Its has been done before, however the metal left between the piston top and the 1st ring land becomes weaker, and typically fail. Ideally try and find a piston where the top ring land is a bit lower down, or buy A15 pistons. They can be had cheap enough. If you are wanting a flat top type piston in an A15, there are several options available. There are a few articles and thread on what others have used in their A15's. Most involve enlarging the con rod little end to 18mm ~ 19mm piston pins though. The rods are up to it.