Stock Commando Cam Specs

Schwany said:

True on the not spending a lot of time at 7000 RPM on the street. However, I thought getting up over 6500 RPM quick was one of the reasons you were freshening up the motor. The need for speed.

On a side note: I'm gearing my little 750 up some more in the primary. It gets to 7000 RPM sooner than I like in 3rd and is wound up more than I like in 4th on the HWY around here. I have to ride in the slow lane at 70mph. Something to think about if you have to ride on the HWY to get to where the fun riding is. A fresh motor should pull taller gears without any trouble. Excuse the useless advice.

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I have a 21 on the gearbox now and have considered going to a 22.

Schwany said:

True on the not spending a lot of time at 7000 RPM on the street. However, I thought getting up over 6500 RPM quick was one of the reasons you were freshening up the motor. The need for speed.

On a side note: I'm gearing my little 750 up some more in the primary. It gets to 7000 RPM sooner than I like in 3rd and is wound up more than I like in 4th on the HWY around here. I have to ride in the slow lane at 70mph. Something to think about if you have to ride on the HWY to get to where the fun riding is. A fresh motor should pull taller gears without any trouble. Excuse the useless advice.

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Higher compression and the 312a cam running on flat followers will raise the torque curve yielding more power from mid range on up. The power on tap at 75 MPH in 4th gear will be quite an improvement over a stock cam. If I really want to walk the dog, shifting to 3rd at 75 will bring RPM to near maximum HP.

I'll point out here for everyone's edification that a cam going "flat" will not change the valve clearance adjustment unless the base circle also wears down. In other words, once the valve begins to lift the valve clearance goes to zero anyway so there is no reason to think that a lower lobe from wear would be noticeable as increased valve clearance.

Dan1950 said:

Higher compression and the 312a cam running on flat followers will raise the torque curve yielding more power from mid range on up. The power on tap at 75 MPH in 4th gear will be quite an improvement over a stock cam. If I really want to walk the dog, shifting to 3rd at 75 will bring RPM to near maximum HP.

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Will a Norton do 75mph in 2nd gear? That seems a tad optimistic to me. 80mph is 7100 RPM in 3rd with my gearing. I think you'd definitely be walking after hitting 75mph in 2nd before shifting into 3rd.

Gearing: I have 21 42 in the primary, 20 42 out back. Primary is single row chain though. Same general plan to put a 22T engine sprocket on. It was in there previously and a little high in town with the 2S cam I had in there. The JS2 cam with the higher compression pistons produces a friendlier torque curve on the street, so I can almost short shift. The 2S worked better holding gears longer. Hoping to get back 400-500 RPM at 75 in 4th, which would keep me out of trouble.

You'll like the fresh motor. Lots of people have already done what you are planning to do, and it works without a hitch. You won't have any trouble with it given how meticulous you seem to be.

Schwany said:

Will a Norton do 75mph in 2nd gear? That seems a tad optimistic to me. 80mph is 7100 RPM in 3rd with my gearing. I think you'd definitely be walking after hitting 75mph in 2nd before shifting into 3rd.

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I never said anything about shifting from second to third at 75 mph.

What I mentioned was shifting from 4th to 3rd at 75 mph.

My current gearing has me doing about 68 mph in 4th gear at 4,000 RPM.

3000 = 60
3500 = 70
4000 = 80

22 front sprocket

motorson said:

I'll point out here for everyone's edification that a cam going "flat" will not change the valve clearance adjustment unless the base circle also wears down. In other words, once the valve begins to lift the valve clearance goes to zero anyway so there is no reason to think that a lower lobe from wear would be noticeable as increased valve clearance.

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True but the lifters wear considerably as the cam is going flat. At least that was the experience I had with two cams that went flat. The lifters were badly worn and could not be salvaged the first time. The second time I caught it soon enough and saved the lifters. Resurfaced them on the valve grinding machine.

Schwany said:

Will a Norton do 75mph in 2nd gear? That seems a tad optimistic to me. 80mph is 7100 RPM in 3rd with my gearing. I think you'd definitely be walking after hitting 75mph in 2nd before shifting into 3rd.

