I am in the process of refurbishing a K100 8V engine and have had to drill-out the exhaust studs. This evening I have tried to tighten new exhaust studs into the head and one of the bosses on the aluminium casting (into which the studs screw) has cracked open and broken off.

I guess that an aluminium welder can create a new boss and thread it for me? But I will first have to remove the head. Or maybe I just remove the head and get a replacement from a scrapped bike.

Anyway, do I have to remove camshafts and valves in order to remove the head? Or can I leave these in place, having removed timing chain, and just take the head off the block?

Grateful for any advice.

Thanks

Chris

Removing the cam shafts and valves is not a "must".
My opinion is that you should leave them on the head, and protect them.
When the welder will do the repair, because of the heat the aluminum will be stressed and the head may warp
Keeping the cam shafts on may prevent the warping. It will also prevent any splash or spatter on the valve seat or where the cam shaft rides on the head.
This is my opinion but I may be wrong.

Why did crack open? Was there oil or water inside when you screwed in the stud? This is a common mistake when installing head bolts. It will impart enough force to crack an iron block.