Combat 2s Cam identification

TomU said:
Interestingly, mine did not have scrolled bushes (nor scrolled cam). I'm not the original owner, but doubt the PO would replace the bushes with 15k on the odo
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That’s probably not good. The cam should have one or the other. Is your combat later production such as 205xxx and above? I heard the later ones may have had scrolled camshafts with plain bushes. My two combats were early production…October and December ‘71.
 
ILLF8ED said:
That’s probably not good. The cam should have one or the other. Is your combat later production such as 205xxx and above? I heard the later ones may have had scrolled camshafts with plain bushes. My two combats were early production…October and December ‘71.
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208618. Cam was smooth. I had it scrolled
 
Thoughts from the curious not the concerned.

The cam I have has 2 S's on the side of the left exhaust lobe, and is scrolled, but has no provision for breather support at the left end, or the hump with the four holes in it. I do not have 1972 Combat cases. I don't need help. I'm just showing some parts that in my warped way of seeing the world are relevant to somebody asking about a 2S cam.

Exploded view drawings on the AN site for 1972 Commando and later cams show both scrolled and unscrolled cams without timed breather support. Only drawings that show cams with timed breather support are for early pre-commando Nortons and 1971 Commandos. AN only sells cams with breather support now. Makes me wonder what the Combat motors actually shipped with in 1972.

Below 2 pics are the SS cam I'm referring to in first paragraph and I was using in modified P11 cases. No breather support in the cam design, and believe me it did everything a 2S cam would. It is a sharp ramped bump stick.

Combat 2s Cam identification


Combat 2s Cam identification


Below pic is the JSM JS2SS cam I am currently using. Note the breather support hump. It is also machined for the timed breather disc at the left end. I don't use the timed breather because it doesn't work as well as the solution I am using in the higher RPM range. The timed breather is however a little better in the lower RPM range. The 1972 cases don't need the timed breather support. The JS2SS cam is different animal from the Norton SS cam. Bigger boost where people say Nortons don't make power anymore. Jim did a good job with this cam, and it is an excellent replacement for a 2S cam.

Combat 2s Cam identification
 
Forgot to mention the JS2 cam isn't what I consider a drop in replacement for any Norton cam. It requires the original tappets be radiused, or the BSA lifter blocks and BSA radiused lifters (or tappets if one prefers), and shortened pushrods. Obviously, it should be less expensive to install a new Norton 2S cam and have the tappets surfaced. I know lots of folks here already know all that, but maybe a person new to Norton ownership might not unless they did some research on Norton cams and JSM products.

Thanks B+ for the pic of the original SS Combat cam. Looks the same as Mensch's cam but no scrolling as mentioned by ILLF8ED.
 
I do not understand this stuff about Commando motors reaching a point where they do not make power any more. My motor pulls hard from bottom and right up through the top. However my exhaust system is designed to avoid cutting the top off the usable rev range and my crank is balanced at about 72%. A standard Commando 850 cam, is a hot cam already. Isolastics do not change the way the crank behaves, when it is revved high. I never believed in the Commando crank and the way I rebalanced it is probably not the best. However I have come to believe that fundamentally the 850 Commando motor is an excellent design.
 
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They still make power as they rev past the power peak at 5800 rpm, but they make less and less power as they rev toward destruction. That might be a bit overly dramatic. I've revved mine past 5800 rpm a great many times without an issue. I think the manufacturer's warning " Do not exceed 7000 rpm" for the 850 is a good one to use.
For getting down the road fast, I now use 62-6300 as a shift point. That is after looking at a few dyno charts for stock 850s. Prior to that I used a higher rpm and thought that was the way to go. Assuming my 850 dyno chart is shaped like the one below, 62 -6300 is going to work better than 66 or 6700 , my old favourite shift point.
The 850 has so much midrange grunt that you really aren't missing much if you shift it at a lower point yet, say 5500 revs. It will be a little slower than with the 6300 rpm shift point but should last a little longer too.

What the rider thinks is happening regarding acceleration and what is actually happening are almost always two different things.
It's probably safe to say that all of the popular Commando high performance cams raise the peak power rpm somewhat. The charts I have looked at seem so show a power peak around 6300-6500 for the PW3 and similar.

A stock 850 dyno chart

Screenshot_20240121_111325_Chrome.jpg
 
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To determine the optimum revs at which to change gear one needs the rear wheel torque curve curve plotted for each gear. The revs in one gear where the torque is exceeded by that in the next gear is where you should change up.
 
hybridracer said:
To determine the optimum revs at which to change gear one needs the rear wheel torque curve curve plotted for each gear. The revs in one gear where the torque is exceeded by that in the next gear is where you should change up.
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Torque is a very strange thing. You can improve it and get no advantage until you raise the overall gearing. With my 850 motor, I use a rev limit of 7000 RPM, but usually see 7,300 RPM on upchanges. My revs never drop below about 5,500 RPM, with the close gears. The bike is quick enough to be competitive when used that way. However it is much lighter than most. I progressively raised the overall gearing while using the bike on a circuit which I know very well. As I do not race these days, the matter is purely academic, but still interesting. In the old days, I would have rebalanced the crank to a factor of 78% and revved it higher. I seem to remember Bob Rosenthal mentioning that balance factor and revs being used with his Atlas. It was quite common to set up 650 Triumphs that way.. When I change gears at 7000 RPM, i back off very slightly to take the load off the gears then pusht the lever - I always got a surge from the heavy crank as it lost momentum.
 
