Re: China Girl Motor Build
I found this information about china girl crank balancing on another forum. I think this is how to static balance the crank. I'm not sure it's 100% accurate, but maybe someone here can look it over with me.
"1) Take the complete weight of the connecting rod big end and the big end bearing - 40.3 grams + 7.1 grams = 47.4 grams
If your engine has thrust washers, add the thrust washer weight to "step 1"
2) Take the complete weight of the connecting rod small end and the total weight of the piston and rings, piston pin, small end bearing and clips - totals 105.5 grams
3) Multiply the figure 105.5, by your desired balance factor. For arguements sake, i'll use a common balance factor figure of 55% > 105.5 x 0.55 = 58.025
4) Add the total weight of "step 1" to the balance factor figure of "step 3" - 47.4 + 58.025 = 105.425
The required bob weight to balance the crankshaft to a balance factor of 55% will be 105.4 grams
5) Assemble the crankshaft by lightly pressing the two halves and crankshaft pin in place, only so it will hold together and you can true the crankshaft on a set of V-blocks for the balancing proceedure.
6) Place the trued crankshaft ends or shafts on parallel knife edged surfaces or use two reasonably sized drill bits that do not have any scratches or deep grooves - you want a perfectly smooth and horizontal surface for the crankshaft to be able to rotate on.
7) Make some sort of hook like device to hang off the crankshaft pin that weighs exactly the same as the figure you've calculated in "step 4"
8) You want to be able to rotate the crankshaft in any position without the crankpin falling downwards.
Generally it seems to be accepted that a slightly heavy counterweight is desirable (the side opposite the crankpin).
If the crankpin falls downwards, you can either remove metal from the crankpin end, or add heavy metal (like brass or malory metal) to the counterweight end, till you get the crankshaft to balance.
9) Once the crankshaft has been balanced, take it apart and install the big end bearing and connecting rod (thrust washers also if the crankshaft used them) and press the crankshaft together and true the crankshaft.
10) Take your crankshaft and put it in a plastic bag and place it it the freezer. After about an hour, take your crankshaft bearings (for the left and right side shafts) and remove the dust seals if they are installed.
Now drop the bearings into a pan filled with hot oil.
The bearings will expand in the oil and the crankshaft will shrink ever so slightly.
Take the crankshaft out of the freezer and prop it up so the shafts are vertical.
With a pair of smooth faced plyers, take a heated bearing out of the oil and carefully drop it on the shaft, don't delay, do the other side as well.
The bearing should just drop straight on, no pressing or force should be needed.
Make sure you have pressed the bearings into contact with the face of the crankshaft before the bearings cool down and the crankshaft heats up.
If they are not seated squarely on the shaft after everything has equalised in temperature, you will have no hope of bashing them flat and straight - you'll only damage the shaft and the bearings.
11) Before assembling the crankshaft in the case halves, along with the clutch shaft, place the assembled crankshaft with it's crankshaft bearings in the freezer and use heat to expand the cases allowing for an easy fit of the bearings into their respective recievers.
Assembly should take place without too much force being applied on the crankshaft.
If you need heaps of pressure to get the case halves to close up, you'll press the crankshaft out of "true" and the engine will vibrate, regardless that it has been balanced.
12) It's best to install the crankshaft seals after the cases are clamped together."
