AMC Gearbox 1967 Atlas

I've recently read that the early Atlas's had a "N" prefix before the gearbox serial number and then it was changed to " NA" somewhere around 1966. Does anybody know what changes were made if any? Reason I ask is that I may need a new case and was wondering if the internals of a "NA" will fit properly into a "N" case? Thanks

Here's what I do when I want to know if parts are interchangeable on some of the popular pre-Commando Norton models. I go to Andover Norton and reference part numbers by clicking on the part in their exploded assembly diagrams. I also reference the Master Parts List 1966-1967-1968 for models 650SS, Atlas, N15CS, P11, P11A, G15CS, G15CSR, G15 MARK II. That way I don't have to read though posts like mine that don't have a particularly useful answer.

Off the top of my head guess after taking a quick peek at the Master Parts List is there's no significant difference in the parts inside the gearbox shells of those years of the Atlas. Only differences I see internally with all of those models and years is with the camplate of the 1966-1967 G15CSR.

Thanks. Good suggestion

this goog came up

"N" Your box marking says it's a early AMC box. N typically is the early style: kicker, kicker spring, and matching inner and outer cover. These "N" parts do not readily interchange with"NA" or commando parts. People have machined them to update them... but a not real necessity.May 9, 2017

Submitted by Alex Arthur on Mon, 19/11/2018
Dave, to add to Anna's ad…

Dave, to add to Anna's advice, your bike would originally have had an 'N****' series serial number box. The later NA series will also fit as will M and (later) MA (Matchless) boxes as they are essentially the same. The later boxes have a better ratio selection for 3rd gear. Commando boxes for RH change bikes will also fit but will need a spacer (1/8" iirc) on the top mounting bolt. As it happens, I have a spare N series box...

IIRC, the major difference is the kick lever return spring. Not interchangeable.

Slick

more details here

Another difference in the AMC boxes was on the clutch adjuster. On the early boxes, the clutch cable was adjusted at the gearbox, so the outer cover has a threaded hole for the cable adjuster to screw into, the later models had the adjustment at the handlebar, so the hole in the gearbox for the cable had no threads, just a hole that the cable bushing would fit into. I believe this change happened in 1957, depending on the model, and the auctual date of manufacture, rather than the model year.

The Commando gearbox was always stamped with the numerical portion on the engine number, and should match the frame as well. 750 models are within a 100000 to 200000 series, and 850 boxes are in the 300000 series. The casing on the Commando was also reduced at the upper mounting bosses to make installation easier. It was thinned and a separate spacer was fitted between the engine mounting cradle and the box. Leaving this out can distort the cradle, so be careful!

In 1968, at number 128646, the high gear pair was modified for strength and durability, and these gears must be a matched set. The tooth profile and number was altered and they will not mesh properly if they are mixed. This also raised the ratio of first, second and third gears. Later on the 850 Commando, the second gear pair was altered starting at number 306591 to enable the bike to pass tougher noise regulations. The test involved passing a noise meter in second gear at a certain speed, so the gearing was altered to allow this to happen at lower revs. Clever those Brits.

Since this is turned into an AMC gearbox on any model thread: The outer cover on my '67 P11 AMC gearbox has the threaded boss for the clutch cable adjustment. Thats how it was when I got it in '73. Nothing to do with the N case prefix stuff, just saying the clutch cable adjuster was still threaded in some AMC gearboxes in 1967 as far as I can tell.

The Norton Villiers White Master Parts List for '66 through '68 pre-Commandos that includes the Atlas does not have any parts changes for Commando gearboxes mentioned in it, so I missed all that obviously. It does not show any differences for the kickstart spring either for those model years. Color me uninformed.

All things considered and not fearing going out on a limb, I'm thinking the working guts/gears and such inside a 1967 NA case will fit into an N case. I'm not sure about the kick start spring, but that should not be an issue if the old inner cover is used.

