All ’77 and later, and some late-’76, models have two small o-rings at the base of the upper studs. You can double check by observing the recesses in the cylinder base around the holes to receive the top studs. If there are no recesses, NO o-rings. If there are recesses, use NEW o-rings. Rather than placing them on the studs, dab them with grease and insert them in the recesses in the cylinder base to avoid pinching them when reinstalling the jug.

Partway through ’78, BMW started using a large rubber o-ring at the cylinder base. Used to be 2 sizes (thicknesses), 2.0 and 2.2 mm, but believe only the 2.0 mm is available now. Make sure to use a new one for reinstallation.