Clutch types

The classic Norton clutch as we know it was designed in 1934.  It had blocks of Ferodo friction material inserted into the steel friction plates, which had tags out.  There were also blocks of friction material in the back of the clutch drum.  There was a shock absorber in the clutch centre, and the clutch spring studs in the clutch centre were hollow, for special spring bolts with a sort of integral washer.  Using ordinary bolts and a washer makes assembly very hard...  The pressure plate was pressed steel, and a 'lifter mushroom' looking like a small flat inlet valve - it ensured even lifting of the plate.

The first modification was for the AMC gearbox. This was largely the same as the clutch for the Norton box.  It got an alloy pressure plate, with an adjuster screw in the centre.  The clutch springs were retained by three special bolts in the clutch drum, and special slotted nuts on the springs.

In the early '60s, the arrangement of friction and plain plates was reversed.  The back plate had bonded friction material, along with all the plates with 'tags in'. The clutch drum, and all the 'tags out' plates became plain.

The Norton boxes used different lengths of mainshaft to gain correct chain alignment with different engines.  I understand that with the AMC box, alignment of the primary chain with various engines by altering the clutch centre so it sits differently on the mainshaft - so you have to be careful to make sure that a newly fitted clutch is the correct version.

As far as I know, the classic Norton clutch always has the same number of plates in road form, but has fewer in its Manx variation, and the drum and clutch basket are narrower. Yes, the clutch fitted to GP-winning bikes was just a cut-down road-bike clutch with heavier springs.

The Commando clutch is a totally different beast, having a triplex chain and using a diaphragm spring.  There''s no cush drive arrangement in the centre.  To fit one to a gearbox made for the classic Norton clutch, you need a different mainshaft, longer with a groove for the circlip to retain the diaphragm spring.  The main variation in Commando clutches is whether it has plates of normal friction material or sintered bronze ones.  They are thicker, so there are variations in how many plates are used.  Some people use a mix of friction material and bronze plates to get perfect preload on the spring.

Paul

- many thanks Paul. Some say that the AMC type pressure plate was a retrograde step as the mushroom in the earlier version helped the plate to lift squarely. I prefer the gearbox action of the AMC, but the clutch of the earler model.

So Mikael - let me start with the AMC versions of clutches that came in with the 1957 Models. All of them share common parts but some parts are peculiar to specific models.  So the Model 50 has seven plates, the ES2 , 88 and 99 have nine plates whilst the 650, 88SS, 99SS, Atlas and Mercury  have eleven plates.  Because they all share the same drum and centre they have different thicknesses of pressure plates to suit with the heftiest on the Model 50 and the thinest on the 650 etc.  The 650 and SS models also share thinner bonded plates, a stronger shock absorber centre, longer spring studs and stronger springs.  The Model 50, ES2, 88 and 99 use the same parts except for there being two plates less and a different pressure plate on the Model 50.  All these clutches have plain steel plates with location tags on the outer rims and bonded plates with tags at the inner rim.  They all appear to have been attacked by a nail gun during the manufacturing process.  The sprocket/drums for all these clutches have a ground surface both sides.  The mainshafts in all these models gearboxes are the same length so clutch adjustment is determined from the pressure plate, spring tension and adjustment screw whilst the clutch position is fixed by the mainshaft and clutch centre machining.  The AMC Manx clutch does not share many items with the road bikes so I mention it only for completeness.

Prior to 1957 all the post-war models used the Norton Burman gearbox and clutch.  These clutches have friction plates with tags at the outer rim and steel plates with tags on the inner. The clutch back-plate is plain so the sprocket/drum also has friction material. Many of the dimensions of these components are the same or very close to the AMC types so there are many opportunities to use odd combinations of parts.  There are many different mainshafts and clutch centres for the various models to fix the  position of the clutch. Some of the early plates used separate pieces of friction material and in the drum whilst many had bonded on material. Also many workshops modified the parts to fit or improve accordingly.  The pressure plates had a fixed centre but many people fitted an adjustment screw. 

So you can see why there are many pitfalls possible with Norton clutches, Pre-Commando of course.  It's a nightmare if you don't buy an assembled item.  Caveat emptor !!  cheers, Howard    

To clarify a point I touched on in my post...  Norton clutches used inserts of Ferodo friction material.  These were inserted into holes cut in the friction plates, and into the flat wall of the clutch chainwheel.  These inserts were pre-formed read to fit, and could be bought as a spare part.  The inserts for the clutch chainwheel were thicker than the ones for the ordinary friction plates  From memory, I think the plate ones were 3/16" and the chainwheel ones 1/4" - these are MUCH harder to obtain than the former.  When fitting them it was important to get them seated evenly.

At some point, either for AMC  or early '60s, the friction material was bonded. 

FWIW Norton bought the design rights to a very good gearbox from Sturmey Archer in '34, and progressively improved. it.  They had no production facility to make the gearboxes, which were made by Burman for Norton under contract.  It was never a 'Burman' gearbox, and should not be referred to as such.

Paul

Sorry Paul - Perhaps I should have referred to it as a Norton/Burman gearbox as opposed to a Norton/Sturmey Archer gearbox or indeed as an AMC gearbox.  I am old school and do not subscribe to the name of "lay down" which sounds like an instruction to a sheepdog or the other silly name "upright ". Ride safe, Howard

Another point about Manx boxes (I've been doing a lot of reading recently, so please correct me if I'm wrong), but their lighter 3 plate clutch was very definitely intended to be run dry. No oil bath chain case but drip feed oil onto the chain. So it appears to be a bit misleading to claim as some report that the Norton clutch is good for competition purposes unless you add the words "provided that it is run dry". Which leaves the matter of lubricating the support bearing race..

As they provide a means to identify the variants of the pre-AMC gearbox. Much of the evolution concerned the positive stop selector mechanism housed in the outer case. The names simply reflect the changes in shape of the outer case needed to accomodate the different selector mechanisms eg Dolls Head, Upright, Laydown. You may not like them but I think they are here to stay!

Ian McD

Re Silly names - Is it little wonder that new younger members seeking information are put off with the system of the names of parts that persist in our Forums when a technical change occurred ?  Dollies head , Lay-down,  Laid-down,  Lie-down (or whatever it is this week) and Upright may well suit less erudite folklore but someone needing to know surely has to be able to specify exactly what gearbox or clutch or whatever the item is giving the problem or needing replacement.  Sturmey Archer, Burman, AMC1/2, Commado and Commando Mk111 are fairly precise.  Silly names will continue upwards, I have no doubt,  and so will the average age of our members........ride safe, Howard

... Burman would be very misleading as they merely made the boxes to Norton's (or even S-A) design. I don't actually see anything wrong with the "silly" names which are at least descriptive and accurate.

I've moved on from Burman to Norton/Burman now Ian......Wakey, wakey .....as Billy Cotton would say.  I shall not contribute to this Silly thread again.  The technical content regarding the various clutches raised by Mikael I have answered and the rest of the thread is just wadding and much of it is incorrect.  Cheers, Howard