You need a bushing for the valve stem to seal.
No, read back in the first post.... You need to bush the back of the valve down to 1/8" instead of 5/32" when you use the MRod poppet, and grind a bit off the end of the stem so you don't lose hammer stroke.... You want the stem to stick out the back of the valve the same distance as the Disco stem, or a whisker less, not more.... Bob