For this purpose the engine frame is made in two parts, one of which carries the cylinder whilst the other is bolted to it. The second part may be composed of several panels and can be removed to give access to the working parts. To facilitate dismounting and re-mounting the gudgeon is fixed to the connecting road small end which may have a thinned body to reduce the liability of the piston to cant in the cylinder. The piston is provided with adjustable bushes.