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If I'm reading you right, and please correct me if I'm not, you are trying to machine out the old stem from the yolk(lack of a better word)?

Is there anything left of the old stem at all?

Assuming I am right about what you are trying to do, and assuming there is anything left of the old stem, here is what I would do.

Using a bridgeport or any vertical mill, clamp a knee to the table. A knee being any right angle plate with enough surface area to span the width of both legs of the fork and allowing room for toe clamps or forged c-clamps.

Mount the knee so it is flush with back of the table on the mill. Clamp the fork assembly to the knee using V-blocks, screw jacks, and either toe clamps or forged c-clamps. The legs hanging down, and the stem sticking up and generally parallel to the spindle.

Using an indicator and the quill, indicate up one side of the stem and true the fork assembly to this degree of freedom. Now rotate the indicator 90° and indicate up and down the stem again. Adjusting until the stem is parallel to the spindle in this orientation.

At this point, you have the fork assembly mounted and true to the spindle. Now, using your indicator again, qualify the stem and zero out the x and y axis on the machine. Select your tooling and have at it.

Bear in mind that when clamped using v-blocks and jacks, torsional rigidity of the part being machined is not 100%. A deft touch and sharp tooling, you'll have it out in no time.
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