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 Post subject: Truing the Crank
PostPosted: Fri Jan 29, 2010 12:10 pm 
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Joined: Wed Oct 22, 2008 4:09 pm
Posts: 10865
Location: Deepinnaharta, Texas

An excerpt by Mark Burgfeldt:
Now comes the tedious part - aligning the crankshaft. You will need two v-blocks and a surface plate or the centers of a lathe or a truing fixture. The main criteria of these tools is that they are perfectly rigid and sta­tionary. I prefer to use my old faithful South Bend lathe. You will also need two dial indicators and the necessary fixtures to mount them in place. Position the indicators so that they contact the main bearing journals of the crankshaft. This setup will allow you to check the run-out of the journals. To check the run-out, rotate the crankshaft using the connecting rod. Check to see the highest reading on the dial indicators. Zero the indicators at that point. Rotate the crank again and check your indicators. If the one needle rises while the other falls, and the crank halves are twisted about the crankpin, they are not concen­tric. This is called vertical misalignment. If this is the case, remove the crankshaft assembly from the centers and smack the crankwheel where it aligns with the high reading dial indicator. Use a large brass hammer and hit it hard. If the needles rise and fall together, the flywheels are not parallel. This is called horizontal misalignment. If the high point is on the crankpin side, the flywheels will need to be spread apart. I use a large modified cold chisel wedge and my brass hammer for this operation. Insert the wedge between the flywheels and tap. Re­move the wedge and recheck the alignment. If the high point is opposite the crankpin, the flywheels are spread apart. Use the brass hammer and smack the outside of one flywheel toward the other and recheck the alignment. As you can probably imagine, this can become a tedious production, especially when you realize that the goal is zero run-out. Those needles on the dial indicators should sit perfectly still when you are done. Don't be surprised if, after a long time, you get very close but can't get it right on the money. If the center lines of the crankpin holes and the center of the halves are not exactly the same distance apart on both crankshaft halves, perfect alignment may not be obtainable. This is most likely to occur when a broken half is replaced. Remember, the closer to perfect you can get on alignment, the more RPMs and power the engine will make.

And here's another good Word to the Wise:
Once the crankshaft is aligned, put it away somewhere where it won't get knocked around. It does not take much to knock a crank­shaft out of alignment. Once it is supported by the main bearings between the crankcase halves, it will be considerably more durable.

I would add that variator removal should always be done with a device to immobilize the fixed pulley. That way the stress applied by, say an impact gun, doesn't have a chance to be transmitted across to twist the crankpin out of true. I agree that the starter motor does this to some extent, but smoothly, not a succession of harsh percussive blows. The crank you save may be your own. :wink:

Most of my money is spent on scooterparts. The rest is just wasted.
Still Seeking Red PCX-150
Flash 9: 2001 Elite SR Contesta 72 ZX Tran, 9:1 Gears, Stock Airbox, Carb and Pipe 58.8 MPH
Punkin: 2010 Vespa/Malossi S78, 58MPH


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