controlled air outflow 16 in a smaller engine compared to a larger engine hence controlling and reducing leakage 19 is desirable especially in smaller gas turbine engines the development of the metal injection molding mim process the direct metal laser sintering dmls process and other additive manufacturing processes allow manufacture of sealing surfaces with detailed shapes and configurations which can be used to reduce cooling air leakage 19 from the intermediate chamber 14 fig3 shows a detail of the first embodiment with a seal for sealing the combustor heat shield 15 against the interior surface of the combustor shell 13 to contain compressed cooling air within the intermediate chamber 14 and impede leakage the seal includes a first sealing surface on the interior surface of the combustor shell having a single rectangular ridge projection 20 and a second sealing surface on the rail 17 on the edge of the heat shield 15 made of an upstream rectangular ridge projection 21 and a downstream rectangular ridge projection 22 the ridges 20 21 and 22 of fig3 are a castellated series of mating rectangular ridges defining a non linear leakage path between them fig4 shows a second embodiment of the invention where the ridges 23 24 are a serrated series of mating triangular ridges fig5 shows a third embodiment of the invention where the ridges are reversed in orientation compared to fig4 in all cases the non linear leakage path impedes the cooling air leakage by increasing resistance to air flow for example modelling of the castellated path shown in fig3 reveals that leakage can be reduced by up to 50 or more compared to the prior art path shown in fig2 the first projections 20 23 on the combustor shell are manufactured using a direct metal laser.