axial line p of the cooling hole 82 as shown at dot dash lines in fig2 a leading portion of the collar 12 is configured in conformity with the semi spherical portion 82 b of the cooling hole 82 namely a leading end of the collar 12 forms a semi spherical end portion 12 c as shown in fig4 the collar 12 has a length l 1 which is somewhat smaller than a total length of the downward hole 82 a and the semi spherical portion 82 b as shown in fig3 a diametrical dimension of the collar 12 is arranged to be identical to the inner diameter d 1 of the downward hole 82 a when the cooling type bushing 14 is secured to the cooling hole 82 as shown in fig6 as shown at the phantom lines in fig2 the collar 12 has an outer surface aligned along the axial line p the collar 12 is arranged to bring the outer surface into tight fitting engagement with an inner surface of the cooling hole 82 instead of the word of tight fitting engagement the word of engagement is used as the same meaning hereinafter unless particularly specified in the meanwhile as shown in fig4 an inner surface of the collar 12 forms a tapered surface 12 d which slants against the axial line p the collar 12 has a thickness which progressively increases.