There was another noise on the Bancroft engine that had been nattering engineers for years. It was a serious thump somewhere in the low pressure side and was attributed to water hammer in the air pump caused by bad design. There was no doubt that it was true the air pump was bad, Roberts had incorporated air chambers in the side to cushion the stroke but they had never been any good. So, I accepted what everyone told me and put up with the thump until one day I was walking down the low pressure side while the engine was stopped with a lump hammer in my hand and noticed that the cotter between the piston rod and the crosshead was bleeding bright red oxide. This is a sure sign that something is loose so I clouted the cotter. It went in almost 1/2"! So I clouted it again and when I restarted the engine the Bancroft thump had vanished but gradually reappeared during the afternoon as the cotter worked loose again.
When the engine stopped that evening I drove the cotter out and had a look at it.
Here are both the HP and LP cotters, both worn but if you look carefully at the biggest one (the LP) nearest the camera you will see that there is a depression worn in the face of it. I got Newton up, he took particulars of both cotters and we replaced them. At weekend Newton came up with the replacement cotters and we installed them together with a thick spacer washer for the spigot on the LP piston rod end because we reckoned that the old fitters had made a mistake and the spigot was bottoming in the crosshead before the cotter had properly got hold. This meant that we had altered the stroke of the LP piston by 1/4" so we ran the engine to check we weren't hitting the back cover. It was perfect, ran like a rice pudding and we had cured the Bancroft thump! It just goes to show, never assume that all the old fitters were perfect, the fault had been there for 60 years and had got worse over time as the hammering deformed the cotter. It never came loose again as long as I was running it....