There's nothing standardised about the many C8's on the market except they roughly look much the same and are similar size. Yes, the pill screws in, but among various makers there are all kinds of sizes of pills being used. So one from say a Convoy C8 might not fit an Ultrafire, etc. Some use aluminium, some brass. And threads and pill depth may differ slightly between makers. It's nowhere near as versatile as a P60 setup if you want to change innards, nor is there any guarantee that pills from various makers will fit all hosts, whereas with P60 hosts most dropins fit mosts hosts with a few exceptions.