appears these  boxes were made in batches. N for Norton and M for AMC. prefix. So they could be sitting around a while.



What is the engine? asking as you may need proper rebuild manuals for engine and gearbox, depending on what you plan to do in the longer term.

In reply to by jan_nelder


The engine is a JAP of uncertain vintage, sporting a Cooper alloy barrel and a mismatched crank (or other component) that makes it approximately 400cc instead of 350 or 500. I've got one or two books for the engine.


The earlier 5 stud (pulling down the head) speedway engine had an alloy conrod which over time got longer, increasing compression and finally broke. Around -48 or maybe -49 came the 4 stud sporting a steel conrod. Had a 5-stud as my first speedway bike. Later exchanged my 99 for a JAWA. Superior frame and less maintenance.


Without looking I couldn't tell if it is a 4 or 5 stud head. 

The bike was made up by my father over about 30 years (he had the idea for it in the '50s, got a frame and bits in the '90s, bike was pretty much 'finished' about 5 years ago, he died 2 years ago. :(

Unfortunately he took a lot of information about it with him.  He had a lot of help from Mick Hemmings (who passed away within months of my Dad :( I saw Mick at my Dad's funeral and we were looking forward to fettling this bike together :( ).  

So: I keep learning new things about the bike. :)  I'm pretty sure that the engine was rebuilt with 'the best' of what is available, but Dad's maintenance of stuff was a little  "oh - that needs doing, I'll get to it soon" .... :D 


Gearbox N12664 dates to early 1959. I cannot help with the model of Norton.


Am I right in thinking that an AMC box with the 1960-on closer ratios is stamped N2, then the box serial number?


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