When fitting saucer type Welsh plugs such as the camshaft plug in some V8's. use a solid flat punch to flatten 2/3's of the dome area in the centre only. Don't reshape from convex to concave, just go to flat as at this point maximum size and sealing pressure has been achieved. Don't use the ball of a hammer to butcher the plug into submission as the quality of the seal will be suspect.
SIMPLY NEATLY FLATTEN 2/3'S OF THE DOME AREA IN THE CENTRE, STAYING AWAY FROM THE SEALING EDGE.