Rocker Ratios

I found in a box of spares a couple rockers that appear to be longer Norton rockers. I lined them up with my 850 Mk3 rockers and the ratio on these is clearly longer, the longer side is on the valve side, the push rod side seems to be the same length. I found an opposite side that matches as well as two others that look like this mystery rocker, but are the same length as my stock MK3 rockers.

Anyone know what this rocker is from or what the rocker ratio is?

Here is a photo with my mk3 rocker on the right with this mystery longer ratio rocker is on the left (there is a rocker spindle holding them on the same axis).

Are you sure it's not an illusion? The central parts look like they're at an angle to the lines on the page.

maaseyracer said:

I found in a box of spares a couple rockers that appear to be longer Norton rockers. I lined them up with my 850 Mk3 rockers and the ratio on these is clearly longer, the longer side is on the valve side, the push rod side seems to be the same length. I found an opposite side that matches as well as two others that look like this mystery rocker, but are the same length as my stock MK3 rockers.

Anyone know what this rocker is from or what the rocker ratio is?

Here is a photo with my mk3 rocker on the right with this mystery longer ratio rocker is on the left (there is a rocker spindle holding them on the same axis).

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Can't view your photo
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hhh said:

Are you sure it's not an illusion? The central parts look like they're at an angle to the lines on the page.

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Very sure about that.

hhh said:

Are you sure it's not an illusion? The central parts look like they're at an angle to the lines on the page.

Click to expand...

I believe so too. Rockers with long bosses were a fitment of Dominators before 1960. Norton shortened the boss to make room for shims and the thackeray washer. They didn't alter the arms on that occasion, at least.

- Knut

mdt-son said:

I believe so too. Rockers with long bosses were a fitment of Dominators before 1960. Norton shortened the boss to make room for shims and the thackeray washer. They didn't alter the arms at that occasion, at least.

- Knut

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I believe you are talking about the push rod side that measures the same size as stock. I have a crude measurement of these and bore to bore there is a difference. I have milled one down and installed it and the ratio is different.

Anyone know what these are off of?

Ductile Iron? Cast steel? The are both heat treatable.

maaseyracer said:

Very sure about that.

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I printed your photo and the centre-centre distance between the ends of each is the same (103.9mm), so, either the photo is lying or you're mistaken.
Cheers

robs ss said:

I printed your photo and the centre-centre distance between the ends of each is the same (103.9mm), so, either the photo is lying or you're mistaken.
Cheers

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Cool. Valve lift is distinctly different, and problematic do to the added length.

Stephen Hill said:

Ductile Iron? Cast steel? The are both heat treatable.

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You know that is a good point, I am going to guess cast steel as they look like the mk3 rockers. For all I know this could be a factory defect, the other set I had in the same spares box has significantly more material on the arms and bodies. I have not machined or measured the other side in actual or even the other set yet.

mdt-son said:

I believe so too. Rockers with long bosses were a fitment of Dominators before 1960. Norton shortened the boss to make room for shims and the thackeray washer. They didn't alter the arms at that occasion, at least.

- Knut

Click to expand...

I have a 1962, matching numbers, Norton 650SS. It has the longer valve rocker bosses similar to that shown at the beginning of this thread. Norvil suggests the changeover was after engine number 114870, which I think was somewhere around 1964. My bike has shims and Thackeray washers, just like later bikes. Do not know why Norton made the change, but later head castings have wider pivot points requiring the narrower rocker bosses.

I also have a ‘74 MK II Commando, but have never tried comparing the ratios of the rockers of the different bikes, so unfortunately cannot comment on that,

It would need to be shorter on the pushrod side, longer on the valve side is no good :.

Britstuff said:

I have a 1962, matching numbers, Norton 650SS. It has the longer valve rocker bosses similar to that shown at the beginning of this thread. Norvil suggests the changeover was after engine number 114870, which I think was somewhere around 1964. My bike has shims and Thackeray washers, just like later bikes. Do not know why Norton made the change, but later head castings have wider pivot points requiring the narrower rocker bosses.

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Ahh, I stand corrected. Now that you mention it, the wider pivots (part of the head casting) and shortened rocker bosses was the essence of an article I read some weeks ago - I just can't remember which one!

The old type of rockers (18249, 18250, 18251, 18252) align with the spigot-type cylinder head casting 23166 - were they fitted to the end at S/N 114869 in October 1965 , or was the changeover before ?

New type of rockers (25332, 25330, 25336, 25334) go with the new cylinder head casting 25316 from the beginning (S/N 114870, October 1965). These part numbers seem to indicate a linked occurence.

AMC and later NV performed a gradual development of the NHT. Change of rockers is not aligned with the adaption of longer valves and shorter pushrods, which occurred at S/N 125871 late in 1967.

There are obviously technical details of the engine we haven't clarified yet - 60 years after they were made! David Comeau ("AtlanticGreen") needs to examine his cylinder heads more closely! and so will I.

- Knut

Shelby-Right said:

It would need to be shorter on the pushrod side, longer on the valve side is no good :.

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Rockers can't be judged by themselves, they have to be reviewed along with the complete valve train and the cylinder head.

- Knut

Stephen Hill said:

Ductile Iron? Cast steel? The are both heat treatable.

Click to expand...

Ductile cast iron is good for big diesel engines, whose max rpm is modest (below 3000 rpm). This is due to higher specific gravity and modest fatigue strength vs. forged steel.
Forged steel alloy is used in petrol engines due to lower specific gravity and higher fatigue strength vs. ductile cast iron.

As far as I know, all classic Norton twin rockers were made of forged steel alloy.

- Knut

mdt-son said:

Rockers can't be judged by themselves, they have to be reviewed along with the complete valve train and the cylinder head.

- Knut

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...but, @Shelby-Right is correct. Without altering cam profile the only way to get more lift on a Norton heavy twin is to shorten the arm on the pushrod side - regardless of the rest of the train.
Unless you have another way - if so, please enlighten us.

PS: I can see no difference in length of those arms in the OP photo, metallurgy notwithstanding