Gearing: I have 21 42 in the primary, 20 42 out back. Primary is single row chain though. Same general plan to put a 22T engine sprocket on. It was in there previously and a little high in town with the 2S cam I had in there. The JS2 cam with the higher compression pistons produces a friendlier torque curve on the street, so I can almost short shift. The 2S worked better holding gears longer. Hoping to get back 400-500 RPM at 75 in 4th, which would keep me out of trouble.

You'll like the fresh motor. Lots of people have already done what you are planning to do, and it works without a hitch. You won't have any trouble with it given how meticulous you seem to be.

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Schwany, aside form the labor issue, I think you are right to change the engine sprocket rather than the countershaft sprocket. Back in the day when there were still Portugese layshaft bearing lurking about we used to argue against much larger countershaft sprockets because of the additional strain on the gearbox. Changing to a larger engine sprocket with the same countershaft sprocket reduces the torque load on the chain as well as the gearbox, or that was the argument we used at the time. Since there was only one commando primary sprocket available the issue was really moot Commandos except I did repair countless gearboxes with exploded layshaft bearings, broken main shafts and badly worn sleeve-gear bushes. All the overhauls got C3 rollers on the layshaft. Layshafts do flex and take out the drive-side ball after a while, along with the case sometimes; even quality bearings, eventually. I don't recall if the owners were running 19T countershaft sprockets or larger but our customers were very good a thrashing gearboxes. My Commando has a 21T countershaft and seems geared about right for most situations even with the 2S cam. It runs about 3300 at 60.

Schwany said:

How long would an 850 last frequently revved freely to 7000 RPM with a stock crank, rods, and cases if it was assembled with care? I get the impression (probably a misinterpretation on my part) that the heavier crank is a gating item on making an 850 wind up that tight all the time. Is 7000 RPM not high RPM on an 850? I've never owned one, so no clue.

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I built a full house 850 in '75 from all ex Norton parts, Development and Thruxton race shop. MkIII bottom end, polished, fully sphered short stroke head, huge valves and matching Omega pistons. It revved to 6800. And made great power there.

One time I deliberately under geared it to go quicker around the twisty parts of one circuit, under red mist I saw 7200 in top as it went bang! It wasn't the bottom end, I reused that for a couple more years, it dropped an inlet, I can show you that!

Schwany said:

How long would an 850 last frequently revved freely to 7000 RPM with a stock crank, rods, and cases if it was assembled with care? I get the impression (probably a misinterpretation on my part) that the heavier crank is a gating item on making an 850 wind up that tight all the time. Is 7000 RPM not high RPM on an 850? I've never owned one, so no clue.

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Apologies for my inability to convey what I was actually attempting to get across. I was simply trying to make the point that the stock CDO cam is a very good design (nice modern O/C ramps, adequate duration, LSA, etc) and is not a limiting rpm factor in a well-prepared street engine.

In general, people tend to blame the stock cam (along with exhaust, carbs, airflow, etc) for a stock engine’s inability to rev, when in fact a stock engine doesn’t have airflow to support operation to 7000 rpm. And no doubt someone will come right back and say “my stock engine revs to 7000 rpm all the time” to which I say, there is a world of difference between revving to 7000 rpm and making peak HP at 7000 rpm, the latter being what I am talking about. Seat-of-the-pants dynos can be quite misleading in this regard.

Anyway, I think people would be surprised how well the stock cam parameters can work when major engine airflow improvements have been made. Perhaps the rider that perpetually operates in the frisky mode would want more cam, whereas the person that packs heavy, rides 2-up, or encounters long mountain grades at moderate speed would be delighted with the stock cams consistent yank in any rpm range.

Then again, I recall a “cam thread” here on the forum several years ago asking what the best Norton cam was. The responses covered every CDO cam ever made, and basically no fault could be found with any of them, i.e., somebody loved every single one of them for whatever application they had them in. Thus, our collective satisfaction, perception, and interpretation of CDO performance is as varied as are the cam options available.

