Standard brass manifold nut. You will probably want a mix of these and the longer 51K1177 to secure your intake / exhaust manifold.
This nut is 5/16" long, whereas the 51K1177 is 1" long.
Recommend: 6 x GFK3436 per Mini.
For the manifold washer, see 12A1211
For the long brass nut, see 51K1177