Crankshaft Truing 2: Checking and adjusting

OK so whether you have tried to replace a big-end bearing or simply have a used crank that's out of true, how do you go about checking and correcting this? You will need a lathe or a truing stand to rotate the shaft and determine the amount and position of any run out. In this description I'm assuming that you are standing in front of the shaft on the truing stand. The dial gauge is positioned on the right and makes contact with the top of the right hand shaft so run out is measured at the top of the right hand shaft.

There are only 4 ways the crank can be out of true and these show at different points in the rotation. Obviously they can occur in combination so they need to be corrected one at a time. So rotate the crank and find the position where runout is at a minimum. Set zero here and rotate again to find the point at which it is maximum. Look at the position of the crank at this position and in particular the position of the big end. This should be in one of 4 pisitions, either vertical, top or bottom, or horizontal, nearest you or farthest away. The position of the crank at max runout tells you the type or distortion you have. These are described below:

Tapering webs
The first type, and the easiest to visualise, is that the webs are not parallel: they may taper in or spread out from the crankpin. Tapering inwards means that max runout will occur when the big end is positioned vertically and down most. Conversely if they taper outwards, max runout will occur when the big end is positioned vertically but uppermost.  Correcting taper-in requires a thick soft metal wedge that can be tapped in between the webs to force them apart a little more: correcting taper-out just needs the webs to be pinched together slightly in a vice (soft jaws). Adjust and check run out again. If you are lucky then such adjustment will eliminate runout completely.

Swivelled webs
The second type of misalignment is harder to explain but occurs when the webs are not in alignment but one is swivelled around the crank pin relative to the other. This defect is manifested by max runout when the big end is horizontal, either forward, nearest the operator or rearward, away from the operator. Correcting this requires a heavy, but soft faced hammer to tap the base of the appropriate web to swivel it slightly around the crankpin. You will need to hold the crank with the crank pin horizontal and away from you, then tap down on the appropriate web nearest you to swivel it round the pin slightly. If it's highest on top right when the conrod is near you, then hold the crank by the left hand web, with the crank pin away from you and tap down on the righthand web. If the runout is highest with the crank pin horizontal but farthest from you the hold the crank by the righthand Web with the crank pin away from you and tap down on the left hand web (see diagram). Retest and repeat.

Runout greatest at top right when big end horizontal and nearest to you. Hold crank as above and strike where shown. 

Runout greatest at top right when  big end horizontal and farthest from you. Hold crank as above and strike where shown. 

If both misalignment types are present, as is likely if assembling a crank without a jig, the worst misalignment will be detected first. Correct that as appropriate until either the crank rotates true or a second misalignment shows itself. Correct that and retest. Keep going until the crank runs true or within 0.1mm of true. I tested a crank I knew to be good and this had a runout of 0.05mm.
I'm taking this approach from the following YouTube video which is very informative.
https://www.youtube.com/watch?v=X34qMzEjOnY