The problem with just looking at torque for shift points on the 850 is that it makes more torque at 2800 than it does at 6000!
Torque alone doesn't get you down the road fast, although it sure is a nice commodity.
For max acceleration the important number is bhp, which means staying as close to the max bhp point of 5800( stock engine) as possible while accelerating. With 4 gears one has to run above and below that 5800 rpm point by a fair bit.

Side note, the chart shown was listed as 46.6 rwhp at max. Converted to DIN horsepower this is 56. It is 55 converted to SAE. That is without any weather corrections, which would likely add a hp or 2.
So very close to the Manufacturer's claim for crankshaft bhp for the 850, assuming they were referring to SAE or DIN horsepower, the two most commonly used standards.

Glen
 
The 2S cam is a good road cam it works a bit better than the standard cam performance wise without over doing it, the motor will run a bit more freely and seeing its got the Combat head I be using the 2S cam profile.
I built my 850 with a 2S cam profile with the ports opened and smoothed out, my head shaved but not as much as the Combat heads were, my lifters had been ground for the cam and my crank balanced at 72% and running 40 oversize flat top Hepalite pistons with a tight final hone and my Amals jetted with 270 mains and manifolds ported to match the head, with stock heads everything is out of port shape, getting everything to flow right is more important than going over board with porting.
My hot 850 is a great road bike and is smooth from low revs around town but open the throttle up it gets up and goes a lot better than when it was just a stock 850 as well your mufflers must be free flowing as well, the 2S cam profile is a good road cam profile as long as everything is matched to it and the Combat heads are matched to the cam compared to a stock 750, if they had the upgraded main bearing in the days they would have been a reliable motor as my 850 with the 2S set up is, haven't had any problem since 1982 running this set up, but it will run freely so keeping an eye on the revs as it's easy to over rev it as it will keep running freely.
This is my opinion on the 2S cam and everyone will have their own opinion and as I say using a 2S cam profile everything else has to work with it, so I say replace it with another 2S cam profile.
Its not a race cam but a cam profile for better performance for the road without over doing it.

Ashley
 
worntorn said:
The problem with just looking at torque for shift points on the 850 is that it makes more torque at 2800 than it does at 6000!
Torque alone doesn't get you down the road fast, although it sure is a nice commodity.
For max acceleration the important number is bhp, which means staying as close to the max bhp point of 5800( stock engine) as possible while accelerating. With 4 gears one has to run above and below that 5800 rpm point by a fair bit.

Side note, the chart shown was listed as 46.6 rwhp at max. Converted to DIN horsepower this is 56. It is 55 converted to SAE. That is without any weather corrections, which would likely add a hp or 2.
So very close to the Manufacturer's claim for crankshaft bhp for the 850, assuming they were referring to SAE or DIN horsepower, the two most commonly used standards.

Glen
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A good point is raised regarding the 4-speed gearbox.
The optimum shift point in any gear is also influenced by the gear you're changing up in to, i.e., not dropping the motor into a flat-spot.
The recent post about a taller geared Combat not achieving high top speeds did make me wonder...
I found the gap between 3rd and 4th on my standard, 19T gearbox sprocket equipped Combat to be rather larger than I would have liked, and I used to rev it higher in 3rd when changing up for this reason.
It definitely didn't feel like the power was falling off a cliff either!
As an aside, my 21T equipped MK2A 850 had a monstrous gap between first and second, and I had to let the revs drop right off before changing down into first.

I understood that the 3S was the Production Racer cam, and the 2S was a fast road cam. I don't know where the 4S cam fitted in, but it never seemed to be a popular choice.
 
B+Bogus said:
A good point is raised regarding the 4-speed gearbox.
The optimum shift point in any gear is also influenced by the gear you're changing up in to, i.e., not dropping the motor into a flat-spot.
The recent post about a taller geared Combat not achieving high top speeds did make me wonder...
I found the gap between 3rd and 4th on my standard, 19T gearbox sprocket equipped Combat to be rather larger than I would have liked, and I used to rev it higher in 3rd when changing up for this reason.
It definitely didn't feel like the power was falling off a cliff either!
As an aside, my 21T equipped MK2A 850 had a monstrous gap between first and second, and I had to let the revs drop right off before changing down into first.
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On Alpine roads, the most important shift is between 1st and 2nd gear, either up or down.
That's why I have a longer (Daytona) 1st gear in my Commando.
 