Submitted by Paul Nicholls on Sat, 16/06/2018

Most likely off a Matchles…

Most likely off a Matchless G12CSR or AJS31CSR as the R stands for close ratio.
Paul

FWIW My Atlas, titled as a '66, has an "NA" shell. The number doesn't match the engine no. I can't tell if the clutch cable hole has threads without pulling it our but my 60 Matchless does.

In about '92 or so I built a G15 CSR out of a G15CS. I had heard that there were reversed cam plates available. Is that the cam plate listed for the CSR? I don't recall the gear ratios being any different. I wanted to flip the shift lever around but not finding one I fitted after-market rear-sets. I believe that bike is still floating around the Puget Sound area. I last saw it at the Isle of Vashon Rally about 10 years ago.

Thomasa said:

FWIW My Atlas, titled as a '66, has an "NA" shell. The number doesn't match the engine no. I can't tell if the clutch cable hole has threads without pulling it our but my 60 Matchless does.

In about '92 or so I built a G15 CSR out of a G15CS. I had heard that there were reversed cam plates available. Is that the cam plate listed for the CSR? I don't recall the gear ratios being any different. I wanted to flip the shift lever around but not finding one I fitted after-market rear-sets. I believe that bike is still floating around the Puget Sound area. I last saw it at the Isle of Vashon Rally about 10 years ago.

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The P11 gearbox serial number does not match the engine number on my P11. I'm not sure if that is typical or not. It starts with P11/ followed by a few numbers that differ from the engine. The P11 shell is unique. Probably a Matchless piece. The scramblers manual I have did come from the Norton Matchless Division. The Master Parts List is from Norton Villiers.

I see maybe 1 Norton a year on the road. Can't look at them closely enough to tell what model they are, cuz they are always going in the opposite direction.

I wonder if the G85 shell is the same as the P11 shell. In the G80 the shell is tilted such that oil runs out of the shift shaft; not so on the Atlas. Don't remember about the G85.

On the G15CSR I mixed internals in the Matchless shell to use a Commando clutch when I fitted a whole Commando primary. I don't remember what gears I used. I fitted the Commando primary because I had serious questions about the clutch and single-row primary chain holding up to a considerably hotted up 750. It walked away from a well tuned Triumph triple on a long uphill straight. I never had a problem with the primary but I'm not at all sure it was worth the considerable effort. I've never had clutch or chain problem with my Atlas that is somewhat warmed up and has been thoroughly thrashed in the Cascade Mtn's and up into the BC Canada interior.

That 3 spring clutch was not able to take WOT on my current incarnation of the P11 engine. It's sitting in a box with brand new frictions and steels, but instead of trying one more time I decided to stuff a belt clutch in the primary. Much better. I tried an NEB wet clutch and never got it to work well either.

The Red Scrambles Motor Cycles manual that covers the G85CS, G80CS, and P11 says All models: Transmission Clutch Gearbox, so yeah the G85CS and P11 gearbox shell is the same. Rebuild kits with the bushes and so on from AN for the Desert Sleds are the same kit for the early Commandos. The parts do fit, so I don't know what it all means. Time to order a pizza

Schwany said:

The P11 gearbox serial number does not match the engine number on my P11. I'm not sure if that is typical or not. It starts with P11/ followed by a few numbers that differ from the engine. The P11 shell is unique. Probably a Matchless piece. The scramblers manual I have did come from the Norton Matchless Division. The Master Parts List is from Norton Villiers.

I see maybe 1 Norton a year on the road. Can't look at them closely enough to tell what model they are, cuz they are always going in the opposite direction.

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FYI. P11 and P11A gearboxes were stamped on the top mounting bolt boss with a number that did NOT match the engine and frame numbers. They also had a prefix of "P11" before the number. For the last batch of P11's, the Ranger 750, the serial number was not stamped on the gearbox, but on the inner gearbox cover. This number matches the engine number and frame number and usually has an "M" suffix