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Pushrod length

Submitted by paul_nicholls on Sat, 26/11/2022
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Hi guys

Does anyone know/can measure the length of the pushrods for a 1931 model 18. 20, ES2?

I have 2 different lengths and both seem a tad short. It's a long rod with no adjusters. should be between 12.5" and 12.75".

Cheers

Paul

Submitted by alan_osborn on Sat, 26/11/2022

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The basic principle of all push-rods is:-You observe the rocker from 'at rest' to 'full lift'. Measure the lift very carefully directly on top of the rocker with a dial gauge. At HALF lift the push-rod needs to be at right angles to the rocker.-You've got that?.......

If any one can explain it better please do... The push rod needs to be at right angles at mid travel to ensure best efficiency, if it is seriously out the efficiency of the engine and any fancy lift with the fancy cam is lost. I haven't done it on any Norton engine but I have done it on a small car engine. Even raising the compression ratio by machining the head is starting to alter the best efficiency of this system and changing the valve length will up set 'best option'.

Submitted by ian_taylor1 on Tue, 29/11/2022

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Paul

I have the same problem on my 31 model 20.

The intake is slightly shorter, why I have no idea other than this bike was raced back in the day and some mods were done to the motor.

What I think happened the valve was replaced with a smaller head thus the stem sat higher above the guide so they shortened the rod.

 

It would be nice to know the correct length as Paul at racing norton is making new pushrods but he also needs the correct length

Submitted by barry_carson1 on Tue, 29/11/2022

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not the same, but on my 1955 es2 build that i wanted to get on the road i couldnt find any pushrods for it to rebuild the engine. i had some 19s ones which were longer. so cut them down same length as the old ones. i did noticed that the original exhaust pushrod was slightly longer than the inlet. by how much i wouldnt know now. on looking at the engine side on, it looks as though the inlet pushrod tube is more or less vertical and the exhaust one seems to lean in, towards the rear. whether thats why i dont know.

 

Barry

I have found the problem. someone had fitted slightly later valves which are 4.3125" overall length and they had shortened the pushrods to get them in. This put the rockers at the wrong angle to the pushrods and the arms were hitting the valve collars too.

The correct valve length for these early Model 20/18 motors 1930-32 should be 4.125". I found a valve spec book. So working back from there, the pushrods should be 12.75" to get the rocker geometry correct.

I have passed this on to Paul Norman and he has come to the same size conclusion.

I am just getting my valves altered to the correct length from blank Inter exhaust valves supplied by Paul. Also I have the new adjusters he supplied.

So hopefully all will be well by the end of next week!

Paul N.

 

 


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