By drilling out part of an end burner, you make it into a 'mini port burner', which burns faster. This will give you the kick to get off the ground. To deep, too wide, causes the grain to burn quite fast, sometimes too fast, with negative results. Altering a ready made motor increases the likely hood of having that motor blowing out the nozzle, the top cap, or both. At worst, splitting the sides open. Not cool.

From and old micro-grain enthusiast.

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