cam timing std cam in a 70 750 using measured lift @TDC on the inlet pushrod

W

watson rider

Joined
Mar 24, 2015
Messages
9
Country flag
Does anyone know what amount of lift the inlet pushrod should have ,measuring at the end of the pushrod,with head off and checking at TDC ?I,m using a bog standard .330 lift cam.with degree wheel accurately set using piston stop method of finding true TDC as per the ED Iskenderian "Valve TIming " bible. I find this method far more accurate rarther than the struggle of checking at the rocker arm ,if it was good enough of Mick Hemming to recommend it,s god enough for me thanks Watson rider.
 
watson rider said:
Does anyone know what amount of lift the inlet pushrod should have ,measuring at the end of the pushrod,with head off and checking at TDC ?I,m using a bog standard .330 lift cam.with degree wheel accurately set using piston stop method of finding true TDC as per the ED Iskenderian "Valve TIming " bible. I find this method far more accurate rarther than the struggle of checking at the rocker arm ,if it was good enough of Mick Hemming to recommend it,s god enough for me thanks Watson rider.
Click to expand...
1970 Commando "S" Type
Intake cam measured .331" max lift @ 98 degrees ATDC (measured at pushrod).
 
Do the opening and closing numbers at 35 thousandth (.035") lift at the pushrods, That will let you do lobe centers using the formular. Look up how to do Lobe centers. Those are Numbers that most racers understand. Rocker arm ratio is 1.13 to 1 so actual lift is about .040 at valve at 0 lash .
 
acadian said:

Camshafts | Norton Owners Club

www.nortonownersclub.org www.nortonownersclub.org

Plugging those numbers in the LC calculator gives a lobe centre of 102 intake/110 exhaust
Click to expand...
Ok, Those numbers are found scrolling down the article. But actually checking lobe centers on any cam can be a good thing. At least that way you know what you have and how it performs. My Combat with a stock 2S cam had ass backward lobe centers. More than likely that is why Combats were not as fast as early Commandos. I did correct the 2S by 5 degrees to more liveable numbers, But still would do a little more. I had a feeling the keyway was cut in the wrong place.



The Norton owner's Club article explains it this way.
" It is worth checking the timing of this cam because at Woolwich it seems they had some trouble with the old cam grinder which had come down from Birmingham, and while the cam lobes came out fairly accurately in relation to each other, they were not at all accurate in relation to the driving keyway."

I don't know if all the numbers shown are just taken from a Norton shop manual ( factory specs) or are actually taken off an assembled engines. You have to take in all the gears and sprockets keyways and splines and cam assembled.
 
Complete specs for 1970 Commando "S" mentioned above...
(stock camshaft - measurements taken directly off pushrods w/ flat tappets)

Intake: .040" lift @ 33 deg BTDC
Max .331" lift @ 98 deg ATDC
.040" lift @ 136 deg BTDC

Exhaust: .040" lift @ 126 deg ATDC
Max .328" lift @ 105 deg BTDC
.040" lift @ 21 deg ATDC

Intake specs (@ .040" lift)
Intake Duration: 252 degrees
Intake opens: 33 deg BTC
Intake closes: 44 deg ABC

Exhaust specs (@ .040" lift)
Ex Duration: 255 degrees
Ex opens: 54 deg BBC
Ex closes: 21 deg ATC
 
A lot depends on what you are trying to achieve. If you are seeking performance rather than fuel economy or noise reduction, advancing the cam can give more torque. However, if you gain more torque, nothing changes unless you raise the gearing . With the heavy crank, the Commando engine always tends to spin-up at the same rate. The shape of the needles in the carburetors has a bigger effect because of the heavy crank. I once spent a lot of time playing with cam timings and exhaust systems - there is a lot of room for variation. But what makes a difference can be deceptive. Everything works together to produce a result.
 
Rockithound, those numbers for the standard S cam are very interesting. Inlet 252 @ .040". Megacycle chart says standard cam is 268 @ .040". Big difference. If 252 is correct then the standard cam is a very good choice for a standard engine. Graham.
 
