Hit and miss engine con rod construction

Discussion in 'General Engine Discussion' started by XD351, Oct 16, 2018.

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  1. Oct 16, 2018 #1

    XD351

    XD351

    XD351

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    I am currently building a hit and miss engine an have hit a cross road with the conrod, I have made a two piece unit as a test piece me to get some practice for silver brazing the crankshaft together .
    The conrod was sent up vertically with the base plate at the top , a liberal amount of flux was applied to the inside of the joint and ring of silver brazing rod sat on top of the base plate where the con rod protruded through . I then applied heat from below the plate so the solder was sucked down into the joint and this formed a neat fillet on the lower side of the plate giving what appears to be a very nice join .
    The spiggot that protrudes through the base plate is 1/2 dia and the plate 1/4 thick .
    My question is will it be strong enough or am I better off machining a one piece unit ?
    See photos attached .
     

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  2. Oct 16, 2018 #2

    el gringo

    el gringo

    el gringo

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    part of the fun is to make everything with as few pieces as meaningful for usage.
     
  3. Oct 16, 2018 #3

    Brian Rupnow

    Brian Rupnow

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    That should be fine. Hit and miss engines are slow runners. I use 6061 aluminum for my rods and have never had a problem.
     
  4. Oct 16, 2018 #4

    el gringo

    el gringo

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    Yes, I have used 2024 aluminm sans bushing in the big end with very good results.
    Ray M
     
  5. Oct 17, 2018 #5

    XD351

    XD351

    XD351

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    Ok thanks for the replies ! I will move along and finish the rod off , it does save o lot of time and wasted materials doing it this way ! Like many model engines once made they tend to become ornaments on a shelf and only bursting into life occasionally this one will probably be no different so it won’t do a lot of running .
     
  6. Oct 18, 2018 #6

    coulsea

    coulsea

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    Most of the force on a con rod is compression, pushing it together not pulling it apart. a pin through it would make sure, just a straight pin through a straight hole and solder, even soft electrical solder so you don't make a mess of your existing joint.
     
  7. Oct 18, 2018 #7

    Goldflash

    Goldflash

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    Most Rods fail under tension and its on the exhaust stroke as there is no cushioning effect as per the compression stroke and that piston wants to just keep going straight ahead . The force generated by a piston hitting TDC at 1,000 RPM is 50 times its initial weight when the engine is not running. at 10,000 rpm its 5000 times greater. 2 strokes are always under compression and rod failure is usually caused other parts breaking first.
     
  8. Oct 18, 2018 #8

    XD351

    XD351

    XD351

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    That’s what was worrying me ! This motor like many hit and miss engines runs a cast iron piston and it is heavy ! I could do a test piece with the plate on one end and a thread on the other , i could then set it up width ways across my vise jaws then use a nut screwed onto the thread to pull tension on the rod to see where it fails . I think the joint would be strong enough to distort the plate .
     
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  9. Oct 19, 2018 #9

    XD351

    XD351

    XD351

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    Today I made up a test piece with the same materials and dimensions of the conrod joint. First I tried using a nut threaded on the end of the test piece to to try and tear the joint apart but it distorted the plate and as I only had a nyloc nut it stripped the 9/16 NF thread out of the soft nyloc nut . I then took it over to the press and tried pushing the rod out of the plate from the back via a hole I drilled in the piece of flatbar I had bolted the rod to , it did start to separate but the distortion of the base plate was enough to tell me this sort of construction is more than strong enough to handle the forces involved .
    See pics .
    Pics are not in order so the mild distortion was from the nut and the rest from the press .
     

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    Last edited: Oct 19, 2018
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