The S30/35's are still a very popular engine today in smaller CL stunt models so there's a huge amount of experience with them. This experience has shown that their weak point is the lower rod bearing which absolutely requires at least 25% all castor if it's to survive for a long time. There's a bushed ST35 rod that can be used as a replacement but it's not a drop in fit (I think the bush has to be reamed very slightly larger).