the latest OPS front intake engines have a ninety eight percent part interchange with the old ones from the late sixties and seventies, the variants are the three port intake design with the sleeve and later five port design sleeve the five port types have windowed piston skirts. but bores and strokes and bearings remain the same. with in these two design variances there are different exhaust port timings, IE 154-160 on the low end which is pattern and sport usage, then 170-190 on the speed and ducted fan and tether car motors, absent the engine having the needle bearing rod setup most parts exchange perfectly, and a lot of the picco engines interchange as well in the 10 cc category, i own a number of these engines and they are remarkably interchangeable. picco engines are for those who aren't aware designed by Gualtiero Picco and he was the "P" in OPS engines. he is still the dominant 10 cc tether car record holder today. this is a direct link with some pictures of the latest designs of sleeve and pistons etc, fun reading

http://www.speedmodelcar.com/