September 2026

Due to Andrew's shift patterns, the September working party was carried out mid-week, from Tuesday through until Thursday. Only Andrew and Dave were in attendance, but Erle also assisted from home.

Having inspected the valve spindle trunnion blocks at a previous working party, we found that the existing trunnions were undersized, which means that the ball bearings which should be a press fit onto them would be loose. Also, the existing blocks have already been weld-repaired previously, not to a very high standard. We have found a 10mm broach at Boston Lodge for making the keyway, so this all makes manufacture of new components rather than repair of the old ones seem desirable and practical.

Dave and Andrew identified a billet of 100mm diameter high grade steel in the work's stock, and after having obtained permission to take a suitable length of it, took it to the band saw to cut off the required length.

Using one of the DSG lathes, Dave started by skimming the outer surface to remove surface imperfections. The end was then faced off to form a datum from which to work. Next, a series of drills were used to open up a hole through the block. Since the trunnion block has to fit snugly over the valve stem, the inside diameter of the hole needs to be to a fairly tight tolerance. Dave therefore used a boring bar to finish the hole to about 0.02mm over 35mm diameter.

Andrew calculated the minimum diameter required to encompass the trunnions (87mm) and Dave turned the body of the material down to this diameter. He then parted the component off. Now mounting the block in the lathe the other way around, Dave quickly skimmed the rear face to the final length (75mm).

The only other turning operation on this axis was to form 5mm long shoulders on the ends of the block. These were each turned down to diameter (52mm) before a radius tool was used to form 1.5mm radii on the corners of the shoulders.

The entire process described above was then carried out again to start the manufacture of the second trunnion block.

Next will be a milling operation to form the sides of the trunnion blocks, followed by a turning operation to form the trunnions. This is a job for the next working party.

While the lathe was set up, Dave found a piece of 50mm diameter scrap steel, and turned up a broaching guide. This will be used when broaching the keyways in the trunnion blocks.

Having started turning the big end bearing inner cover plates at the previous working party (we used then to help align the connecting rods), Andrew used the Elliot lathe to finish turning them. These cover plates still require holes drilling for the fixing bolts, but this will be better done when we have the rotary table set up for milling the big end bearing outer cover plates.

Attention then turned to the big end bearing outer cover plates. We were still undecided on the thickness to make these plates. Their thickness is one of the factors which influence the lateral position of the return rods (alongside the return crank thickness and the return crank pin seating). Although not in Wales with us, Erle was able to assist us by updating his motion assembly drawing with the final big end bearing inner cover plate dimensions. He could then show us the theoretical position of the return cranks. The big end bearing outer cover plates are already quite thin, so we decided to make them to their nominal dimensions, and to use the return crank thickness and crank pin seating to provide any adjustment required in the return rod lateral position.

Andrew therefore started turning the big end bearing outer cover plates, and has now finished one of them. The second should be turned at the next working party, when we can drill and mill them on the rotary table to finish them completely.

Another turning job which Andrew undertook was the re-machining of the insides of the crosshead bridge pieces. These components straddle the gudgeon pin retaining bar, and carry the pin for the union link bearing which is on the centreline of the gudgeon pin. As such, they need to be precisely located on the crosshead. The crosshead castings have two curved raised shoulders machined on their outer faces, centred on the gudgeon pin.

The bridge pieces should be a close fit on the raised shoulders on the crosshead. However, ours were very loose. To remedy this problem, Andrew had welded up the inside faces of the bridge pieces at the previous working party. Now, using the union link bearing pin to hold each bridge piece in the lathe, he could bore out the inside face of the bridge piece until it is a close fit over the shoulders on the corresponding crosshead. With both bridge pieces turned and now fitting precisely, the repair of them is almost complete.

Although Andrew and Dave spent most of the three days working on lathes, the work was sufficiently varies that we didn't get bored. And several of the more important tasks are now in progress, which is rather satisfying.