Rockithound said:
Complete specs for 1970 Commando "S" mentioned above...
(stock camshaft - measurements taken directly off pushrods w/ flat tappets)

Intake: .040" lift @ 33 deg BTDC
Max .331" lift @ 98 deg ATDC
.040" lift @ 136 deg BTDC

Exhaust: .040" lift @ 126 deg ATDC
Max .328" lift @ 105 deg BTDC
.040" lift @ 21 deg ATDC

Intake specs (@ .040" lift)
Intake Duration: 252 degrees
Intake opens: 33 deg BTC
Intake closes: 44 deg ABC

Exhaust specs (@ .040" lift)
Ex Duration: 255 degrees
Ex opens: 54 deg BBC
Ex closes: 21 deg ATC
Click to expand...
Lobe centers Intake = 33 +44 +180 = 257 divided by 2 = 128.5 - 33 = 95.5 Lobe center

Lobe center Exhaust = 54 + 21+180 = 255 Divided by 2 = 127.5 -21 = 106 .5 Lobe center

A degree or two make a big difference in lobe centers.

These are numbers I would expect from an early cam. I would recheck the intake numbers. Lobe centers do not change much at the openings or closing if you use numbers from .030" - .050" Different manufacturers use different values of opening lifts as well as valve lash. WebCams uses .050.
Some of their cams. I use has smaller valve lash than OEM Norton. .006 intake and Exhaust
 
velo54 said:
Rockithound, those numbers for the standard S cam are very interesting. Inlet 252 @ .040". Megacycle chart says standard cam is 268 @ .040". Big difference. If 252 is correct then the standard cam is a very good choice for a standard engine. Graham.
Click to expand...
These measurements were taken off of a very "well used" (but not totally knackered) stock camshaft that was replaced during a complete top & bottom end rebuild. I took the opportunity to measure & record things to satisfy my own curiosity & for comparison purposes to other camshafts.
 
To add to discussion... another set of measurements taken during recent rebuild using new AN 06.1084 camshaft & new AN 06.7820 solid tappets. Measurements taken off pushrods.

Intake: .040" lift @ 29 deg BTDC
Max .332" lift @ 99 deg ATDC
.040" lift @ 131 deg BTDC

Exhaust: .040" lift @ 129 deg ATDC
Max .332" lift @ 103.5 deg BTDC
.040" lift @ 27 deg ATDC

Intake specs (@ .040" lift)
Intake Duration: 258 degrees
Intake opens: 29 deg BTC
Intake closes: 49 deg ABC

Exhaust specs (@ .040" lift)
Ex Duration: 258 degrees
Ex opens: 51 deg BBC
Ex closes: 27 deg ATC
 
Rockithound said:
These measurements were taken off of a very "well used" (but not totally knackered) stock camshaft that was replaced during a complete top & bottom end rebuild. I took the opportunity to measure & record things to satisfy my own curiosity & for comparison purposes to other camshafts.
Click to expand...
Thanks for the info. I shall measure mine when i get the chance. 72 standard 750 cam, also well used. Graham
 
Rockithound said:
These measurements were taken off of a very "well used" (but not totally knackered) stock camshaft that was replaced during a complete top & bottom end rebuild. I took the opportunity to measure & record things to satisfy my own curiosity & for comparison purposes to other camshafts.
Click to expand...
Hi, just checked your figures for the first 70 S cam. Inlet duration should be 257, not 252. Pretty close to the new cam. And advanced about 4 degrees, which would be ok. Graham
 
velo54 said:
Hi, just checked your figures for the first 70 S cam. Inlet duration should be 257, not 252. Pretty close to the new cam. And advanced about 4 degrees, which would be ok. Graham
Click to expand...
You are correct Sir! My measurements seem OK but my calculations aren't. Given specs cited intake duration should be 257 degrees. I have corrected my post.
Thanks.
 
