Skip to main content
000000 000003 000006 000009 000012 000015 000018 000021 000024 000027 000030 000033 000036 000039 000042 000045 000048 000051 000054 000057 000060 000063 000066 000069 000072 000075 000078 000081 000084 000087 000090 000093 000096 000099 000102 000105 000108 000111 000114 000117 000120 000123 000126 000129 000132 000135 000138 000141 000144 000147 000150 000153 000156 000159 000162 000165 000168 000171 000174 000177 000180 000183 000186 000189 000192 000195 000198 000201 000204 000207 000210 000213 000216 000219 000222 000225 000228 000231 000234 000237 000240 000243 000246 000249 000252 000255 000258 000261 000264 000267 000270 000273 000276 000279 000282 000285 000288 000291 000294 000297 000300 000303 000306 000309 000312 000318 000321 000324 000327 000330 000333 000336 000339 000342 000345 000348 000351 000354 000357 000360 000363 000366 000369 000372 000375 000378 000381 000384 000387 000390 000393 000396 000399 000402 000405 000408 000411 000414 000417 000420 000423 000426 000429 000432 000435 000438 000441 000444 000447 000450 000453 000456 000459 000462 000465 000468 000471 000474 000477 000480 000483 000486 000489 000492 000495 000498 000501 000504 000507 000510 000513 000516 000519 000522 000525 000528 000531 000534 000537 000540 000543 000546 000549 000552 000555 000558 000561 000564 000567 000570 000573 000576 000579 000582 000585 000588 000591 000594 000597 000600 000603 000606 000609 000612 000615 000618 000621 000624 000627 000630 000633 000636 000639 000642 000645 000648 000651 000654 000657 000660 000663 000666 000669 000672 000675 000678 000681 000684 000687 000690 000693 000696 000699 000702 000705 000708 000711 000714 000717 000720 000723 000726 000729 000732 000735 000738 000741 000744 000747 000750 000753 000756 000759 000762 000765 000768 000771 000774 000777 000780 000783 000786 000789 000792 000795 000798 000801 000804 000807 000810 000813 000816 000819 000822 000825 000828 000831 000834 000837 000840 000843 000846 000849 000852 000855 000858 000861 000864 000867 000870 000873 000876 000879 000882
English French German Italian Spanish

Inter gear ratios


I need to exercise my new machine before making changes but already I have ordered a replacement pair if pinions to lower 2nd gear. Have others done this? It's a question of keeping originality or making it more convenient.

Original ratios with kick start are

1.0  1.1  1.33  2.33

And with new gears

1.0  1.1  1.61  2.3

...has anyone else bothered?  And, if so, was it worth it?


I think it will make a better ratio set, with gaps of 1.4:1 between 1st - 2nd, 2nd - 3rd.  It would be even better if you get a trials 3rd pair, if they can be had.  Try ex-WD suppliers. 

But I haven't actually put a box together with these ratios to try them



Thanks Paul

One of the gears arrived yesterday.  The dogs are a bit bruised on a leading corner, so I hope they don't jump out if I do fit it.  Before I do anything, I'll compare it with a spare AMC set I have - in case the AMC individual gears fit after all.

The picture shows my calculation for force on the ground from each gear, with a rev limit of 5500.  The torque curve is imaginary, but the ratios between the heights of the lines will be the same whatever the curve shape.

'Standard' is standard Inter kick start box.

The difference is clear and should make it more tractable at the low speeds it was not intended for.




When I work with gearboxes, I start with making diagrams like this. Where blue is original and red is your suggested set.

Inter gearing


Thanks Mikael - that looks an improvement, although there's now (obviously) a very jump from 2 to 3, which your plot emphasises.  Anyway I think I mightgive it a go!  I've not been inside a doll's head box - I'll practice on my 16H, where the effort won't be wasted because I think a thrust washer is worn which (someone here said) can cause uncertainly going up the box from second to third gear.  That'll tell me how long the job will take, and I shall have gained something on the way.

When the bike spent some time with a restorer a decade or so ago, it went with an Inter box (this one) and came back with a standard box (naughty!).  I suspect the standard one would have been better all along of course!


