Here we have a selection of engines for various scale size models from 1/16 to 1/8 scale
Size of engine usually relates to model it is intended for as follows
.05 to .09 for 1/16 scale
.12 to .18 for 1/10 scale
.21 & upwards for 1/8 scale nitro models.
Petrol models usually have 23cc to 28cc 2-stroke engines fitted