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Seeking any experience on crankshaft cracking prevention.

Discussion in 'Norton Commando Motorcycles (Classic)' started by johnm, Sep 2, 2019.

  1. johnm

    johnm VIP MEMBER

    Joined:
    Feb 26, 2008
    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.
     
    Last edited: Sep 2, 2019
  2. texasSlick

    texasSlick VIP MEMBER

    Joined:
    Jan 2, 2013
  3. johnm

    johnm VIP MEMBER

    Joined:
    Feb 26, 2008
    Thanks for that

    Not exactly the same situation because I'm talking about the drive side crank not the big end journals, but Brooking 850 and Matchless have relevant comments thanks. Drilling out and pressing a shaft right through as suggested by Brooking is a possible on the old crank. Its basically what many Brit singles do as standard but the porkchop is not that thick and there is not a huge amount of support there.

    It is certainly one of the options we have considered.

    Matchless is basically making the same suggestion as we have thought about
     
  4. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    I hadn't heard DSIR mentioned for a long time and thought of Phillip Sherry reading that quote.

    The crank out of my bike cracked the other way, from the cheek out to the rotor thread.

    Maybe it is in your post but what about a reverse radius.

    065 - Copy.JPG

    Fwiw... even though there is next to no radius and as mentioned in other posts.
    I have always used moderate heat to install both inner and others along with a weight until cold so said part will not creep (lift) as a general thing regarding bearings.

    There is a remote possibility that if the crankshaft inner (850 or roller bearing) is well seated against the cheek it might offer some support to that transition (or not)
    That even with a 0.001" or two gap (bearing inner rear face to crank) cheek) might allow flex at the pin transition and at a certain harmonic oddities might happen.
     
    Last edited: Sep 2, 2019
  5. johnm

    johnm VIP MEMBER

    Joined:
    Feb 26, 2008

    " Maybe it is in your post but what about a reverse radius. "

    Yes that's exactly what we are suggesting. and what Matchless was suggesting too.

    You and I must have been in the same places !!! I actually worked for DSIR for a short time. But in the Geological survey.

    For the non NZers. DSIR - Department of Scientific and Industrial Research. Before it was got at by various government reorganisations.

    We put the shaft in the freezer and the bearing inner in a low oven !
     
  6. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    Oddly enough I am not a fan of cold and heat, a chilled part can produce condensation when paired with the heated part which can remain in the seating area's. :D
    I think it would need nitrogen to get any real result on a solid shaft.

    Taumarunui.
    Hamilton.
    Auckland.
     
    comnoz likes this.
  7. johnm

    johnm VIP MEMBER

    Joined:
    Feb 26, 2008

    Makes sense.

    Taumarunui huh !!!

    Ohakune. !!!!

    Wellington
     
  8. gortnipper

    gortnipper VIP MEMBER

    Joined:
    Nov 11, 2013
    @johnm Suggest you give Plazz (Tony) a call at Aero Machinists in Hamilton. He may have some value to impart.
     
  9. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
  10. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
    Ken Canaga said : You are cutting the groove into the face of the cheek, not into the shaft. I use a .090" radius tool to cut the groove. That's what the crankshaft guru's recommended back in the '70s. It's also necessary to polish the groove and/or shot peen it to reduce surface stress risers. I use a wooden stick with valve grinding paste for the task. You can see the groove pretty well in this picture.
    https://www.accessnorton.com/NortonCommando/commando-crankshaft-porn.7673/page-3
     
    johnm likes this.
  11. Dances with Shrapnel

    Dances with Shrapnel VIP MEMBER

    Joined:
    Jul 8, 2011
    Cutting a greater radius into the cheeks on the main journals is an accepted practice to enhance durability, that along with polishing to reduce stress risers and rolling or shot peeling to increase residual surface compressive stresses. I recall a reference book, maybe Internal Combustion Engine - In Theory and Practice by Taylor, where there is a table of filet diameter to shaft diameter ratios as compared to increased durability. It was quite impressive. There was also a table showing main journal to rod journal overlap/underlap and it’s impact on crankshaft durability, also quite impressive.

    I would investigate the effectiveness of nitriding the factory crank cheeks as they are cast steel. I understand the effectiveness of nitriding on billet and forged steel - just don’t know if nitriding cast steel will give you the results you want.