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acotrel said:
...Isolastics do not change the way the crank behaves, when it is revved high...
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No they don't, but they do allow the entire motor/gearbox/cradle assembly to act as a partial counterweight to the pistons and rods' small ends which changes everything with respect to selecting the crank counterweight percentage.
 
ashman said:
The 2S cam is a good road cam it works a bit better than the standard cam performance wise without over doing it, the motor will run a bit more freely and seeing its got the Combat head I be using the 2S cam profile.
I built my 850 with a 2S cam profile with the ports opened and smoothed out, my head shaved but not as much as the Combat heads were, my lifters had been ground for the cam and my crank balanced at 72% and running 40 oversize flat top Hepalite pistons with a tight final hone and my Amals jetted with 270 mains and manifolds ported to match the head, with stock heads everything is out of port shape, getting everything to flow right is more important than going over board with porting.
My hot 850 is a great road bike and is smooth from low revs around town but open the throttle up it gets up and goes a lot better than when it was just a stock 850 as well your mufflers must be free flowing as well, the 2S cam profile is a good road cam profile as long as everything is matched to it and the Combat heads are matched to the cam compared to a stock 750, if they had the upgraded main bearing in the days they would have been a reliable motor as my 850 with the 2S set up is, haven't had any problem since 1982 running this set up, but it will run freely so keeping an eye on the revs as it's easy to over rev it as it will keep running freely.
This is my opinion on the 2S cam and everyone will have their own opinion and as I say using a 2S cam profile everything else has to work with it, so I say replace it with another 2S cam profile.
Its not a race cam but a cam profile for better performance for the road without over doing it.

Ashley
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I think you're right here. It's a combat so I'll keep it original and put a 2s in.

I have a 71 with the stock cam and I just love the low down torque especially around town so I was thinking about a stock cam. But I'll go with originality
 
The cam in my 850 motor is close to stock. Looking at the timings - it is a race cam. The gearbox is a torque converter - the wide ratio standard gear box of the Commando combined with a low crank balance factor is a major limiting influence to it it's performance. But as a road bike, it is probably good. As soon as you ride any motorcycle, you should know where the power band is and how to use it, and also whether it oversteers or understeers when you gas it mid-corner. When I built my 850 motor, I did not care what the cam was, because I knew I could get it to work. I simply adjusted everything to suit it. - I had a very happy childhood. - I learned to race by crashing. The slightest restriction in the exhaust will usually take the top off the usable rev range with any cam. It is always a mistake to look at things as being single issues. Everything works together as a pattern, and the effect is a pattern.
 
Functionality shapes the bike and the rider adjusts to the bike. A road bike is not a race bike. If you try to race a road bike, you will usually become an instant dud. The best way to learn to race is by riding the best race bike. Somebody else has had all the crashes.
 
worntorn said:
If using the Combat cam, based on forum observations, it might be good to go with a 19 tooth sprocket as well.

Glen That might be true unless most people are making the same mistake. A lot depends on the type of roads on which you use the motorcycle. Where I live the roads are long and boring, so I would use higher overall gearing. If I lived in England, I would use lower gearing. Your mind adjusts to the landscape.
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2S cam and 19t front sprocket is the only way to go, my 850 will sit on 75 MPH and up to 90MPH all day, in fact it loves it, but who is going to sit on 90 MPH all day when our highest speed limits in Aus is usually 110 km unless you live in the NT where they have higher open speed limits (might have changed by now) but sitting on high speeds all day will affect the body and most high speed runs on the road are usually short and sweet.
I am not interested in going flat out in top gear, maybe a few times in my younger days but getting to the ton and over is a thrill and the 2S cam and work I have done to my motor gets it up there pretty quick, quicker than a stock 850, I ride with another mate who I build his 850 to stock except for rebore and new bearings and head cleaned up, new guides fitted, new valve springs, valves relapped and ports just cleaned up to the owners wishes and Amals rebuilt, in a straight run my 850 out revs his every time, 1st, 2nd and 3rd is so much quicker compared to his near stock motor, we even exchanged bike to do the same run and my 850 performed so much better, but for a stock motor his runs pretty good and he is happy with its performance the way it is.
As I say my Norton runs pretty smooth around the suburbs with the 2S cam it will happily sit on 40mph in 4th gear any lower than that and I have to drop down to 3rd as it will be labouring too much, but from 40 mph I can open the throttle up in 4th and with the big spark from the JH maggie it just gets up and goes, and just a little over 4k RPMs the cam really shows its worth, but most of the time if I am cruising around in 4th at 40 and I stick it into it I will drop it back to 3rd for even quicker get up and go.
I really love how my motor works and how quick it gets going even with the stock gear box, it's not a race bike but it's a pretty quick old road bike that still has a big punch when needed and surprises a lot of modern bike riders with its punch, I just love it and it's a well sorted bike has done me so well in the 42+ years running this set up, but the Joe Hunt made it even better now.
So keeping the Combat at its original specs, I am sure you will love it.

Ashley
 

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