Hi Aldo. My method is to clamp a short piece of steel about half to an inch square to the bench grinder's tool rest. I set up the steel block with a square so that it is exactly at right angles to the side of the wheel. Then I hold the valve stem firmly into the corner between the tool rest and the base of the steel block, so it is guaranteed to be square to the wheel both horizontally and vertically. Hold the valve head to apply light stem-end contact with the side of the grinding wheel and rotate the valve.
Tecumseh's service procedures specify the same method but with a short V block clamped to the tool rest, instead of the square block.
Do not expect good results from anglegrinder-based solutions. The wheel of an angle grinder does not run especially straight: the bearings have slack and are too close together. The usual wheels are also much too coarse.
