johnm
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- Joined
- Feb 26, 2008
- Messages
- 1,837

Recently I had my low mileage 1974 850 crankshaft crack tested and it failed. There was a microscopic crack around 30 % of diameter of the shaft at the right angle from the drive side main bearing shaft to the drive side "porkchop" .
I use the words right angle because that is exactly what it was - and in the words of a mechanical engineer and 30 years experience in the materials lab of the old NZ Physics and Engineering lab of the DSIR - one of the best examples of exactly how NOT to machine a change of section he had seen.
Direct quote " Those damn Brits should know much better than to machine a crank like that in this day and age. " I pointed out to him it was a 1974 machine and he said - " I mean in the 20 th century !!!!!!!! " They learnt not to do that making railway wheels in the 19 th century !!!! "
I have since inspected a few more cranks and " Yes they are all like that Sir !!!!
Its pretty common for Norton cranks to break at the big end after a regrind if the correct radius has not been been ground but engineering shops here know that and do it properly. And perversely the timing end does have a small ground radius from the porkchop to the shaft.
But not the drive side !!!
The inner races of main bearings have a small radius ground in them to allow for this so who knows why Norton didn't bother on the drive side !!!!!
I have two options I'm currently following. The first is a new one piece billet crank which is having its own issues but not the subject of this post.
Plan B is a second hand crank I have bought. Right now it is being crack tested.
If it passes - engineering friends have suggested the following:
Machine in a small radius at the drive side bearing shaft to the porkchop change of section.
Mask off the shafts and shotpean the change of sections
Possibly consider nitriting ?
Reassemble with new hardware, check dynamic balance, run out etc and then install.
Has anyone had any experience with this and any further suggestions ?
I'm not planning to race this bike but I also want total peace of mind and am prepared to pay for it. I do not want to take this engine apart again.
I use the words right angle because that is exactly what it was - and in the words of a mechanical engineer and 30 years experience in the materials lab of the old NZ Physics and Engineering lab of the DSIR - one of the best examples of exactly how NOT to machine a change of section he had seen.
Direct quote " Those damn Brits should know much better than to machine a crank like that in this day and age. " I pointed out to him it was a 1974 machine and he said - " I mean in the 20 th century !!!!!!!! " They learnt not to do that making railway wheels in the 19 th century !!!! "
I have since inspected a few more cranks and " Yes they are all like that Sir !!!!
Its pretty common for Norton cranks to break at the big end after a regrind if the correct radius has not been been ground but engineering shops here know that and do it properly. And perversely the timing end does have a small ground radius from the porkchop to the shaft.
But not the drive side !!!
The inner races of main bearings have a small radius ground in them to allow for this so who knows why Norton didn't bother on the drive side !!!!!
I have two options I'm currently following. The first is a new one piece billet crank which is having its own issues but not the subject of this post.
Plan B is a second hand crank I have bought. Right now it is being crack tested.
If it passes - engineering friends have suggested the following:
Machine in a small radius at the drive side bearing shaft to the porkchop change of section.
Mask off the shafts and shotpean the change of sections
Possibly consider nitriting ?
Reassemble with new hardware, check dynamic balance, run out etc and then install.
Has anyone had any experience with this and any further suggestions ?
I'm not planning to race this bike but I also want total peace of mind and am prepared to pay for it. I do not want to take this engine apart again.
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