Rockithound said:
Complete specs for 1970 Commando "S" mentioned above...
(stock camshaft - measurements taken directly off pushrods w/ flat tappets)

Intake: .040" lift @ 33 deg BTDC
Max .331" lift @ 98 deg ATDC
.040" lift @ 136 deg BTDC

Exhaust: .040" lift @ 126 deg ATDC
Max .328" lift @ 105 deg BTDC
.040" lift @ 21 deg ATDC

Intake specs (@ .040" lift)
Intake Duration: 252 degrees
Intake opens: 33 deg BTC
Intake closes: 44 deg ABC

Exhaust specs (@ .040" lift)
Ex Duration: 255 degrees
Ex opens: 54 deg BBC
Ex closes: 21 deg ATC
Click to expand...
Correction to above specs...

Intake specs (@ .040" lift)
Intake Duration: 257 degrees <== corrected due to my inability to accurately perform simple math
Intake opens: 33 deg BTC
Intake closes: 44 deg ABC
 
This should be close to what you need as the JS0 cam is very close to the stock cam specs.

Set valves at .006” tappet checking (and hot running) clearance on compression stroke. Rotate crank only in the direction it runs. At TDC the intake valve should have APPROX .015” MORE LIFT THAN THE EXHAUST. For example – if the intake lift is .140" then the exhaust lift should be approx .125" - or if the intake lift is .130" then the exhaust lift should be approx .115" etc. Adjust your cam timing accordingly. This will give the correct intake lobe center of 102 degrees.


Always visually check the valves at TDC. Looking in through the port; with a tiny flashlight (magnification helps) – you should see that the intake has a little more lift than the exhaust (both valves level is acceptable). This is assuming that the valve seats are level as with new stock heads.
 
Carl H said:
Ok, Those numbers are found scrolling down the article. But actually checking lobe centers on any cam can be a good thing. At least that way you know what you have and how it performs. My Combat with a stock 2S cam had ass backward lobe centers. More than likely that is why Combats were not as fast as early Commandos. I did correct the 2S by 5 degrees to more liveable numbers, But still would do a little more. I had a feeling the keyway was cut in the wrong place.



The Norton owner's Club article explains it this way.
" It is worth checking the timing of this cam because at Woolwich it seems they had some trouble with the old cam grinder which had come down from Birmingham, and while the cam lobes came out fairly accurately in relation to each other, they were not at all accurate in relation to the driving keyway."

I don't know if all the numbers shown are just taken from a Norton shop manual ( factory specs) or are actually taken off an assembled engines. You have to take in all the gears and sprockets keyways and splines and cam assembled.
Click to expand...
why early Commando.s went better than later (inc C0mbats) has more to do witn cylinder heads than any thing else,check out the port sizes and shaping.this was explained to me directly by a ex factory road tester,recounting his days riding his own early green Fastback,regards Watson Rider
 
Please note that all Norton heavyweight twin camshaft lift and timing data are based on the crankshaft angle and NOT tdc as measured from the piston position (unless otherwise specified by an aftermarket cam manufacturer). On the Atlas/Commando engines tdc of the piston is about 1 degree advanced compared to the crankshaft owing to the rearward offset of the pistons.
 
I do not really understand what you guys are doing - the main thing with a cam is that it is not a valve-dropper. When I had the cam ground for my 850, I went to an experienced cam grinder and asked for their best Norton grind. I did not care what it was because I knew I could get it to work. From the timings, it is not much different from a Commando cam. Because I use a 2 into1 exhaust system, I advanced the opening points by 12 degrees, then tuned the carburation and ignition advance to suit the timing and fuel type. The main thing is to get the gearing right, so the power band and power delivery suits the circuit. When you have increased torque, the Commando motor still spins up at the same rate, but has the ability to pull higher gearing. With wide ratio gears, the situation is hopeless. With a 2 into 1 exhaust system, any restriction at the outlet of the collector can easily cut 1000 RPM off peak revs.
 
There was a theory that the best motor in the best frame should be the best motorcycle. It does not matter what you build, it is only the beginning. The rest is about adjusting the variables to suit the use. The first time you race an untested motorcycle is when you are most likely to crash it. If I get around one lap of a race circuit, I usually know what the motorcycle is doing.
 
Back
Top