The intake / exhaust manifold should be secured to the head with brass nuts and these very thick washers.
These washers are intended to not only spread the clamping action, but the thickness allows them to absorb the heat without deforming!
1" wide for 5/16"studs. To be used on the 4 inside studs.
Recommend: 4x 12A1211 per Mini.
For the short brass nut, see GFK3436
For the long brass nut, see 51K1177