I am trying to confirm the measurement I am taking on a triumph Rocker arm ratio
What is the rocker arm ratio? I checked the 1969 service manual and found no specific information

I am measuring 1.22:1
I can adjust placement of my dials around and I get a range of 1.18-1.22:1

I am looking to Neumann cams for their Race grind for this rebuild and I do not want to over-lift the valves - By my observation the lift with Neumanns .348" lift cam means I should have just over .417 at the valve - Meaning I may hit the guide and ruin the performance springs I spec'd for it (the springs are listed as a max lift of .416")

Did I get this right? Is the rocker ratio on a TR6/T120 a 1.2:1
The engines in question are originally from 1969 and 1971 for reference.

While it's an interesting set-up, the gauge on the valve side in particular (but also the other) will give you a much longer arm where you measure it. Ideally measurements should be taken from where the pushrod and valve adjuster action is, which would be on the opposite side. The projected arms should be a line from the tips of the ball or adjuster respectively and through the middle of the shaft. If you draw a line from the top of your adjuster way up there and through the center, you'll see it's much longer than the actual imaginary line.
A common quoted figure is 1.1:1 at mid lift, but I can't vouch for it. There was a table presented on here recently, someone will find it for you...

Unless the DTI stays perpendicular to the tappet adjuster through the full movement I do not think you are getting an accurate figure. I would be measuring the radii from the rocker shaft center to the pushrod cup end of the rocker center and also to the tappet adjuster center, dividing the 2 numbers will give the ratio.

The pushrod button has a small hole for oiling. You can use a close fit pin in the hole and align it vertical then measure from there to the shaft centre. Same for the adjuster side, use a close fit pin in place of the adjuster.

As Stein said, the rocker arm ratio given by Triumph is 1.1 to 1

At .348" the rocker arm would lift the valve 0.382". Then you need to check that at 0.382" lift you have at least an additional 0.060" before coil bind or top collar hitting guide.

While we are at this the cam timing figures given by Triumph are based upon valve lift, not tappet lift. Most after market cams give their timing figures at tappet lift.

The simplest meaning of "rocker ratio" would involve measurement of the radial distances from the centre of rotation, to the centre of adjuster pin tip (with adjuster set as it would be in use) and to the centre of the ball (to be pedantic the centre of the sphere, which is the axis of movement between pushrod cup and rocker ball).
This will give the simple lever ratio. This ratio (call it SLR) doesn't convert a certain cam lift into a certain valve lift, because of other geometric variables involved, all of which result in a lower valve lift in reality as calculated using the SLR and cam lift.

The losses of movement in this system result from the (nearly) vertical movement of the cam follower converting to the rotational movement of the rocker ball (analogous to the piston/rod/crankpin arrangement) and the rotational motion of the rocker adjuster converting to the linear movement of the valve.
Both of these geometric losses add together. Consequently, the valve lift is always less than cam lift X SLR, even when the rocker geometry is ideally set.

I don’t know if Triumph’s rocker ratio (as stated by John Healy) is the simple lever ratio (SLR), I’ve never measured it myself, or whether it may be Triumph’s real world ratio between cam lift and the consequent impact upon valve lift, allowing for those losses through geometry?

The rocker arm is a simple class 1 lever. The fulcrum is in the middle between the movement of the push rod and the movement of the valve. It is typical for the long arm of the lever to be on the valve side. Thus the movement of the valve will be greater than the push rod. Rocker arm ratios are supplied to allow calculation of how much lift you will get with with a given ration. THE STATED ROCKER ARM RATIOS for a Triumph 650-750 twin is 1.1 to 1.

So with the poster cams having a 0.348" lift multiplied by 1.1 will give an actual valve lift of 0.382". If the longer arm of the rocker was on the push rod side the valve lift would be 0.314."

The ratio, through out the rotation of the rocker arm, is not a consistent figure. The maximum increase in lift, per degree of rotation, will be at right angles. It will be less either side of right angles with the least at the beginning and end of the rotation. It is the total increase that we are interested in. This figure gives us a figure in which we can calculate the needed clearance before valve spring coil bind and clearance at full lift between the under side of the top valve collar and the valve guide (or valve guide seal). In the end, no matter the physics, all we are trying to do is stop from breaking the rocker arm or shattering a valve spring.



I know nothing of the "losses" of which you speak. I do know that the ratio Triumph gives the Real World relationship between the cam, and this tappet and push rod, lift and the increase in actual valve lift. Just a practical figure to allow some very basic checks when installing a cam with more lift than stock.

Thanks John, I wasn’t contradicting what you’d said.
It was just to question whether the stated 1 to 1.1 ratio was the actual physical ratio of the 2 arms of the rocker, which can be easily measured with a rocker out on its own (all 4 of mine are stuck in an engine so I don’t have that possibility at the moment), OR whether that ratio is a real world figure that represents the calculation of valve lift from cam lift.

You have answered that question by confirming the latter, as a practical relationship between cam and valve lift, and any error in its application errs on the safe side at higher lifts.

The rocker arm may in itself be a simple lever, but the pushrod operating it, and the valve at the other end, render the entire system more subtle. Yourself mentioning right angles and non-consistency of ratio throughout the whole movement illustrates that you know what I meant by losses, I’m sure.

But just in case:
Whatever the physical measurements of the rocker lever arms (say 1 to 1.1), the valve lift can never be quite as much as that calculation would suggest, because of the angles to the vertical of the pushrod and the movement of the adjuster pin in an arc upon the flat surface of the valve tip. The closest it would get would be when the rocker pin/follower and adjuster/valve tip angles are aligned at mid lift, which is what I think you were saying.

If the geometry is significantly away from this, even more lift will be lost, let alone more unnecessary stress put on the system.

How many Angels can fit on the head of pin? Any argument pertaining to Triumphs containing microscopic measurements is about as nonsensical as the aforementioned statement. These old turds are dinosaurs, not rockets to the moon.

Well with the head of that pin around a 1/16th of an inch (.060"), not many! But we are talking about sixteenths, that even with my eyes at 80, I can see without glasses.

Now Denis's question about installing a cam with more lift is a fair one. It is an intelligent question from someone who has done head work before. And he is right, you have to dig deep into Triumph literature to find the answer.

I am here to tell you that if you assume anything on one of these dinosaurs, as you refer to them, you are going to get a lesson in humility. Putting in a higher lift cam is just one of those opportunities to get your fingers burnt.

And we haven't even got into discussing valve to piston clearance.
And then there are those who like to understand what makes things work...

Excellent, thank you. I am trying to make certain this engine will not crush it’s springs and damage it’s guides.
...correct..the listed cam specs are at the lobe..not valve which I find the standard