Raising CR on MK2a 850

Anyone calculated out the forces and elongation of "plastic deformation" in Norton alloy rods ..... !!!

wakeup said:

Stand by to be gobsmacked. I worked at NV in 69/70,

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This would have been for the Atlas type motor in the early Commandos. ?
These didn't even have any record of crank problems ?

wakeup said:

This is the first time I've seen fatigue failure in this thread, obviously that will apply, its extent will be governed by the weights of the con rod, gudgeon pin (wrist pin to you Yanks) piston, circlips etc) I've forgotten how to calculate this but I'll accept your guess. I'm sure that there would be someone here who can quickly knock out the various stresses on a conrod.

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Fatigue failure in Commando rods.
Lets see, some Commandos are on record as having done 200,000+ miles.
The pistons will have done similar miles, and at a 3.5" stroke, thats about 4.x billion revs on a back of a stamp rough calc.
So about 10 billion stress cycles, in round terms.
With no significant record of failures with those miles, so obviously the fatigue life is significantly beyond 10 billion cycles...

acotrel said:

Eddie, have you weighed your new pistons and compared them with the ones you have replaced ? The rods must stretch, otherwise they would break. If you over-rev a motor and the rods stretch so much that they exceed the elastic limit, plastic deformation occurs and the damage is irreversible. If they don't break immediately, they can do that at any time afterwards when fatigue completes the process. Keeping piston weights low is a good thing to do, and if the high-comp pistons are heavier, it is a backwards step fitting them.

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Good point, and I agree. The difference is quite small, for a Triumph man at least! It is nothing like what you'd see co,paring stock T140 pistons to Powermax's for example! They are actually 6 grams heavier, but I reckon I can relieve material from the piston, and possibly machine a taper into the wrist pin and get them close to the stock weight without to much difficulty.

What is the actual weight of the Omega pistons. ?

As Rohan states and every single rod failure I've heard of last 15 yr of paying attention - that was really investigated had nothing at all to do with any Norton issue rod but something else broke first, so in a street bike the Norton rods are all so over built there is no fatigue flex limit life time. The rods like in TC engines are fine to handle huge torque cycles up to 8000 rpm a long time is another example ya can't ignore. Of course in real world every item is not made perfect so certainly some rods must of let go first but so very few its like worrying about a lightening strike riding. When I wear out JMS light pistons with the Carrillo rods I'm going back to Norton rods and longer skirt pistons and drop CR to 10-ish and hope for 20+ yr tooling around in public with Drouin to get some pay back on the investment.

Rohan said:

What is the actual weight of the Omega pistons. ?

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402g against 396g stock, I think, not near m notes at present!
FE.

10.5:1 920cc conversion. Some off the head, some off the jugs

Rohan said:

I worked at NV in 69/70,

This would have been for the Atlas type motor in the early Commandos. ?
These didn't even have any record of crank problems ?

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Yes it was the early Commando, you know when they were looking at electric starters, disc brakes, oil consumption problems, noise issues (which you would have to believe only apply to non USA manufactured motorcycles)
Just because there are no recorded crank problems doesn't mean that there wasn't work going on..........

cheers
wakeup