Thank you for allowing me to join the group.
I've been riding for well over 25years started with Honda big red the last quad I owed was a Suzuki LT500R had it when I was 15 sold it when I was 17 I just bought my 1988 Honda trx250R it was used for racing all its life with tons of mods from the exhaust fmf to the cool head which I was given the head that is in it now (not sure which size) plus a dome marked 18 for it.
It has a Keihin flat slide carb and is bored 30 over as well.
I'd like to know what do I need gaskets o-rings to change from the dome in it to the 18
Sometimes the shell needs to be resurfaced flat, that is internally on the dome sitting shelf. These shells get Hobbed over extended use, and when a new square & flat dome is installed; they tend to leak or fail.
Otherwise; you just need to order a new 18CC dome, it will come with two new o-rings. Plus a new OEM Metal head gasket.
My suggestion is always perform a UCCR when changing compression or fuel or both...safe is Best IMO.