Conrod Bolts. Looking for bolts 5.5 mm or close.

Discussion in 'HMEM Forum Support & Suggestions' started by PeterLM, Jul 13, 2018.

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  1. Jul 13, 2018 #1

    PeterLM

    PeterLM

    PeterLM

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    Hi Forum

    I got summer holiday, and for family reasons Im going nowhere. This means that I got time to spare, and a project is about to begin.

    I will have to make some conrods for a small 4-stroke im modifying.

    For this I will need some conrod bolts. I would rather use standard bolts, but until now I have not been able to find any that is close to my dimensions.

    The width of the conrods will bee 8 mm. So the bolt-head must be no wider than this. The shank of the bolt should be close to 5.5 mm (to allow the same x-section area on the seat as the shank).

    The threads could be metric or imperial.


    Since im making designing the conrods I can modify it to accommodate the right conrod length.



    Does anyone at this forum know a good source for bolts like this?
     
  2. Jul 13, 2018 #2

    GailInNM

    GailInNM

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    An imperial size of 12-24 or 12-28 socket head cap screw would be very close although you might have to skim the head diameterr a tiny bit. They are not as common as they used to be but are available in the US without much trouble.

    Remember on a 4 stroke engine that the loading on the conrod cap screws is very low compared to the conrod so you could probably safely use M5 screws quite safely.

    Gail in NM
     
  3. Jul 14, 2018 #3

    Cogsy

    Cogsy

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    Making exactly the bits you need is most of the fun isn't it? I'd whip up some M5's with whatever head size you prefer, something like these :


    IMG_0756.JPG
     
  4. Jul 14, 2018 #4

    PeterLM

    PeterLM

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    Gail
    I kind of disagee on the load of the conrod cap bolts.
    My calculations says that a M5 bolt will have a stress of 286MPa at top RPM. To get a decent saefty factor i have to compromise on the cyclic strength or I have to be able to controll the material and hardening with high accuracy. Hence the wish of purchaced bolts.
    The conrod is easier since i got more dimensional freedom, so i can reduce the maximum stress by optimising the geometry.

    I have to agree with you Cogsy that making the parts is half the fun. But making bolts that are only just good enough is not my kind of fun. I would rather pay for getting bolts that are really good.

    So again. If anyone knows of a source of conrod bolts with a shank diameter around 5 mm please let me know.

    Peter
     
  5. Jul 14, 2018 #5

    GailInNM

    GailInNM

    GailInNM

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    Fair enough, Peter. Your calculations are undoubtedly correct. for your engine.The only information I knew on it was that it is a 4 stroke.

    Again, take a look at the imperial 12-28. Converted to metric the diameter is about 5.48 mm with a a 0.9 mm pitch. In a SHCS version, the head diameter is only slightly over over your 8mm max, and most of this oversize is in the knurling that is on the head OD. Since that contributes nothing to the sytrength it coould be shaved off to your 8mm max safely. They are available in both alloy steel and stainless.

    Gail in NM
     
  6. Jul 15, 2018 #6

    PeterLM

    PeterLM

    PeterLM

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    Gail
    I will defenently have to look into this 12-28 size. The fit is allmost perfect.
    Thank you a lot.
    Pete
     

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