Missing dowel/bushing between cylinder and head

[mb848]

So I removed the head from the horizontal cylinder of my 1978 900SD Darmah to replace the valve seals... removed the rockers, got at the seals, cleaned up the head since it had a lot of burnt oil residue inside it (using plastic and brass brushes on a Dremel... even with the brass I was wary of hitting the valve seats since I think it's also a soft metal?). When I was looking at what o-rings to use putting it back together I realized there are supposed to be two dowels/bushings (some websites seem to list them as dowels, but the parts diagram calls it "bush"), one 8mm in diameter and one 12mm in diameter. The 12mm one is there, but the 8mm isn't in the cylinder or the head and I'm positive I didn't drop it anywhere since I have a picture I took right after removing the head and there's a visible excess of some kind of sealer squished past the old o-ring where the dowel would have been if it had been there.

Is anyone familiar with the purpose of these dowels? They're cheap so I went ahead and ordered some from Ducati Omaha (looks like the same ones were used on even much later 2V Ducatis), but since it seems it didn't have any issues running without it I'm not actually sure what the purpose is since it's just a hollow tube within a hollow channel to allow oil to pass through... the only thing I can think of is that it's to ensure alignment of the oil channel between the head and cylinder and that the o-ring is on the correct surface of each, but it seems like the studs would enforce that.

Pictures below... it looks like whoever had the head off the last time used some kind of blue sealer stuff to augment the o-rings. There was also this plastic ring on the head where I think the o-ring is supposed to go around the 12mm dowel... it's not shown in the head or cylinder parts diagrams, but it also seems to be made to fit perfectly in the head like it's supposed to be there, so I'm not sure if I should remove it and replace it with an o-ring or just leave it *and* use an o-ring (I'm guessing remove it since it fills the recessed surface for the o-ring).

Would appreciate any thoughts!

Missing dowel (blue gunk inside o-ring)


Plastic gasket that was around 12mm dowel