The Act of Crank Balancing

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I sucessfully ported my engine yesterday and took it for a test run today. I got her up to 8500rpms. The low end was a bit slower but I will try to compensate byy increasing the header of my expansion chamber because I have it turned for high rpms right now. After about 5500 rpms I believe the power band kicks in and then I'm off straight up to 8500rpms smooth with a 40t sprocket. I calculated it to be about 40 mph or so. At 8500 rpms the vibrations are pretty bad. I'm going to get man hands on a cobalt bit today if I can to lighten things up.

Jag I like your idea about the connection rod. Are you sure it shouldny weaken things to much?
 
Good work on the porting!
Like the wrist pin, the connecting rod can handle way more than these little engines can put out. I wouldn't hesitate at all to drill a few holes in it (upper area only).
 
Yea.. got the cobalt bits. Only got half way through the pin. The other half was virtually un-openable. Even with a drill-press, and a pneumatic drill, the suck wont budge. I will test it with just a half of the pin gone. I am also in the process of a totally new bottom end remake with better bearings, etc. So I will just drill a hole or to in the upper crank arm, as well as in the flywheel. That should definitely level things out.
 
If you drill holes in the flywheels then you can always fill them with lead pellets and seal the ends with JBWeld. Test your half pin and see how it goes. Next step could be drilling holes in the connecting rod (just the upper part of it). Taking too much off can leave you with too much vibration just as well but then you'd be scratching your head not knowing if you took off too much or too little. That's why I wouldn't drill the flywheels except as a last recourse.

I had a machine shop drill out my wrist pin. I didn't hang around to watch and see how they did it but it turned out nice.
 
Cool. Also let me point out this fun fact. The stock wrist pin bearing that I had was approximately 6 grams. The replacement bearing that I got from Treatland.tv was 4 grams. So that's 2 grams off. I'm on my way home now from the lab to measure how much reduction in weight is from the half drilled wrist pin. Yea, perhaps I will just wait and see. No sense rushing things if your gonna do a half *** job you know?

Drilling this wrist pin is an ABSOLUTE challenge. I'm here for about 3 hours trying to take my time and I only got half way... ahh
 
Drilling hardened steel takes time and burns up cobalt drill bits, but it's the only way to get through such material and accomplish the task, unless you have access to a machine shop with diamond grinding tools.
If paying for shop time, the boring of the pin will possibly cost more than the engine is worth, so it's back to using your own time and cobalt drill bits.

The only problem with twist drilling is that it leaves stress risers which form a perfect surface for cracks to propagate.
A failed wrist pin isn't conducive to engine reliability.
 
Ok so I measured how much the pin weighs now. Its about 9.5 grams. Originally it weight around 15.5. So with half the diameter bored out it lost about 6 grams.. Incredible. Its pretty dense material. Now this coupled with a lighter wrist pin makes about 8 grams off the top end. I'm gonna connect it up in a couple hours. I'm really excited about getting rid of the vibrations they really become unbearable after like 6000 rpms. IN addition, the bike feels like it wants to rev higher but the vibrations are stopping it. I reckon it can probably go up somewhere around 9200 rpms. Actually when I rev it without load my tack reads 9200rpms, but the vibrations are insane once again.

Also on another note. I know its off topic but how can I get new bearings inside a crank case without bashing them in? I read somewhere that I put the crankcase in the freeze and the bearings in hot oil, then it should slip on. What degree of truth is this? Or any other suggestions.
 
Go to your local "BOC Gasses" or "Supagas" and purchase 2 kilograms (5 pounds) of dry ice pellets, storing them in a small esky.
Using your assembly method, throw the bearings in dry ice pellets for half an hour, shrinking their size slightly by reducing temperature to around minus 80 degrees Celsius (minus 112 degrees Fahrenheit).
Whilst the bearings are cooling, heat the cases in the oven to around 120 degrees Celsius (250 degrees Fahrenheit).

Take one of the cases out of the oven and one of the bearings from the dry ice and simply pop the bearing into the case.

"Job Done" - No bashing or smashing or swearing or cursing.

My preference is to freeze the crankshaft in dry ice and heat the bearings, then assemble bearings on crankshaft without any fuss.
Next job is to freeze the crankshaft and bearing assembly and clutch shaft, whilst separately heating the cases.
This makes assembly of the bottom end a simple and hassle free process, as everything just fits together nicely without neanderthal force being required.

Please remember to use protective gloves when handling parts frozen by dry ice as it will burn your skin just as easily as heat will burn your skin.
 
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Good work on the porting!
Like the wrist pin, the connecting rod can handle way more than these little engines can put out. I wouldn't hesitate at all to drill a few holes in it (upper area only).
Instead of drilling the rod, I suggest an old racer trick. Grind the parting line and polish the beam if it's accessable. This removes a considerable amount of weight, and actually makes the rod stronger. The small-block chevy guys did that for years before aftermarket rods became available. Amazing what you can do with a dremel and a few sanding rolls.
 
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