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

J

JVROOM

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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::geek:
 
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.
 
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.
 
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).
 
There are dynamics involved, it might be that torque is improved at the expense of absolute power and slightly reduced crankshaft acceleration.
 
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.
 
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
 
Have you tried drilling out the wrist pin to lighten it up? (9/32" bit)
 
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
 
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