One thing that graph shows is that a Norton needed to have a five speed box to make those ratios work properly - all the standard ratios and the second, higher second as well



Norton spaced the gears as they did because air resistance is proportional to the square of speed. At high speed most of the engine power is used to overcome drag, thus high revs is needed. At low speed the small drag means that lower revs gives enough power for acceleration. Reason why higher gears are more closely spaced.


Good point, Mikael. The engine has plenty of torque for practical purposes even on the high second gear at moderate revs. I'll get used to it before doing something drastic. Thanks to you all for the observations.

Tragically, the bike's previous owner, Bernard Harding, who rebuilt it and from whom I obtained it and who was my never-ending source of technical advice for over 40 years died just two weeks ago, and the conversations I enjoyed and relied upon so much have now finally ceased.


It's great that you had such a long-term relationship with the previous owner of your bike.  Forty years is amazing.

The ratios I suggested give a spread of 20%, 33% and 43%

There's a cluster offered by RGM for the AMC case, which uses Manx sleeve pair, AMC 2 third, AMC 3 second, Daytona first.  That gives ratios 1:1, 1.15, 1.52, 2.05, giving spreads of 15%, 32%, 32%

I would be interested to know what the actual tooth numbers are in a Daytona first gear pair.  Someone published that in Roadholder recently, but I forgot to note it down and don't remember which issue.  Does anyone other than RGM offer them?



Thank you, Paul.

I have just been to the shed and pulled out my old box of spare Norton AMC gears. I now find that the gear I bought off Ebay, with its 1930's part number, is identical to a 1960's one I already own! Lesson still not learned: never buy replacement parts before taking the machine apart first!  Is the doll's head cluster identical to the AMC box? We read the later box was developed from the earlier, but that doesn't help the amateur like me.

Now for the spreadsheet calculations of all options. Meanwhile this will remain theoretical for a while.


Making it a bit more complicated. My previous comments refers to the ideal world (racetrack) where you want best acceleration and top speed. For road use you need a low first for congested traffic. Then you need a fairly tall high gear keeping revs low when effortless cruising around the legal speed limit. You also need a gear where you have more revs and power for overtaking annoying slower lorries and cars. So the ideal set of gears depends on how you use the bike.


David - you ask if the AMC cluster the same as a Norton one, eg for a Doll's head box.

The AMC cluster is derived from the Norton one, but you can not assume that a gear will interchange from one box to the other.  Besides having different chain lines, the mainshafts are obviously not interchangeable, because the Norton boxes use a thrust washer between the 3rd gear pinion and the sleeve gear to control the end thrust due to lifting the clutch.  The AMC box has a nut on the cover end to lock the shaft to the outer ball bearing. 

That also causes differences in the sleeve gear - you have to have a ground face for the thrust washer to press against.  Also the Norton box has roller bearings at this end of the sleeve gear.  This was one of the 'improvements' over the Sturmey Archer box, which functioned well without them.  The AMC lost this, and relied on two plain bushes without ill effect. 

Also, from memory, the dogs between sleeve gear and 3rd gear pinion are 3-dog on AMC, and 4-dog on Norton.  You have to look carefully at each pinion to work out if they can interchange

Another difference is the first gear mainshaft pinion.  Because of the different method of controlling end-float on the AMC, its first gear mainshaft pinion has a shoulder on its right hand side, to act as a spacer between the toothed segment and the ball bearing.  The International first gear pair is 14:27, and the AMC first gear is 14:28.  I expect that these two 14t gear pinions are a different size and tooth form, but have not checked.  It's important to make sure that you use a compatible pair of pinions.

Traditionally, Norton used pinions pairs which always added up to 42 (apart from that Daytona first, which was fine-toothed to get the highest possible first gear with a kick starter).  With the AMC 2, they started mucking about with the tooth totals and gear forms.  It makes it much harder to be certain which pinions will mate satisfactorily.

One AMC upgrade which can be made to transfer is the cam-plate.  If you machine out the main casting, it will go into a horizontal box, but must be mated to a long-travel cam-plate plunger.  For the Doll's Head and Vertical box, the cam-plate spindle with integral pinion are larger, and have larger splines.  So if you want the superb indexing of the AMC on an early Norton box,  you would have to make up a special spindle with the smaller splines.

So you see, it gets complicated when you try mixing and matching



Thanks Paul. All best to leave things alone I suspect. It does however look like the dolls head second gears are unchanged. But I see from 30s parts lists that even those have more than one part number each!


