Building the 'Mad Mallet' 'River Avon'


the 'Mad Mallet  'River Avon'

click on image to enlarge

click on image to enlarge
The 'Mad Mallet' began as a joke on the Lynton and Barnstaple forum about articulated locomotives that could been built. I was stuck with my work and had a spare hour to get bored and watch telly, so I drew an L&B Mallet on my laptop with the company of Judge Judy. Rather embarrassingly, I fell in love with the design.

line drawing of 'River Avon' and 'River Brue' - click on image to enlarge
I have always rather liked Mallets. As a kid I had the privilege of seeing Challengers blast through the Rockies. You never forget a thing like that! Garratts might be a better design but I have never warmed to seeing locomotives with two backsides.
The Mallet has its origins in France. The design minimised the number of flexible high pressure steam pipes and limited their movement by having the rear power bogie fixed to the boiler so only a small flexibility was needed. The design really favoured the use of compounding. The system reused the low pressure steam to operate a secondary drive using much larger cylinders. By the time this engine was built (1920s) greater efficiency would have been extracted by the use of super heating.

Mallet locomotives were used with success on a number of European lines, mostly narrow gauge, but really came into their own in the USA. Here they were a huge success as they permitted the Americans to have the biggest in the world without needing any imagination of their own!
The heavily graded line of the L&B required double heading if any sizable trains were mustered: an expensive business. Had traffic been greater, it would have been quite possible that Eastleigh would have contemplated building more powerful traction units that still had to operate within the loading gauge and go round the numerous tight curves.
The L&B benefited from the development experience of Welsh narrow gauge railways. The one mistake made was the relative lightness of the track. The only way to get more power would have been to have more wheels. The Mallet could have been considered.
This is not  a complete step to step instruction sheet, but hopefully a guide as what you can do with etched kit bashing, in this case a Backwoods Miniatures Manning Wardle was attached to two ROCO outside frame chassis.  'Avon' is freelance. Its looks ring my bell but not necessarily those of others. Hopefully, though, it does give a stimulus to others to 'have a go'. Anyway, if you mess up the Backwoods chassis (like I had), this is a possible use for the remaining body!
Sadly, the Roco chassis gear mechanism turned out to be 'not fit for purpose' and after a few hours of excellent running began to develop gyrations every bit as good as binge drinkers falling out of a pub! I had intended to rebuild this locomotive with the new N Drive outside frame six coupled chassis. I have seen the prototype and in my view it is an excellent basis to start with. Sadly, some production delays have arisen and the rebuild will use two Bachman/Grafar 08 class diesel no 371-015 incorporating the cylinders and valve gear from the scrapped Roco chassis.
I do not have specialist equipment and this locomotive is made using the most basic of tools. Most were bought from EXPO tools.
25 watt soldering iron with 2mm end.
rail cutter
needle nose pliers
larger needle nose pliers
set of small screw drivers
set of small drills
fine craft saw and modelling knives
small set square
steel rule
dental probes
small clamps
small engineers block
wife's best scissors
Dremel tool with attachments
set of small burrs
Badger air brush
thin multicore solder
plumbers liquid flux
5 minute epoxy
Remove the two cab sides from the fret. The cab sides of the MWs as delivered are used.

After removing the bunker top, squarely cut off the rear bunker and solder into position. Cut out two pieces of brass 10mm X 22mm. Tack solder them accurately together and transfer the curve of the original bunker to the replacements. Sand the new bunkers to that profile with a Dremel and separate. The new bunker can been seen in the photo above, top right. These are then soldered to the rear of the cab sheets. I solder panels with the work lined up against a steel rule to ensure straightness.

Now cut the front of the cab down as shown in the photo above.

Remove the vertical bars from the cab side windows and file to profile and solder to the cab sides. Solder in the bunker doors and the cab side handrails.
Now cut the tank sides, 65mm X 12mm. Solder these accurately to the cab sides, 2 mm from the bottom.

The bunker back is fabricated from two of the etches, including the one which has etched lines to make following the curve easy. The excess is cut off. The cab back has had the lower section removed to allow for the mechanism. The small return is saved to give strength.

The cab front is soldered into place. Great care is needed to ensure that the whole assembly remains aligned. I always solder on the spectacle frames is first. I line these up by pushing a paint brush handle through the hole!

