Engines come in bewildering series of variations; these are .60s. L-R: K&B .65 is ABC, non-Schnuerle; SuperTigre .61 is Schnuerle ported, ABC; blacke-headed Rossi .61 is high rpm, high compression, rear exhaust; O.S. rear-exhaust, long-stroke .61 produces enormous torque at lower rpm and has fuel pump.