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Hi Doug, Using a mill to true up what looks similar to a engine cylinder casting You might be putting a lot of trust in the quality of the casting to be true - they have a habit to be a bit all over the place. This is what makes castings such challenging, but fun , beasties to machine. The first thing to think about, when we machine a casting, is where the master datum should be to get the best out of the outside shape of the casting. On a cylinder I would use the outer curve of the cylinder rather than the port face. I know that sounds like an odd place to start but its actually easy as you can hold the cylinder in a 3 or 4 jaw chuck on the lathe by the outside of the cylinder and that allows you to skim one end, turn it round, put the casting solid against the chuck, then both face the end and bore at the same time and that end of the cylinder from this point onwards is the master face which faces towards the crankshaft. Yours as it is a generator it requires both faces to be true so I would be tempted to mount it on a mandrel when taking it to length If you get a chance it may be worth having a wander through some of the threads to see how other people have machined their casting sets. It is always fascinating and you will find that we use different techniques than are common in commercial work. Jo