Cylinders

Castings can be purchased for the cylinders which usually include the cylinder covers too. The cylinder covers as very simple and it's probably just as easy to machine these from a short length of Bronze bar, say SAE660 Leaded Bronze. If the castings for the cylinders can be obtained separately, that might produce a saving. The whole set can cost in excess of £500 for Bronze ones.

There are two options for materials, Cast Iron or a Bronze, and the choice of best material is subject to debate. Many insist the Cast Iron is the best and in terms of wear that may well be true. The problem with Cast Iron is that it can rust when the locomotive is laid up for any length of time. If care is taken to dry out the cylinders and lubricate them then this need not be a problem but the use of a Bronze material removes that possibility altogether. If you don't mind spending the time and you have a stout milling machine you might consider making the cylinders from bar stock which currently costs less than £200 for SAE660 and a similar amount for Meehanite (Cast Iron). One reason for machining from bar stock is that the material is likely to be more homogeneous than a casting which are sometimes of poor quality.

The picture shows the first stage in machining the cylinders from bar stock using a CNC milling machine. Similar results could be achieved using a manual milling machine and a rotary table but would take much longer.

Machining the cylinders
Whether the cylinders are made from bar or castings, the challenges of holding them for the machining operations is similar. The piston and valve bores need to be set in the correct places relative to the flat rear face and perpendicular to the front. A faceplate or 4-jaw chuck can be used to hold the cylinder block in the lathe for facing and boring the holes. The order you machine the various faces will depend on how the positions of the holes have been set out. Toolmakers buttons can be used to locate the centres but a reference surface needs to be machined first and the obvious candidate for that is the flat back that bolts to the frame.

Use of Fixtures
There are many operations that require the cylinders to be held in a number of different orientations and this poses a problem for holding them. One solution is to make a mandrel that locates inside the bore and is secured either by a through bolt or by using the tapped cylinder cover holes to hold it. The mandrel pictured uses alternate holes used to secure the cylinder cover to hold it on. The flange on the mandrel includes a spigot that locates in the hole for the valve sleeve. That's been added because the mandrel was used to support the bar while machining the cylinders from solid and the cutting forces were expected to be large. There's also a through bolt that was used at that stage before the cylinder cover screws were added.

Although there's a bit of effort required to make the mandrel, the time is easily recouped by the ease with which later setups can be achieved.

Here's the notoriously difficult setup required to machine the passages from the ends of the cylinders to the valve bore. A tilting rotary table make this an easy setup while holding the cylinder on the mandrel.

Design
Cast cylinders have recesses for insulation and this can be added on cylinders machined from solid if so desired. The cylinders can be held on a mandrel in a rotary table to achieve that, or they can be CNC machine like the ones in the picture. It's doubtful if the insulation makes much difference to the performance of the locomotive. Plain cylinders could potentially be just painted but it might look better to use cleading. The position of the cylinder cover screws needs to be looked at because the plans show the botom ones coming next to the drain cock holes. These have been moved to either side of centre on the cylinders pictured here, and the ones either side of the steam passages have been moved further apart too. The holes are therefore not spread evenly and this needs to be taken into account if a mandrel is used.

The passages from the cylinder ends can be drilled and then manually made into a slot using a Dremel or similar tool. Alternatively, it's possible to use a long series slot drill to join up the slots. This needs to be done very gently due to the long overhang of the cutter and the multiple fine cuts makes this an ideal job for CNC. On both the end passages and the exhaust ones, you only need to go 27mm deep but ther cutter needs to hang out a long way if the chuck is to miss the work. Another variation on the original design is possible in the form of a cavity behind the exhaust hole. This shortens the exhaust passages so that they can be fully machined as slots from the inside of the piston valve hole. The cavity reduces back pressure and is intended to improve the flow of steam to the blast pipe.

It may be necessary to add a small drain pipe to lead any condensation and oil away from this cavity and away from the rails. Another idea might be to add a small piece of Stainless Steel Pan Scrubber to help remove more oil from the steam. This is yet to be tried.

Some designs have a similar shaped piece to that of the petticoat pipe to smooth the exhaust flow from the cavity to the exhaust hole in the frame. A thin gasket covering the whole of the rear face can be used form a seal.

The cylinder bore on the cylinders shown is slightly less than shown on the plans, now being 38mm. The boiler for this particular locomotive is going to be redesigned to a slightly smaller size so that there is room for insulation under the cleading. The bores are large for this size of locomotive so it was felt that a small reduction to bring it in line with typical locomotives of a similar size was appropriate.

The standard castings don't have the spigots for the three drain cocks but these have been added to the parts machined from solid. The middle one is a dummy.

Cylinder Covers
The covers on speedy call for a gland nut to seal the piston rod this is quite unlike the original and not a very good design so an oring can be used and the cland made to look more like the original.

Click on the link above to download a copy of the drawing

Piston Valves
The plans show a solid bobbin style of valve which is a precision fit in the valve body. There are no seals so the fit is critical to keeping losses to a minimum. The design uses a locknut at either end of the bobbin which make it tricky to set the valves.

An alternative is to use PTFE rings in place of the solid bobbin but it may be necessary to add more bridges to the ports to prevent the rings from trying to enter into the port as they pass. On larger locomotives, these are often in the form of triangular openings that are staggered.

Solid or split rings....

Most PTFE rings are solid and are made to seal when they are up to temperature. This can be done by heating the piston liner in an oven and machining the PTFE rings until they are a good sliding fit without being too stiff.

It is possible to use piston rings but this is more complex to achieve because they don't have enough residual spring to keep them pressed outwards. Thin rings can be used which are subsequently slit with a craft knife. This has been tried successfully.

Drain Cocks
There are three drain cocks on 1501, the centre one being for the piston valve chest. The middle one is set slightly to the rear ie it's not exactly between the two end cocks. There's no need to fit this but a dummy could be included.

1501 uses a different type of cock to that drawn on the plans. It consists of a poppet valve that is opened by a push rod. This can be modelled but it's much easier to fit the LBSC type.

1501 has automatic drain cocks on the cylinder covers and these are quite prominent. They can be made as dummies or functioning.