Add the tank tops, 7.5 mm X 52 mm

Solder in the  tank bottoms  7.5 mm X 64 mm. and the tank backs 65mm X 12mm. Then close off the back of the bunker as shown below. Cut excess off flush once soldered.

Now build up the bunker sides using strip.
Prepare the boiler barrel, solder on the bands made in thin strip and solder the barrel into place. Solder in the floor and enlarge the central hole to accommodate the chassis.

Now add the boiler supports, water fillers and the smokebox, (made from the same material, slit at the bottom and opened out a little.

9 hours into project
The tank is drilled to take the handrails, and lamp irons and foot steps are fitted. Cut the cap off the chimney and reduce height so the complete casting remains in loading gauge and re-solder. 
The MW smokebox is just too small. A plain disc was made by  cutting one out of thin sheet with scissors (wife's best), drilling the centre, spinning in the Dremel in a mandrel and sanding until round and the right size. Then a second disc was made, a tad smaller than the first , one side covered with solder and sanded on the Dremel until the right door profile was obtained.
Drill a hole in the smoke box top in the correct position and glue the chimney into position.
The two steps on the tanks' fronts are soldered on.
The new door is then epoxied into place. The door hinge and straps are made from thin strip and wire, soldered up and glued on.

The rear coal bunker retaining rail , whistle, safety valves and interior cab details are then fitted . The roof is then soldered on having been trimmed to size and the roof vents soldered in. The sanding pipes are then soldered into position. The 'Russell' sandpots will be mounted on top. Also fitted is the plumbing for the steam draincocks. The front bogie support footplate has been added. The thin wire rubbing strip has been soldered underneath in an arc the radius of the front chassis pivot.

click on image to enlarge
17 hours into project

click on image to enlarge
The front pilot is built from spare bits of Backwoods cabs (with an etched fold line) and awaits the cowcatcher. The pilot will be attached to the front chassis.

Piping strips are added, using very thin brass strip. The 'Russell' dome is not attached until after painting.


supporting brackets soldered onto pipework

finished bar one sandbox - click on image to enlarge


I hate white metal vacuum connectors. This is particularly the case with MW locomotives which have these items unsupported on the front buffer beam. I always make these in brass wire and wrap 3 amp fuse wire. Flanges and connectors can also be modelled by wrapping a ring of fuse wire at the location of the flange and touching with solder.

When soldering a small bit onto a larger bit, tin both surfaces. Sand the surface of the larger bit smooth, add flux, place on the small bit and heat with clean iron (wipe off solder with cloth) until you see the solder fuse.
If you get solder where it should not be, (and you will), it will sand off with the sander attachment of the Dremel or using a burr, you can use 150 grade freecut sandpaper, and for small spaces use a modified craft blade, break off end and use as a scraper.

First things first. True Mallets are compound locomotives, so the front cylinder shape has to be modified. From the Backwoods fret, select another rear bunker which has bending lines and cut two rectangles to the correct size and bend them to obtain the cylinder shape.

Sand or file off all the details of the cylinder ends of the front chassis.

the retrofit of two new Grafar 08 chassis

The Grafar chassis is easy to dismantle and the first was cut back and the high pressure cylinders fitted with epoxy. Great care is needed to position the cylinders to ensure that the correct movement of the valve gear is possible. They have to be as close to the front crank as possible. The rear coupling rod pin was removed and the hole opened out slightly on the crank to accommodate a brass wire pin. The chassis is well made and the drive is on the centre wheel. This does mean that additional strain is taken on the rear axle with the valve gear.

The Roco valve gear easily grafted onto the chassis and seems to run very smoothly.

I have experienced several loose wheels. This is what I suggest:

On initial stripdown, degrease axles and wheels and run into the wheel boss, a small quantity of Locktight 603 and leave overnight. This should prevent these stupidities.

Several phosphor bronze are often on in contact with the wheels....check for this..

Positioning of the Roco cylinders. Remove motor. Use 5 minute epoxy to attach and have the return crank already fitted into the rear crank. There is then a short window of opportunity to ensure that there is free movement of the gear.

Once the cylinders are fastened, push the chassis along to check for any binding between the coupling rod and the slide bar support bracket. Only then replace the motor but glue this in position as well, as the extra weight of the valve gear can cause the motor to 'jump' and strip the gears.

the new chassis arrangement with lead weighting

the loco after successful trials and ready to return to service - click on image to enlarge