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The Great Western Railway Castle Class

A review by Dave Stick

When Andrew Pullen of Aster UK told me that their new loco was to be a GWR / BR Castle my delight knew no bounds! It’s the first Swindon engine for a long time so I couldn’t wait to see how Aster had interpreted what to me was a stunning looking engine. There are many new features included resulting in a loco that has proved both a delight to build and operate. Andrew kindly loaned Ted Chatfield of South West Steam and me the prototype to take to the Exeter Garden Railway Show and it proved a big crowd puller! 

Both GW and BR versions of 5015 Kingswear Castle are fitted with the original elegant tall single chimney. But the shorter version, introduced in 1936, would be a simple substitution by shortening the base casting. The chosen prototype is of the 4th batch Lot No. 280 built in 1932 and numbered 5013 to 5022. The 5th and 6th batches appear outwardly identical and are numbered from 5023 to 5042, but from 5033 a speedo was fitted.  

The design of the new Aster Castle is similar to the King of 1990 but is a completely new detail design incorporating many lessons learned during the intervening years. Those of us who own a King recognise its visual shortcomings and that regular and careful maintenance is required. Hard run, poorly maintained Kings have shown reliability issues and for the new Castle these areas have received particular attention. The wheels, crank-axle, eccentrics and valve gear have all been redesigned but retain the close to scale feel for which Aster are famous.  

The kit is well packaged ensuring that an undamaged set of parts is received. Included is an excellent instruction manual containing the specification, an inventory of parts and detailed instructions on building and operating the loco. There is a splendid set of assembly diagrams showing each stage of the build on which all parts are clearly marked. This attention to detail places Aster products head and shoulders ahead of the competition. Note that parts are not stamped with the identification numbers shown on the diagrams. Hence, it’s important to compare the drawing with each part and where the parts are similar, the diagrams show the key dimensions. An example of this is seen in the top right hand corner of the Section 1 drawing.  

At each stage of construction builders should only unpack those parts necessary to complete the section shown in the diagram and examine each part using the inventory. There are a few occasions when you may have to temporarily ‘borrow’ parts from a later section e.g. to conduct air testing. Remember to restore these borrowed parts to the correct box on completion of the task.

Here are the four cylinders and slide bars with crossheads fitted to the piston rods.

Section 1 starts with the assembly of the cylinders and, as the loco has slide valves, lapping the port faces and valves must first be completed. The diagrams show how this is best accomplished the essential fine surface finish. Next, before assembling the pistons into the cylinders pour a little steam oil over the assembled piston head and rings and trickle a little down the walls of each cylinder bore and into the steam chests. This will ensure good lubrication of the assembly during the short air test required later. It also aids the swelling process needed by the Rulon piston rings, normally complete after 2 to 3 hours running in steam. These rings have been used by Aster since the mid 1980s and have proved hard-wearing and reliable in service. I have never had a failure in 23 years and given a little patience and understanding, you will find that any initial evidence of blow-by quickly disappears. 

When assembling the gaskets to the cylinders for both the covers and the steam-chest be extremely frugal with the application of silicone sealant. Take a small spatula and spread a THIN, EVEN coating of sealant to both sides of the gasket. Pick the gasket up using tweezers and carefully place it on the surface to be sealed. Failure to take these precautions will inevitably result in sealant getting into the steam ports and cavities! 

When assembling the crosshead to the piston rod use a SMALL amount of nut lock to ensure that the piston and rod don’t work loose in service. Be very sparing with locking fluids and only use them where parts are subject to vibration or rotation. For example, use it on gudgeon pins and other bearing pins in the valve gear, being VERY careful not to get it on bearing surfaces.

DON’T use super glue! I recently had to correct a very stiff chassis brought to me for rectification and found that many chassis bolts had been glued in using super glue. I had the devil’s own job getting it apart and cleaned up! 

Finally, leave the inside and outside cylinder slide bars and their brackets fairly loose at this stage. When you come to assemble them to the chassis later these will need to be adjusted to avoid any binding. Store assemblies upright where they will stay clean and dust free and the oil will not run out.

The eccentrics and links, leading bogie (with side control springs),

and the buffer beam after adding the number transfers.

In Section 2 the bogie assembly is completed. This is a beautifully executed model of the full size loco providing axle-box, bogie stretcher and side control springing. When assembling the axle-boxes make sure that they slide well in the horns without rocking. Sometimes the paint in both horns and suspension holes makes them a little tight and this must be gently relieved using a fine file. Smooth operation in all planes is essential for good performance. 

The buffer beam needs no comment but next come the expansion links which require precision in your work. Many of the parts are laser cut and all bearing surfaces need smoothing. Polish the surfaces of both the links and the trunnions so that a fine finish is achieved without removing too much metal. The trunnions must slide in the links without rattle and remember to polish the slotted ends of the radius rods. 