For midrange pull , as in climbing a mountain pass with a loaded bike, the stock cam can't be beat.
For roll on races and such, it can be outdone, but not by a large amount. Even there, the valve train may have to pay a price, all in pursuit of a few HP at top. It's all for fun anyway, so sometimes that extra few HP at top is exactly what the owner wants.

Glen

Dan1950 said:

If I'm reading the text correctly, the stock Commando cam has an intake closing at 48°ABDC.

This would result in an effective/dynamic compression ratio of 6.9: 1 at 8.5:1 static compression.

Compare that to the 7.5: 1 effective/dynamic compression ratio of my 10.2:1 static compression combination with the Web 312a cam running flat followers.

My main concern is cylinder pressure at the bottom of the RPM range and the detonation that could result from that cylinder pressure being too high.. A . 6 raise in dynamic compression should result in an engine that would not be significantly more prone to detonation than a stock engine. Furthermore, it should also have low end torque similar to the stock combination.

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Yes, you interpreted the posted opening/closing results correctly.

Regarding your comments (here and in subsequent posts) about compression increasing torque and the 312a shifting the torque curve to a higher rpm range, your observations are undisputable. But as always, you don’t get something for nothing, i.e., if the torque curve is shifted to a higher rpm range, you’re giving up some low-end grunt. Your comment about torque, “it should also have low end torque similar to the stock combination”, might generate some debate. Torque may be similar (but not the same), and as you note will occur in a higher rpm regime. The 312a cam run with flat tappets is clearly a performance cam (very similar valve motion to a JS1), as it adds about 17% more area under the lift curve (which is a very significant increase) relative to a stock CDO cam and has considerably more overlap, due to both increased lift and duration. This is one of the reasons why some of the "wise ole sages" here opt for the 4” radius lifter over the flat follower, i.e., to moderate the valve motion by shortening the effective duration (but absolute open to close duration remains the same) and reducing overlap, thereby preserving low end torque and keeping it from shifting significantly to higher rpm. To quantify the change of moving from a flat tappet to a 4” radius tappet with the 312a, we can again compare the area under the lift curve to the CDO cam that reveals the radius tappet provides +11% increase in curve area vs the flat tappet that provides +17% increase in curve area relative to a CDO cam.

Please don’t misconstrue any of the above as being critical of any of your component choices or reasoning, since your rationale and component choices are sound, have been run by myriad others, and resulted in high satisfaction with the outcome. Maybe you’re a frisky guy that rides frisky 24/7/365, in which case you’ll love your frisky set-up.

As noted above, your combination of parts is not new and has been employed by many previously, so perhaps some of them will weigh in on how they like the resulting performance. One project that stands out in my mind as being almost identical to yours is Fast Eddie’s 850 that had a Comnoz big valve head, a JS1 cam, and 10+/1 CR. He’s usually not too far away so he may provide us with a refresher on how he faired and possibly even a dyno curve.

WZ507 said:

Apologies for my inability to convey what I was actually attempting to get across. I was simply trying to make the point that the stock CDO cam is a very good design (nice modern O/C ramps, adequate duration, LSA, etc) and is not a limiting rpm factor in a well-prepared street engine.

In general, people tend to blame the stock cam (along with exhaust, carbs, airflow, etc) for a stock engine’s inability to rev, when in fact a stock engine doesn’t have airflow to support operation to 7000 rpm. And no doubt someone will come right back and say “my stock engine revs to 7000 rpm all the time” to which I say, there is a world of difference between revving to 7000 rpm and making peak HP at 7000 rpm, the latter being what I am talking about. Seat-of-the-pants dynos can be quite misleading in this regard.

Anyway, I think people would be surprised how well the stock cam parameters can work when major engine airflow improvements have been made. Perhaps the rider that perpetually operates in the frisky mode would want more cam, whereas the person that packs heavy, rides 2-up, or encounters long mountain grades at moderate speed would be delighted with the stock cams consistent yank in any rpm range.

Then again, I recall a “cam thread” here on the forum several years ago asking what the best Norton cam was. The responses covered every CDO cam ever made, and basically no fault could be found with any of them, i.e., somebody loved every single one of them for whatever application they had them in. Thus, our collective satisfaction, perception, and interpretation of CDO performance is as varied as are the cam options available.

