Great technical articles from Jaguar, question about piston wrist pin location

Discussion in 'General Questions' started by JVROOM, Oct 25, 2012.

  1. JVROOM

    JVROOM Member

    Enjoy the many great articles by Jaguar. Years ago I turned wrenches on a dragster. The owner's father use to race flat track in Orange County CA on Indian motorcycles. He told me about how his father had made a piston for his bike moving the wrist pin location closer to the top (I think) of the piston. I can't remember the details, but he said something about it made the piston "flip" faster and nobody could beat him anymore. Everyone was trying to figure out what he had done, but never did, and he kept it a secret. My friend Jim, the owner of the dragster, used to say other than electronics, there's nothing new in racing, everything that comes around has already been done before. Has anyone ever heard about this piston modification?

    After reading some of Jaguar's articles, I'm planning on changing the upper connecting rod needle bearing. Assume this would be pretty easy, is it just removing the head and the barrel, does this require complete disassembly? Thanks for everyone's help, sincerely, John :bowdown::bowdown::detective:
     

  2. professor

    professor Active Member

    I was real impressed with reading Jaguar's stuff. Great of him to share his knowledge.
     
  3. Greg58

    Greg58 Member

    Replacing the upper bearing is a fairly quick job, you don't need a ring compressor just use your finger and thumb to get the cylinder back on.
     
  4. Purple Haze

    Purple Haze Active Member

    I think what you are talking about is setting the deck height. In most apps it is best to set the deck height @ 0 (top of piston even with deck) that will give you optimum quench and squish with a good closed-chamber head. The benefits of quench has been well documented since the beginning of the last century. But, this only applies to a wedge-type combustion chamber. The stock chamber on these HT's is a hemi, which works more like a swirl action. Therefore, the only thing you accomplish by moving the wristpin is changing the compression ratio.
     
  5. jaguar

    jaguar Well-Known Member

    What I don't like about this design is the connecting rod being more than twice as long as the stroke, which according to one technical article brings it out of the "zone" which delivers best power. Once more the chinese did everything they could to weaken this engine. Since the stroke on a 48cc is 38mm the rod shouldn't be more than 76mm long (eye to eye).
     
  6. Ludwig II

    Ludwig II Member

    There are dynamics involved, it might be that torque is improved at the expense of absolute power and slightly reduced crankshaft acceleration.
     
  7. jaguar

    jaguar Well-Known Member

    Within a certain range there is maximum torque/power due to the angle of the rod to the lower rod pivot point. Too long a rod (in this case) puts less lateral stress on the piston but wastes downward force.
     
  8. JVROOM

    JVROOM Member

    I wish I would have paid closer attention, when my friend was telling what his father had done to the Indian motorcycle, unfortunately they have both passed on. But I've been thinking about it and wondering if maybe it was a shorter rod with an offset piston?

    Also wondering if anyone has balanced the engine assembly, I'm still trying to get my 66cc slant head dialed in, suspecting an intake vacuum leak, but still having way too much vibration? Jaguar CDI, expansion chamber, Reed valve kit, waiting on Walbro carb. Thanks for everyone's help and consideration, John
     
  9. jaguar

    jaguar Well-Known Member

    Have you tried drilling out the wrist pin to lighten it up? (9/32" bit)
     
  10. JVROOM

    JVROOM Member

    Thank you for your reply, no I haven't tried that and I haven't changed the bearing either, will that help reduce vibration? Anxiously awaiting a Walbro carb. Sincerely, John
     
  11. jaguar

    jaguar Well-Known Member

    Drilling out the pin usually does the trick. But engine compression is a big factor also. Everyone should change out the wrist pin bearing.
     
  12. JVROOM

    JVROOM Member

    Okay I guess I'll try that, drilling out the wrist pin, and changing the bearing, wow this is like starting over. Got to find a better way to go than this, John:shout::shout::shout:
     
  13. jaguar

    jaguar Well-Known Member

    Since no one sells the lighter wrist pins then we need to look online for a source.
    Here's the dimensions of the stock wrist pins:
    48cc piston: 33mm x 10mm dia.
    55cc piston: 37mm x 10mm dia.
    60cc piston: 37mm x 10mm dia.
    Looking just now I found these:
    Honda Hobbit 33mm x 10mm wrist pin $9 [site]
    Yamaha QT50 33mm x 10mm wrist pin $9 [site]
    Generic 33mm x 10mm wrist pin $9 [site]
    Generic 36mm x 10mm wrist pin & bearing $13 [site]
    Generic 36mm x 10mm wrist pin $9 [site]
    I'm going to ask the sellers for the inner diameters of these pins.
     
  14. jaguar

    jaguar Well-Known Member

    the QT50 wrist pin is perfect for our use since it has a bigger 7.5mm inner diameter. All 48cc engine owners need this item unless they like engine vibration.
     
  15. JVROOM

    JVROOM Member

    Would this be the same for 66cc slant head motor? Thank you John
     
  16. jaguar

    jaguar Well-Known Member

    You'd have to measure the wrist pin. To see if using a lighter one would help you should stuff the standard one with something metal to see if the vibration gets worse. If so, then you should try a lighter pin.
     
  17. Gear_Head_717

    Gear_Head_717 New Member

    What about usuing a 5/16 bit? Would that be to much? Would a tittanium bit work or would it have to be carbide?
     
  18. Fabian

    Fabian Well-Known Member

    A longer rod has been proven to increase torque by allowing greater piston dwell at top dead centre; holding the piston at top dead centre and slowing down piston decent for a greater period of crankshaft rotation, however it slightly reduces peak power compared to a shorter connecting rod, for various reasons.
     
  19. Fabian

    Fabian Well-Known Member

    A single cylinder engine cannot be balanced. All you can do is play around with the balance factor to alter the rpm zone where the engines gives the least vibration.
    That said, a single cylinder engine will vibrate (by design), no matter how much you alter the reciprocating weight.
     
  20. Greg58

    Greg58 Member

    That is true, from my experience the 48cc engines have less vibration in the useable rpm range for normal riding. My grubee 48 vibrates the least of the three engines I own, after notching and ramping the piston it is fairly light.
     

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