I'll leave this thread with the attached.  It shows comparisons between the thrust on the road (torque at back wheel) in each gear.

The standard Inter box appears, not surprisingly, to be aimed at getting as much thrust down onto the road as possible between about 60 and 90mph, at the expense of 40 to 60.  Whereas on today's roads, 40 to 60 count for more.


Maybe a job for a wet autumn weekend.


.. will, I expect, have similar ratios to the 16H - at least will if as I assume it is standard. I really like the close 3rd and 4th which allow easy switching between them on 50-70mph roads, but there's a yawning gap down to 2nd. I'd really like something between what I have and what I see from your Inter curves.

Although when I say I would like it I'm not sure if I'd like it enough to pay for it......


Just looking at Bacon's Norton Singles. The post-1960 AMC box  has almost exactly the same ratios as the standard 1935-on Norton box. The early AMC box had a bigger gap from 3 to 4, but they must have decided they had made a mistake and the later one, like the 16H box from 1935  had the bigger gap from 2 to 3. Maybe you could lower third by using 3rd gear cogs from an early AMC box. You're on your own from here on! Paul Standeven's mail above might help!

It's all too complicated. Anyway I hope to ride mine this weekend now the working week is over!


I have just spoken to the man at Russell Motors, who deals with classic spares.  He says that the 21:21 third gear is in fact not WD, but they are all gone because the Inter owners bought them all up.  To do my mod.  Damn, other people had the same idea as me, and have used them.


I've just changed the First and Second gear ratios in the laydown box of my 1957 350cc Inter.

The original ratios were (as you first said) 1  1.10  1.33  2.33.

They are now 1  1.10  1.61  2.70. I changed the first and second gear pinions to those of the standard road box. I've been out on it this afternoon and can say that it makes it much easier to ride on the road.



A club friend told me to learn to love it! But he does not live in quite as urban a location as I do. I'll leave it for a while. My 16H gearbox needs a new thrust washer somewhere or so I believe, so I'll learn about the doll's head there first. As far as I can tell it's only a couple of hours work...but I've thought that before!

Hi very interesting reading all your comment on the gear box ratios as I'm at a loss trying to identify my gear box that been siting on the garage shelf collecting dust, wondering if you could help identify it?

The serial number is M11969 the gear teeth are interesting thou, M 23-21-18-14.L 18-21-24-28. main sprocket teeth 16.. The gears shifter forks are bronze in color too .this box is like new no wear or damage, very little use by the parts condition. if this box is of any interest to any one in the Norton club will gladly sell it for beer money.

Sorry for the spelling I'm on a Canadian computer spell check.


Tecwyn Roberts   


I am wondering which gearbox you have.  Is the outer cover a sort of triangular shape, ie an AMC box, or is it a sort of flat oval, the horizontal box?  The ratios read as a mix of AMC1  and AMC2.

I am interested in having the gearbox - I presume you are in the UK.  Is it collectable distance from London?  And what is 'beer money'?


Thanks Paul for the quick reply. If you give me the Postal Code will get a exact shipping cost for you  .want the hole box or just the gear set? and beer money is what you think it worth,( its no good for me sitting collecting dust when some one can really appreciate it and but it to good use) I can take picture if you like to answer your questions. I think its an AMC box (give us a day as I'm traveling thou) . 





Hi Tecwyn/Paul,

   I think the M might indicate this 'box was originally fitted to a Matchless, which might mean the lugs and mainshaft length are different. These gearboxes were fitted to Norton, AJS and Matchless, all part of the AMC group. If it is not the correct one you will probably be able to do a swap or re-sell it quite easily.


I beg to differ, my ES2 had a Matchless box fitted when I bought it, as far as I could see the only visible difference was the angle it mounted at because of the differing mounting lug positions and one mounting lug had been reduced slightly as i5 was wider on one side than the Norton AMC box.  I’m rebuilding that box at the moment and am interested in the gear ratio malarkey as I’m building a Scrambles bike with one of Mikes shortstrokes, I’m thinking that closer ratio gears may help as wind resistance will be less of a factor at the generally lower speed we race at (or me anyway!) But there’s enough people with Norton boxes in Metisses to find out what they run! 


Norton Owners Club Website by White-Hot Design

Privacy Policy