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WZ507,
You got your point across. I knew what you were talking about. I think most people did. I don't have first hand 850 experience though. My comment was merely to get some Norton stalwart discussion rolling.

The stock cam in my pre-commando worked and seemed fast enough on the street in the 70's. Not sure what it would feel like today. Probably really nice in stop and go traffic with 930 Amals on it. Not going to ever know though.

Sun is out which is really unusual for this time of year. I need to get out my seat of the pants dyno, the old Norton hot rod, and go for a ride. I also have a seat of the pants suspension tuning dyno that will get some use today. Old dog old tricks in a misleading fog of delusion.

Thomasa said:

Schwany, aside form the labor issue, I think you are right to change the engine sprocket rather than the countershaft sprocket. Back in the day when there were still Portugese layshaft bearing lurking about we used to argue against much larger countershaft sprockets because of the additional strain on the gearbox. Changing to a larger engine sprocket with the same countershaft sprocket reduces the torque load on the chain as well as the gearbox, or that was the argument we used at the time. Since there was only one commando primary sprocket available the issue was really moot Commandos except I did repair countless gearboxes with exploded layshaft bearings, broken main shafts and badly worn sleeve-gear bushes. All the overhauls got C3 rollers on the layshaft. Layshafts do flex and take out the drive-side ball after a while, along with the case sometimes; even quality bearings, eventually. I don't recall if the owners were running 19T countershaft sprockets or larger but our customers were very good a thrashing gearboxes. My Commando has a 21T countershaft and seems geared about right for most situations even with the 2S cam. It runs about 3300 at 60.

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I'm seldom right about anything on the internet. lol

Soon as I get around to ordering parts including a longer primary chain I'll put the 22T engine sprocket back on. I know this will sound like BS, but the little P11 would lift the front end up just a bit on hard 2nd gear acceleration with the 22T engine sprocket and 34mm Mikuni carburetors on it. Rear gearing was 19/42 when it did that. That gearing did seem to work well with the 2S cam everywhere but on the HWY. Will see how the taller gearing in the primary goes with the JS2 20/42 gearing in the rear and bigger carburetors. Probably won't pull hills that well short shifting, but be less frantic on the HWY. I'll add a 21T gearbox sprocket to the list for the future.

My guess is overly tight rear chains chew on AMC gearbox bushes and bearings as much as anything else. I've shifted into 1st one too many times thinking I was going into 3rd, due to forgetting the shift pattern. That I'm sure is not good for the box either.

WZ507 said:

Please don’t misconstrue any of the above as being critical of any of your component choices or reasoning, since your rationale and component choices are sound, have been run by myriad others, and resulted in high satisfaction with the outcome. Maybe you’re a frisky guy that rides frisky 24/7/365, in which case you’ll love your frisky set-up.

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I am an "adrenaline junkie". I love to twist the throttle and feel the power build as revs climb. That doesn't mean I want to constantly redline in the gears. On the contrary, a robust feeling of power can satisfy my addiction without frequently pushing it to or near the limits.

23 teeth gets you 112 in third .
Exact instantaeneous Accurate R P M figure shown .KROBER . \
You can SEE IT there , down below .

Now , If its good enough for Pete , Who do you think you are , to disagree .
Around 80 in second . 75 anyway ( No Exact figure ! thereabouts )

76.4705882353 if its 1.7 : 1 . for a fact .

THE PROBLEM IS , one should not downshift - fetching past 3/4 redline. Or 2/3rd .

AND ONLY Throttle On .

Fropple Off , the main loads TENSION . 2 year olds let out of the city downshift their 1600 escorts ,
into second ( because they havnt realised theyre NOT doing 30 mph )when coming into a country town at 80 mph . With Similar Results. Par for the course . B A N G . clatter clatter CLUNK .

DONT lend your vehical to a teenager .
=========================

Reading the Cam Specs ?? , saying Valve Springs & the Redlines 7500 .
Id run 7000 regularly WFO on a Std Cam ( Head & carbs trick ed up ) .

