We have had many. Failures were linked to o-rings unable to support the heat. Markisches Werk analyzed the o-rings and supplied us with a high temperature replacement.
Ben Pacas from Markisches can help you out. <email@example.com> Motor services can also supply these parts. Mladen Malovic <Mladen.Malovic@mshs.com>
During 16k overhauls we pull and replace all exhaust seats and o-rings. Since then we have had a significant reduction in seat leaks.