October 2026

The October C2 working party took place over 7 days in total, which meant that we got a lot done.

Andrew started off by completing the turning of the big end bearing covers. He had previously completed the internal shape and overall thickness of each plate, to allow the plates to be used to measure the alignment of the connecting rod big ends. Now he completed the external surfaces.

Andrew also spent some time measuring the piston ring grooves. He and Dave had previously measured the valve ring grooves, so it won't be long before we are ready to order new piston and valve rings. It had been our intention to start cleaning up the ring grooves on a lathe in readiness for the rings, but a change of plan for the working party gradually evolved.

Dave arrived mid-week, and suggested that he temporarily set up the expansion links in readiness for Erle's arrival towards the weekend, so that Erle could measure alignment of the return cranks and rods. The first step towards this was to fit the expansion link pivot bushes to the motion brackets. Andrew assisted as time permitted, as he was also booked to drive the Works Tour shuttle trains on Wednesday.

The expansion link pivot bushes (made of Vesconite) were a rather heavy fit in the holes prepared for them, so Dave took them to one of the DSG lathes in the machine shop and skimmed their outer surfaces. The plastic didn't like having very fine cuts being taken, so Dave ended up using emery cloth to take a thou or two off the outside diameter. Making use of an improvised press tool, Dave then pressed the bushes into their housings in the motion brackets.

Since plastic is a lot softer than metal, pressing the bushes into the holes caused some contraction of their internal diameters. This meant that the expansion link trunnions, which had been a perfect fit on the bench, were now rather too tight. Andrew therefore used some fine emery cloth wrapped around a suitable length of plastic piping, to open out the internal diameters a little. After some hours of work, Andrew was able to mount the expansion link cover plates with their trunnions in the bushes.

This revealed a couple of minor issues. Firstly, the shoulders on the expansion link cover plates are not always perfectly perpendicular to the axis of the trunnions. And secondly, the space between the expansion link cover plates was rather less than the thickness of the expansion links themselves.

To solve the first problem, Andrew removed the expansion link cover plates and bolted them together in pairs (without the expansion link between them). He then removed the bed extension from one of the 17 inch throw DSG lathes so that the cover plates could be spun on the axis of the trunnions. With care, Andrew skimmed the shoulders on the cover plates to ensure that they are perpendicular to the trunnions.

Re-fitting the expansion link cover plates to the motion brackets showed that skimming the cover plate shoulders had increased the gap between the plates. On the driver's side, the gap is now about correct. On the fireman's side a few millimetres will still be required. This can be achieved by removing a millimetre or two from the flanges of the plastic bushes. However, before we do that we want to check the alignment of the return rod and valve spindle, as it may be better to remove different amounts of material from each bush to move the expansion link laterally.

For the moment, the expansion link cover plates are flexible enough to spring them apart sufficiently to fit the return rods. These connect to the bottom of the expansion link cover plates using stainless steel pins which Dave had made a while back.

By this time Erle had arrived and started checking the alignment of the components against what was shown on his detailed motion assembly drawing.

Temporarily fitting the return cranks onto the driving crank pins showed that the rear faces of the cranks was not perpendicular to the axis of the bore in the crank. Dave had machined these faces flat after building up with weld, but had only made them parallel with other faces on the crank. Erle found a piece of 60mm diameter steel bar which acted as a perfect dummy driving crank pin upon which to mount the return cranks. The dummy crank pin could then be set vertical in a milling machine to allow the rear faces of the return cranks to be skimmed perpendicular to that axis.

Re-fitting the return cranks, and comparing to the position of the bearing end of the return rods, showed that while the driver's side seemed to line up quite well, the return crank pin was offset outwards by several millimetres on the fireman's side. The offset was compounded by the return crank pin not being correctly seated in the return crank.

The drawings of the original design of return crank (with plain motion bearings) showed the crank and pin to be a single forging. However, the Chinese modified the design when they fitted roller bearings to the return rods, so now the return crank pin is a separate component welded into the return crank. Erle measured the diameters of the return crank pins and found that they were a tiny fraction of a millimetre over nominal size, so ideal for pushing roller bearing inner races onto. This means that we don't need to re-make the return crank pins.

To correct the improperly seated crank pin, we wanted to remove it from its return crank. If a full penetration weld had been carried out on the rear of the pin, it would have been almost impossible to remove it without destroying it. Fortunately, Erle found that just milling away the weld on the rear of the pin allowed it to be knocked out of the return crank with no damage to the pin or crank. After cleaning up the components, it will now be possible to correctly locate the pin in the return crank. Correctly seating the pin will also improve the alignment between the pin and the return rod on the fireman's side.

