Cam failure

 Too long between oil changes. etc etc

Sometimes the engine can be noisy when the cam is/has failed. Other times it will give no sign of  failure, the engine exhibiting no undue noise. The engine has obviously been apart at some stage in the past fifty odd years (at least the top end has)  If the cam has been replaced in the past it is possible that the cam run in procedure was neglected and/or the followers weren't refaced at the same time.  1970 bike, does it have the spin on filter?    

Once you're thru the hardening the cam will disappear pretty quickly. Suprising it ran at all really. All sorts of reasons for failure from poorly manufactured camshaft to head oil feed failure to poor assembly.

Bronze guides are fine for exhaust. Seals can be pushed over guides without a lip and will stay for a while. Even floating around, the seal will do a little bit.

The tappet clearance is measured on the base circle so disappearing lobes aren't noticeable. The clue is usually on a magnetic drain plug if one is fitted.

It usually seems to be the drive side exhaust lobe which has the most marginal lubrication, but once a lobe has scuffed, it can indeed disappear quite quickly. It is indeed remarkable how well they can run with presumably one cylinder doing most of the work.

Are your guides perhaps Colsibro (a copper/nickel/silicon alloy, rather than copper/tin)?

I have a set in my Fastback, run without oil seals on either inlet or exhaust. I can't comment on really long-term use, but three years' experience suggests minimal effects on oil consumption, etc. 

Most of the BIG name Norton Engine restorers and tuners recommend fitting Colisbro guides all-round. The RGM set comes with inlet guide seal lip grooves and boot seals whereas the exhaust have plain shoulders. Care must be taken when fitting Colisbro guides as the inner bore can narrow and need reaming back to the correct running clearance. If this clearance is poor then the valve stems will pick up in the guide which in turn can overload the camshaft.

The downside of a worn camshaft lobe(s)  is the trail of very abrasive debris pumped around the engine as a consequence. An inline filter will have helped, if fitted, but I would be looking at changing the main bearings and possibly the big ends while the motor is dismantled. The oil pump and any bushes would probably need checking out as well.

In the 1970s the factory fitted a soft cam, I questioned this at the time with the factory, they said the cams were the same material as they had always been, what they didn't say their new hardening process actually made them softer!! So 1970s saw a whole fiasco of soft cams from the factory (I know the 'hard' way). AS Richard says above-you adjust the tappet gap on the BACK of the cam. So the cam could completely disappear, but your tappet gap will stay the same. The other notice of a worn cam is a lack of top end performance. Loosing the timed breather is no big deal, BUT you must have some breathing-see Commando Service Notes for 750 breathing.

It is not soft cams that fail, but Norton's ever increasing desire to produce harder and harder cams and followers. The race to ever increasing hardness was their undoing, due to an odd unrelated comment I made to a hard facing company it resulted in uncovering on why cams fail. 

gonnae tell us why Ashley ????

Cam followers should have ground finish. For Norton camshafts, grind flat, square to the follower body and in the direction of travel.

Camshaft is lubricated by oil running down the pushrods tunnels. It should be a river. Correct installion of cam followers and rocker spindles is essential. Likewise a good oil pump, good seals and no blockages in feed pipes, unions etc.

Before starting after a lengthy layup (winter), squirt some oil down the pushrods via the exhaust tappet covers. This also applies to newly assembled engines. Moly grease, aaargh, no. Proper assembly paste if the engine is going to sit around a while before you start it, otherwise oil is fine.

The camshafts and followers that Andover sell are fine. I wouldn't look anywhere else.

The earlier bikes like the 88/99/650  had little issues with cam wear even though  the rocker boxes recieved only a dribble of oil from the very low pressure system. MY 99 is still giving better than new performance on the orriginal parts after 50k miles. I put this down to a mild state of tune ,  a functional cam trough and a bit of wet sumping that ensures the cam gets a good splash immediately on starting. I dont fully drain the wet sump now but leave plenty in there . Hard valve springs and fierce  cam profiles must be a factor. It should be possible to end feed the cam and have oilways in the cam flanks? perhaps thats the way to go ?.

and engine oil with proper zinc content will help too.

Sure am, but you'll have to wait until September.