Another cylinder head

If it has a 4S camshaft, chances are the head had already had quite a bit of material taken off the gasket face. If the valves don't hit the pistons and the pistons don't hit the head it'll be ok but whether it's what you want is another matter altogether.

If you want a race engine then read the tuning leaflet in the Commando Service notes on this website (https://www.nortonownersclub.org/roadholder/roadholder/load?rh=Svs_Release, page 114), measure the clearances and proceed from there. If you want a more standard bike, swap the 4S cam for standard, adjust the compression with one or more compression plates under the barrel, check the clearances and enjoy.

Personally I would use the simple solution, use a standard cam and live with the little extra compression if the head has been previously skimmed. The 06.1084 cam is very good and will work well with more compression. Unless you are enlarging the valves the 4S cam in an 850 is truly pointless. 

Seems strange to fit a 4S cam into a MKIII. IF you want to 'go faster' then just take that extra electric motor off! In fact I would have thought a lot more Earlier Commandos would take the 4S a lot better. As Ashley says.

I have to take issue with Ashley's assertion that, with standard valves "the 4S cam in an 850 is truly pointless". Such an 850 competed successfully against CBR600s in the 90s, so far from pointless.

But that's a digression. To get back to the original question, a 4S engine could be running 10.5:1 compression. With a further 0.012" skimmed of that could be knocking on 11:1. The OP really needs to measure what he's got and where decide he wants to go.

You would need to do a lot of work to an 850 to compete with CBR 600. I have one, if not, the cam with the largest lift fitted in my MK3, the cam designed by PW just before his passing. Many experts who saw the lift and LSA said it would not even work as the bike would not start or even tickover, guess what, they were all wrong. 

My bike with full auto head and standard valves was dyno'd and the rev range was still the same as the standard 850 as the 850 cannot breath through the size of valve that is standard. Even in this configuration it only managed just shy of 50 Hp, any suggestion that an 850 without a lot of work could compete with CBR 600 is nonsense. A standard 850 struggles to compete with a well sorted 750 Commando. 

Compression is very subjective, we have a MK3 here with PW3 cam and our HC Omega's fitted, the pistons some racers are afraid fit due to the increase in compression they give. And yet where others say it would be awful would be surprised to find like many that know the bike actually rides very well. 

So unless the OP is going to fit HC pistons, and fit large inlet valves, then yes - the 4s cam in an 850 is truly pointless as I suggested above. 

I had to use a copper  cylinder spacer and locktite sealer to get my engine right after too many head skims

What do you intend to do with the bike? ride around at wide open throttle? or just want a nice reliable tourer? if the answer is the second one replace the 4S  with the standard cam.

Leaving the 4S cam in it will give the valve gear a harder time, it will be a bit 'cammy' and just generally not as nice to ride. It will also use more petrol. 

Head wise, get it flat and the valves sorted and fit with a copper gasket, any slight increase in CR will be ok. If you have a burette, measure the CR and see what you have, then decide how to address if it needs it. 

interesting read    manuel_norton_1974_850_hautes_performances_anglais.pdf (motos-anglaises.com)

I am not sure what all the fuss is about regarding a 4S camshaft.  I fitted one to my Mk.III in 1988.  Machine runs faultlessly, is easy to ride, not ‘cammy’ at all, regularly returns over 70mpg from my one Amal 32mm concentric carburettor. Valve clearances are in the region of ‘ordinary’, i.e. not quite the 6 and 8 standard specifications. Having heard another Commando with a 4S, this time set to 12 and 16 on a 750, I watched as the acceleration was phenomenal, especially as it turned out to have a 19 sprocket [are you out there Dr Dan?]. I rarely exceed 70mph and have a steady right hand which means my bike is usually very reliable. If I wanted to ride much faster I would buy a different bike. Incidentally, I bought the 4S as there were no standard camshafts available at the time and, as usual, I just wanted to ride my bike…

As ever, your mileage, and experience with camshafts, may vary.

I've had a MKIIa 850 for many years now. When purchased it had the black box air box, down pipes with the balance pipe (very poor fit) and pea shooters. The cylinder head had been replaced sometime in the past for an earlier version with the 32mm inlet ports, RH4 rather than the original RH10 if I recall correctly. (I have the original head with striped exhaust ports).

The bike ran fine but the power peaked at 6000rpm, tapering off and not keen to head towards the redline. I replaced the air filter for the early type and altered the carbs accordingly. I probably replaced the down pipes for straight through items around the same time. Massive improvement, the power just keeps building all the way up to 7000rpm and would happly carry on going if I let it.

After the standard cam lost a lobe several years ago I replaced it with a 4S. I also had the head gas flowed by Pete Lovell at the same time. Pete also machined pockets in the pistons for valve clearence. The head also had a slight skim following a thread repair (bronze insert). I expected to lose some flexibility and gain a little at the top end but I was wrong, it was a win win all round! Better from the bottom end right through to the redline with improved fuel consumption to boot. (Pete told me the gas flowed heads needed a leaner mixture and it certainly did). From memory the main jet went down 2 sizes and the needle jet might be smaller as well.

The only down side is wider, (therefore slightly noisier) valve clearences, I stick to 10  thou now. I know it should be a little more but I know somebody who rides a lot harder, faster and further than I do on 10 thou clearences who has never had an issue. I accept the valve train has a harder time but I'm not likely to put another 100,000 miles on the bike in my life time.

After the work had been carried out I bought a bigger, much faster modern bike. Consequently the Norton isn't ridden so hard now so standard trim (without the black air box) would be fine. It still has the odd run up to the redline though, a lot of fun!