 The problem comes about because in "forward" running the major thrust face of the piston is on the back face of the cylinder which, in a two-stroke particularly, is the coolest and best-lubricated part.The forward face of the piston in a trunk engine is less well-suited to be the major thrust face since it covers and uncovers the exhaust port in the cylinder, the hottest part of the engine, where piston lubrication is at its most marginal.The front face of the piston is also more vulnerable since the exhaust port, the largest in the engine, is in the front wall of the cylinder.