850 RPM redline

Fullauto · Nov 2, 2010

850s had a 21 tooth sprocket as standard, not 20. MK3s had a 22tooth.

Myford · Nov 2, 2010

I thought I read somewhere that the old Burman/AMC/Norton/Commando box was being asked to do an awful lot coping with the Commando output, & that it was wise not to go too long legged on the gearbox sprocket as the extra torque was too much for what is basically a 1930s gearbox designed for engines of half that output.
where did I get that from? or did I dream it (don't answer that!).
Terry

mike996 · Nov 2, 2010

I seem to recall something like that myself but have no idea where I might have heard it. Actully, based on my current memory ability, maybe the first time I heard it was in the post above!

Reggie · Nov 2, 2010

myfore wrote;

I thought I read somewhere that the old Burman/AMC/Norton/Commando box was being asked to do an awful lot coping with the Commando output, & that it was wise not to go too long legged on the gearbox sprocket as the extra torque was too much for what is basically a 1930s gearbox designed for engines of half that output.
where did I get that from? or did I dream it (don't answer that!).

Taken from Norton Service Notes;

When you think that the AMC box was designed to cope with 30 bhp from a 500cc
machine, it is not surprising that on 828cc machines giving about twice the power the reliability of the box
is not what it should be

mike996 wrote;

I seem to recall something like that myself but have no idea where I might have heard it.

Taken from Norton Service Notes.

It seems to me very unfortunate that they did not change Commando gear ratios by using alternative
engine sprockets, because with a larger engine sprocket and retaining say, a 19T on the gearbox (as was
standard on all Norton Nortons from Model 50 to Atlas and Manx) the speed of the box would be
increased without such a heavy journal load on -the bearings and tooth loading on the pinions
themselves. > >

comnoz · Nov 2, 2010

Myford said:
I thought I read somewhere that the old Burman/AMC/Norton/Commando box was being asked to do an awful lot coping with the Commando output, & that it was wise not to go too long legged on the gearbox sprocket as the extra torque was too much for what is basically a 1930s gearbox designed for engines of half that output.
where did I get that from? or did I dream it (don't answer that!).
Terry

I don't know if tall final drive gearing has much influence on gearbox life or not. Some say yes, some say no, I have to think the engine only makes so much torque and it will all be applied to the gearbox anyway.
The one thing for sure is if you want to take some torque off the gearbox then go to a taller primary ratio. I use the 1.75 ration and a 19 tooth gearbox with a 42 tooth rear. Works well for a highway bike and has cured broken trannys. Maybe age has had an influence also. Jim

Govig · Nov 2, 2010

Having ridden a 850 Mk3 for well over 30 years I'm puzzled why anyone needs to rev past about 5500 in normal road riding. 7000 is asking for it.......

it =

kraakevik · Nov 2, 2010

I've been running a 24-tooth sprocket with a taller 18-inch tire for decades without undue gearbox trauma--second is a 0-100 mph gear if you're willing to overrev, with an indicated 89 mph at 4000 rpm in fourth. The current Combat head TC magically ported for me delivers acceleration in the top half of second that's almost unbelievable.

The layshaft fourth gear breaks every 50,000 miles or so--the sleeve gear usually lasts about twice that. The first, second and third gearsets seem practically bulletproof despite occasional abuse if properly bushed with good bearings and the primary on the loose side. When you consider the built-in headwinds Norton faced in manufacturing issues the gearbox is a semi-modern marvel.

Tim Kraakevik
kraakevik@voyager.net
Three Commandos

maylar · Nov 3, 2010

Fullauto said:
850s had a 21 tooth sprocket as standard, not 20. MK3s had a 22tooth.

My '74 Roadster came from the factory with a 20T. Maybe the Interstates had a 21?

Fullauto · Nov 3, 2010

maylar said:
Fullauto said:

850s had a 21 tooth sprocket as standard, not 20. MK3s had a 22tooth.

Click to expand...

My '74 Roadster came from the factory with a 20T. Maybe the Interstates had a 21?

The only one I've ever heard of with a 20 tooth as standard.

Flo · Nov 3, 2010

My 850 Mk.1 came with a 20 tooth & I can remember seeing a sticker on it saying to change to 21 tooth for motorway use. That was an Interstate.

tpeever · Nov 3, 2010

Fullauto said:
maylar said:

Fullauto said:

850s had a 21 tooth sprocket as standard, not 20. MK3s had a 22tooth.

Click to expand...

My '74 Roadster came from the factory with a 20T. Maybe the Interstates had a 21?

Click to expand...

The only one I've ever heard of with a 20 tooth as standard.

I thought I remember reading somewhere that all the 850s shipped to the US had 20-tooth sprockets. Someone more knowledgeable than I may be able to confirm. My 850 had a 20-tooth and was original as far as I can tell.

ewgoforth · Nov 4, 2010

When i got my '73 850 in 1995 it had been sitting since 1983 and it had a 21 T sprocket on it. This was with 9000ish miles on it, so I imagine it was the original sprocket.

-Eric

debby · Nov 4, 2010

I thought my '73 850 came with a 21 from the factory, but I'm not sure anymore. That bike is long gone. I do remember there was a half link in the chain, and with a 20 tooth you don't need the half link.

Maybe it's another one of those situations where they fitted whatever parts they happened to have on hand at the time.

I have a 21 on my current '74 850 and it seems just right for the riding I do.

Debby

850 RPM redline

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