This is probably "old hat" to you pro's I found it a very useful solution.
On the GXV140 the valve heads were so small I couldn't find a suction cap that would work. I stumbled across the idea to stick the two valves back to back with a foam sticky pad. see picture. You use the valve stem sticking up to rotate the one being lapped. The foam gives it just enough flex to keep the valve running on the seat. The glue on the pad held up until I'd finished both valves. The pad could then just be peeled off. The only thing to watch is to use the right valve to lap the right seat!