To locate the return cranks correctly on the driving cranks, small spacer rings are required which fit between the return cranks and the inner races of the big end bearings. Erle found some suitable steel tube and turned a couple of spacer rings which he then parted off to the correct length. These spacer rings are now fitted to the driving crank pins (held on with grease) ready for the return cranks to be pushed up against them.

While making spacer rings, he also made two pairs of rings which fit between the roller bearings (inner and outer races) of the return rod bearings. They were made in the same way as other spacers, turning the inner and outer diameters and then parting off to the correct length. The outer spacers also needed a short gap cut in them, so that grease can be injected into the bearings during maintenance.

With the spacers made, the best way of keeping all the parts together seemed to be to assemble the return rod bearings. Erle did this, using the hydraulic press to push in one of the outer roller bearing races, then laying in a spacer ring before pressing in the second roller bearing outer race. The inner races will fit onto the return crank pins, but we're not quite ready for them yet, so the inner races and corresponding spacers went back in the cupboard for now.

With the big end bearing cover plates turned to their final profiles, the only remaining operations required to complete them were machining the recesses and holes for the bolts which hold the plates onto the connecting rod big ends. Dave had carried out similar machining operations on the coupling rod bearing cover plates some years ago, so dug out the jigs he'd made back then to try and remind himself of the process he'd used.

Andrew helped Dave locate a billet of steel (actually the end of a standard gauge axle) from the scrap pile, which was of suitable diameter and length. Dave then designed and, using the Colchester lathe, turned up a jig which could centre a pair of big end bearing cover plates on the rotary table.

The jig locates in the hole in the centre of the rotary table. The rotary table is then centred under the spindle of a milling machine by using a pointed bar of stainless steel held in the chuck, and locating it in a centre hole in the jig before clamping the rotary table to the bed of the milling machine.

When Dave machined the coupling rod cover plates he had used the Bridgeport milling machine, which is sadly now out of action. So he had to use the smaller Chinese milling machine next to it. This resulted in an unexpected problem when he found that the machine didn't have sufficient vertical travel to get the regular pointed bar into the chuck with the rotary table and jig in situ. The solution to this was to make another, shorter, pointed bar; not difficult but an extra task that hadn't been anticipated.

With the rotary table finally centred and bolted to the milling machine, a pair of cover plates, outer plate uppermost, were clamped to the rotary table with machine clamps. Offsetting the bed of the milling machine by 82mm gave the correct pitch circle diameter for the fixing holes, and the digital readout was re-zeroed at this position. The bed was further offset before, with a 25mm milling cutter in the chuck, the bed was wound back to the zero position to cut the recesses for the bolt heads. Each recess was made using two 2mm deep cuts. The rotary table was then rotated by 90 degrees before the process was repeated for the next recess, and so on.

With the four recesses complete, a centre drill was used to spot the four bolt hole centres. A 6.8mm diameter drill (tapping size for an M8 thread) was then run through both plates at each bolt hole position. After removing the plates from the jig, Dave used a pillar drill to open out the holes to 9mm diameter (clearance size) in the outer cover plate. Meanwhile, Erle tapped the holes in the inner cover plate to M8 thread form.

The whole process was then repeated for the second pair of big end bearing cover plates.

While Dave was milling the recesses in the outer cover plates, Erle found some suitable M8 bolts we had in stock, and trimmed them to the correct length. The bolt length is critical, as there is very little space for them to project through the cover plates. With cover plates and bolts complete, Erle then assembled the cover plates onto the connecting rod big end bearings, with spring washers under the bolt heads to prevent them coming loose.

We hadn't really planned on making such significant progress on the big end bearings, return cranks and rods at this working party. But things seemed to fall into place and so we kept working on them. The big end bearings are now complete, and other components such as the return cranks and expansion link mountings are progressing rapidly. This is all very pleasing.

The working party ended with Erle greasing the big end bearings and return rod bearings and wrapping them in plastic bags to keep the clean, while Dave had a tidy up of the workbench in the C2 Shed.

To further check the alignment of the expansion links, we want to get the valves installed in the valve chests. We need valve rings to be able to do this, and we'll order these at the same time as the piston rings. So Andrew's measurement of the piston ring grooves at the start of the working party was an important part of the plan after all!