just tried another pump , same as the last one ,melling 77, and it seems to be about 5 or 6 mm longer in the shaft, just measured from the shaft slot to the dizzy base on my other motor, a small block and that only seems to engage bout 5 mm, so....... it looks like the first pump i tried was a reject! i suppose 5 mm is enough for the shaft to engage? the small blocks been run like that for bout 2 years !