All but one of the four valves had been smashed off the engine at some point in the past. This not only broke off the 5/8"BSP threads but also significantly distorted the case and therefore the ports within. It would have been easy at this stage to scrap them and replace with modern valves but that would have completely changed the appearance of the engine.
What remained of the spigot was turned up to form a plain boss which was axially in line with the remaining undamaged thread and threaded spigots were made to fit over the newly turned boss. From the picture the extent of the distortion to the case can be seen by the off-centre hole in the turned boss.The bore was machined to return the internal ports to their original roundness. This meant that the internal bore was now larger and so new spindles had to be made.
The last stage of the thread repair was to silver solder the new threaded spigot onto the turned boss of the valve case and clean up with a needle file.
The last difficulty was the odd valve packing. Under analysis, it turned out to be hemp soaked in grease and impregnated with graphite then packed very tightly into the case and in between the ports. Once the new spindles were made it was just a matter of reassembling the valve and fitting it to the engine.