A Problem is Cable drive Magnetic TACH. LAG & innacuracy .
Touched 7200 only a few times . BUT owdya KNOW its 7200 .
Theres a hesitancey .

With A IMPULSE Tachometer , it is INSTANTAENEOUS & Exact . ( if its set exact ) . V D O & Krober - which was Std. 750 fare - in the day .

ooops , so THATS where they WENT ! ( )



Gotcha . THATS a white One . And the funny electronic THINGS in the back of it , too !
not for the hoi poloi .
A gentlemans guide to Smiths tachometers
. https://img1.wsimg.com/blobby/go/1975f84f-4935-4131-8404-5a914da1afb7/downloads/A gentlemans guide to Smiths tachometers V2.pdf?ver=1576555388667

So , if you WANT it to EXPLODE , Downshift full out , and shut the throttle as you declutch .

RESULTS OF OVER-REVVING​

Over-revving can cause serious harm to your engine, even in the few seconds it takes to realize that you meant to shift to fourth gear but wound up in second instead. The good and bad news is that if you seriously damage something you’ll know it almost instantly.

Valve Train Damage​

Over-revving can cause damage to your valve train by causing a valve to stay open for too long. This leads to valve float. Valve float occurs when a valve is stuck in between open and closed. This will cause an immediate loss of power. Valve float is the only one of these problems that can be "quiet" and cause problems later, but you’ll be able to tell by either the engine misfiring or losing power at higher speeds. To fix valve float you just need to replace the springs, which is a great thing to do as part of routine maintenance anyway.

Throwing a Connecting Rod​

Throwing a connecting rod is the number one cause of catastrophic engine failure. The connecting rod is a metal piece that looks almost like a wrench and connects your piston to your crankshaft. It’s crucial to the operation of the vehicle, but that’s not why it causes catastrophic engine failure. Connecting rods are located in the center of the magic that is an internal combustion engine.

The connecting rod is a solid piece of metal, with the pistons and cylinders above, and the crankshaft below. No matter which way you toss your connecting rod it’s going to take a lot of important stuff out with it, and it’s going to throw your engine out of sync. The only reason the thousands and thousands of mini-explosions that occur in your engine don’t destroy your car is a perfectly timed series of actions that contain and utilize every bit of power from those explosions.

If one thing falls out of choreography though, it doesn’t take more than a second for the entire routine to be ruined.

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Dan1950 said:

I am an "adrenaline junkie". I love to twist the throttle and feel the power build as revs climb. That doesn't mean I want to constantly redline in the gears. On the contrary, a robust feeling of power can satisfy my addiction without frequently pushing it to or near the limits.

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Have you bent any valve stems on this "adrenaline junkie" project yet?

Making any progress? Still waiting on some machine work?

Schwany said:

Have you bent any valve stems on this "adrenaline junkie" project yet?

Making any progress? Still waiting on some machine work?

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Waiting on JE Pistons, graphite coated, underside of crowns machined to reduce weight. (running them at full deck height)

Sent my cam in to Web Cams a few weeks ago for 312a grind

After the cam comes in I can at least assemble the cases and check valve head interference clearances. I want to run flat followers to reduce low RPM cylinder pressure to prevent detonation as well as boost the power band RPM. (slightly more duration/overlap)

Dan1950 said:

Waiting on JE Pistons, graphite coated, underside of crowns machined to reduce weight. (running them at full deck height)

Sent my cam in to Web Cams a few weeks ago for 312a grind

After the cam comes in I can at least assemble the cases and check valve head interference clearances. I want to run flat followers to reduce low RPM cylinder pressure to prevent detonation as well as boost the power band RPM. (slightly more duration/overlap)

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I am sure you know, but be sure to run cooler plugs.

I don't recall seeing if you ended up flow porting and getting +1.5mm vv on your RH10?

You may want to start on the fairly rich side on your jetting when you first get it going.

Schwany said:

Have you bent any valve stems on this "adrenaline junkie" project yet?

Making any progress? Still waiting on some machine work?

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Pistons came yesterday. Web will be sending the cam in about 1 week. It has been welded up but not ground yet.










Since I am using the pistons at full compression height, some material was machined from inside the crown to reduce weight.