When assembling the eccentric straps note the joggle in the Pts 2-4 & 2-13 which are handed. The joggle aligns the bearing in each end with its expansion link. After assembling the expansion links to the 2-1 and 2-3 ‘bridging’ plates ensure that the links all rotate freely in their bearing hangers and that the trunnions are free to move over their full range. Any off set or out of alignment built in will result in stiff operation and will be detrimental to valve performance. Last in this section is the fitting of the balance weights to the main driving wheels. No problems here but remember that the bolt heads are on the inside of each wheel. 

Section 3 starts assembly of the frame and frame spacers which should be completed on a flat sheet of glass to ensure everything is true. The section then installs the inside cylinders between the frames together with the valve gear sub-assembly to which the front drivers are then attached. The right-hand crank pin leads the left when looking from back to front in the frames. PTFE strips are wrapped around the eccentric prior to fitting the straps and I smear silicone grease over them during fitting. These bearing surfaces have proved both hard-wearing and easy to fit in all recent products and in the future renewal will be a breeze. 

The inside connecting rods are next assembled on their crank pins after first fitting the PTFE bushes. The technique for fitting these bushes is not difficult if the correct procedure is applied though it seems quite brutal at first sight! 

The bush is made with a split in it into which a screwdriver blade is inserted to ease the bush open. It is then stretched further open until it can be pushed over the crank pin. The bush has a memory and will return to its original shape aided by its enclosure within the connecting rod big end straps. 

The wheels are retained in the horns by horn keeps also acting as dummy springs under the axle boxes. The suspension is provided by coil springs located on spigots in the top of the horn guides and in a small recess in the top of the axle box. The tuning of the chassis to get correct weight distribution and a level footplate is achieved by adjusting the length and hence preload of these springs on each axle. 

The weight shaft and levers connecting to the radius rod are next assembled. Operation of the assembly must enable full range of movement of the trunnions equally either side of the centre pivot of the expansion link. You will now be able to judge how well you have prepared the bearing surfaces in the valve gear! 

Section 4 tasks include fitting the remaining driving wheels and keeps, at which point the final adjustment of the springs should be made. Then fit the outside cylinders and slide bar supports, frame spacer-mounted axle pump and the associated bypass valve, cylinder lubrication tank and a frame spacer with additional weights to improve weight distribution over the axles. Aster returns to a pressurised cylinder lubrication system with this loco, as was used on the Duchess. I have been much impressed by this system and modified my King to this design some years ago. 

The axle pump eccentric is also fitted with a PTFE strip bearing surface and the performance of the pump can be further enhanced by the fitting of 1/8” dia. nitrile balls replacing the 3 mm stainless steel balls provided. The assembly of the pump is straightforward but take care to keep the silicone sealant to the merest smear over the banjos faces to avoid ingress to the ports. 

Next, having fitted the outside cylinders, the slide bar supports should be carefully adjusted so that the crossheads will slide over their full range of movement without resistance. The bolts left loose earlier may then be finally tightened. 

Lastly, when assembling the bypass and oil control needle valves, smear grease over the threads to avoid damaging the O rings. Chamfer the mouth of the threaded holes using a Slocomb centre drill twisted between the fingers. This will guide the O rings and help to avoid ring damage.  

Section 5 completes the motion and outside rods and great care should be taken in fitting the reverser gear and weight shaft assembly. Set up precisely in mid gear as the next section depends on your accuracy here.  This section also includes the fitting of the bogie stretcher and bearing pads and finishes with attaching the bogie itself. Personally, I leave the bogie off until much later but a trial fit now is recommended to check clearances.

The careful setting up of the valve gear in Section 6 is absolutely crucial to good performance. On the Castle the outside cylinders derive their valve gear motion from the inside cylinder’s valve gear via the rocking levers fitted in the previous section. Hence any error in setting the inside cylinder valve gear will be transferred to the outside motion. Read Aster’s instructions very carefully making sure that before you start on setting the outside gear you are satisfied that the inside is right. Make sure that as you set each slide valve you have the associated crank at exactly the right angle and have allowed for any backlash by coming up to the right position from the correct direction of rotation. Make sure the axle doesn’t move whilst you set the valve and make the leading edge of the valve line for line with the port leading edge. Having achieved perfect setting in forward gear, when set for reverse you may have to make a small compromise. Rarely will the gear be exactly right in both directions. Since this is an express loco it is unlikely that you will run in reverse except when running light engine; hence I always favour forward gear setting but ensuring reverse works satisfactorily.