    Speaking to the OPs remarks about peace of mind and willingness to spend a bit to get there.......after the expenses of searching for purchasing, testing, assembling, machining, heat treating (nitride) and balancing a factory cast steel crank cheek or two, the OP will really only be looking at an incremental cost (maybe) to just purchase a billet crank and put this concern to bed.

    The durability of billet steel is considerably greater than that of cast steel.
     
    johnm likes this.
  12. jseng1

    jseng1

    Joined:
    Nov 26, 2009
    When I was racing my cranks were cracking at the PTO shaft so I started radiusing the stubs per Ken Canaga's advice (see thread above) and the problem was solved. You have to shape a .090" radius cutting tool and keep moving it sideways back and forth a little to avoid chatter. Then use a brass rod mounted in a high speed die grinder. dipping it frequently in valve grinding compound for the final polish ( I have dremel bits that also work). I do this to all my cranks now (including street).

    [​IMG]

    If you're using stock cases then they will be cracking next. Reinforce them with pressed on plate (tight on the bearing housing OD) similar to what Herb Becker did below.
    [​IMG]
     
    Last edited: Sep 3, 2019
    Tony Pierce and johnm like this.
  13. Brooking 850

    Brooking 850 VIP MEMBER

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    Oct 3, 2011
    Johnm PM me
     
  14. johnm

    johnm VIP MEMBER

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    Feb 26, 2008
    Gentlemen

    Thanks very much for all this information

    The radius method advocated by Ken Canaga and then confirmed by several posters is exactly what we were considering and it is great to get some real experience. So I shall certainly do this if I go with an original crank. Interested you polished with grinding paste. Some people here here have suggested rolling which is evidently what BSA did when they were fracturing cranks.

    It is very frustrating that Norton created this mess in the first place but great to learn the radius fix has worked for several people.

    However further to Dances comment about billet cranks this is indeed my current plan A. Unfortunately not that simple because billet cranks don't grow on trees. I did a lot of searching around the world and tracked down two billet cranks. But one was lightened and machined more for race use. I really want the Commando smooth flywheel torque feel for my street bike. The other was from the guy who bought Dave Nourish's company and we have not had good experiences with him down here.

    But I did get onto a crank from a reliable US supplier which I thought would solve all my problems. Unfortunately they were let down by their supplier and received only partially finished cranks. It has cost them a lot of money and they are hard at work trying to solve the problem. They are totally in the right and it was a supplier who screwed them over. I'm not going to give their name because they deserve the chance to try and fix things which were totally not their fault.

    So they have sent me photos of the unfinished crank and I have bought it from them and hope to finish it down here in NZ. I'm pretty sure I can get it finished here. It has now been delivered here in NZ so I will now make a decision on which way to go.

    Pros and cons both ways.

    The billet will be stronger but will need lots of time, work and money to finish and its hard to get a really good flywheel shape with the same weight/ inertia properties of a standard Norton crank. Using standard crank involves a bit of running around but could be finished in a couple of weeks. It being crack tested now. Then machine and polish the radius, then shotpeen, reassemble with new hardware and check dynamic balance.

    None of this is cheap but I hope I'm almost there now with the final plan.

    Again thanks for all the experienced information guys. Much appreciated.
     
    Last edited: Sep 3, 2019
  15. johnm

    johnm VIP MEMBER

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    Feb 26, 2008
    Have done so

    Cheers
     
  16. johnm

    johnm VIP MEMBER

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    Feb 26, 2008
    Thanks. I talked with the guys in Hamilton a few weeks back. They are indeed one of my options but now I have a second hand crank and the part finished billet I will probably go with one of those two options
     
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  17. johnm

    johnm VIP MEMBER

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    Feb 26, 2008
    Hi Jim

    Do you radius the timing end as well? I’m just about to get this done

    John
     
  18. jseng1

    jseng1

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    Nov 26, 2009
    There is much less stress on the timing end. I wouldn't bother.
     
  19. lcrken

    lcrken VIP MEMBER

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    Mar 15, 2009
    As far as I know, most racers just radius the drive side. As Jim said, there's really no significant bending stress on the timing side, as there is on the drive side.

    Ken
     
  20. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    Royal Enfield Interceptor crankshaft for a comparison

    DSC08314.JPG enfield crank.jpg
     

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