The next task is the air test in section 7. Cylinder covers and associated steam pipes are fitted taking great care not to get sealant into the steam ways. The smokebox saddle is fitted as are the superheater and the test adaptors provided. All bearings are oiled making sure that you have steam oil in the cylinders and valve chests. This, together with limiting your run to a few minutes, will avoid any danger of scoring on valve port faces. The chassis is then run on air using a compressor. After the test return the fittings borrowed to the right boxes and remove the screw reverser from the footplate until the cab is fitted. 

Section 8 finishes the chassis detailing and includes fitting of the footplate and there are no surprises here and nothing difficult. Section 9 builds the C type boiler and is standard Aster practice. It uses a dummy backhead in front of a sight glass fitted with a blow down. There is a backhead clack to which I fit a 1/8” dia. nitrile ball. A fountain is fitted to the boiler top and feeds a pressure gauge, the blower and the oil tank. The regulator is of the usual screw-down type. Under the boiler is the firebox which is lined with ceramic sheet. 

The superheater and blower are fitted to the front tube plate next and not in Section 11 to enable a boiler pressure test to be conducted prior to fitting the boiler to the chassis. These are attached trapping O rings that seal the high pressure steam from the smokebox. Failure of either of these two O rings will reveal itself as either a steam/water leak from the blower or by-passing of the regulator! I always bed these O rings in silicone sealant inside the O ring housing on the forward tube plate as a back-up, but being very careful not to allow any sealant into the steam passages. When the boiler is completed it should be pressure tested to check for leaks prior to fitting to the chassis.  

Section 10 tasks mount all of the detail parts to the boiler outer casing and installing the boiler. The oil supply pipe to the cylinders is routed under the casing along the right side of the boiler. Care is needed here to avoid damaging the pipe as the boiler is slid into the outer casing. It’s also important to make a good job of sealing the front end of the boiler to casing joint using ceramic sheet and yarn. Leaks from the smokebox would badly influence steaming.

The front end with the steam pipes.  This shows the split smokebox arrangement.

Section 11 fits the boiler to the chassis and here some builders have had difficulty on earlier locos when fitting the main steam supply pipes between the cylinders and the superheater header. Care is needed to avoid cross-threading the banjo-bolts and also getting sealant into the steam passages. Next the oil delivery pipe protruding from the back of the boiler outer casing [see previous para.] is attached to the oil tank, as is the steam supply to the oil control valve. This section is then completed with the fitting of nice-looking dummy brake gear. 

Section 12 tasks complete the fitting of the smokebox together with final few external attachments to both boiler and smokebox. The latter is a split design as used before and enables far easier access to the contents within! Both the top and bottom parts and the smokebox door are lined with ceramic sheet which is glued in place using silicone sealant. Care should be taken to mount this lining accurately to avoid air leaks which would otherwise destroy the partial vacuum created when running.  

Different smokebox doors are provided for the GW and BR models the latter having a number plate mounting pad. The smokebox door front mounting ring has a dummy steam lance cock mounted on it and this doubles as a lock for the smokebox door. Before leaving this section it is imperative to check that there is correct alignment of both the blast and blower nozzles with the chimney. Because of the very high tolerances maintained by Aster in their manufacturing process the chimney and blast nozzle are very unlikely to be out of alignment but it’s important to check. The blower however, may require a tweak to bring it into position as indicated in the diagram provided. A small length of suitable straight brass wire helps in sighting the alignment here.

Section 13 completes the engine by mounting the cab and associated fittings. Make sure you have full range of movement on the screw reverser and that the reach rod doesn’t foul anything. Make up the burner as recommended but remember that after a few hours running you will probably have to remove one or two wicks as they swell in use. 

In Section 14, 15 and 16 the beautifully detailed tender is assembled. Axleboxes that are free-sliding are essential to good running and a light polish with a fine Swiss file on the outer faces of each axlebox is usually sufficient. Tune the chassis spring length to ensure that the chassis running plate is level and that the axleboxes do not bottom in their horn cheeks. Seal the water tank seams using silicone sealant on a cotton bud before mounting the pump and assemble the steps to the back wall of the tender before fitting the tank inside. The top deck of the tender is removable on the Castle and this enables the meths tank and water pump to be accessed easily if required. 

There you have it, one of the most beautiful G1 locomotives ever, with the details and paintwork accurately portraying a Great Western Castle at its finest. It has none of the mistakes made with the King at a time when Aster was still learning their trade. Judging by the running experience I have had so far it will also be one of the best performers too!

Permission to post this article by Dave Stick on the Southern Steam Trains website has been graciously granted by the author, Dave Stick.  Also, we express our thanks to Nick Rudoe and Peter Trinder, Editors, Gauge 1 Model Railway Association’s Newsletter and Journal.  Originally published in the G1MRA Newsletter and Journal (Issue 220) Winter 2008/9. 



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