Construction of Model

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Diesel Locomotives of Sri Lanka


7¼  inch Model Construction


April 2008 – Lynnsport Miniature Railway

 Construction Log – M7 Project.

October of 2005

I made contact with Brush Traction to request information and if possible drawings of the 16 Brush Bo-Bo locomotives that were made in 1981 and exported to Ceylon. These engines were of similar design to the Alco S1 switchers that were very popular with the American Railroad Companies. I really wanted to build a British engine that was different and had character so that it would be a “talking point” to arouse interest from it being on display and whenever operating at other Club tracks.

May of 2006

A two week holiday was arranged in Sri Lanka with Vinodh Wickremeratne and Ifthar Rizvi that allowed me to access these locomotives, take pictures and measurements where needed. However a picture is worth a thousand words, if it shows the actual detail, since drawings often show what was designed but not what was built! How big will it be when finished? About 6 foot 3 inches long, 15 inches wide and 20 inches high at the cab roof.

Construction started

Work started on Brush M7 number 803 locomotive less than one week after returning home from Sri Lanka. The pre-formed chassis deck plate had been made some weeks previously was modified to reflect the shape of the steps at the front and rear sides of the loco and the front buffer beam supporting angle frame had been cut and bolted to it. The lower part of the buffer beam that also serves as a cow-catcher, would be a one piece part that will have strengthening struts very similar to what are fitted on these engines. In practice they will have to be strong enough to support the whole weight of the engine in case of a derailment. (about 250 lbs) but that excludes the traction batteries and two auxiliarry batteries that separately power the air compressor, the lighting and amplifier unit. These batteries weigh another 100lbs in total.

Power plant

This engine is powred by two Bosch 750 watt x 24 volt motors, one on each power bogie, a total of 2 hp of direct traction, powerful enough to pull about two tons on level track at about 12 mph. The engine was originally designed with dual controls so that there would be no need to reverse or turn the engine when off-loading onto another clubs track. It worked ok but made fault diagnosis a lot of bother so when the 2010 refurb was done the duplicated circuitry was removed. After completing the traction testing trials I changed one of the drive gears which then gave the locomotive a top speed of about 9.7mph.

Braking systems

A dual braking system was designed with the mechanical system already installed on both bogies. The engine has working straight air brakes that are controlled from a control valve mounted on the purpose built driving truck. Brake blocks are made from cast iron and the actuating rods are controlled from a centrally mounted dual cam system within the bogie equalising bar frames. The actuator rams or pistons act in a lever / crank type mode to apply the brakes using compressed air at about 30psi (2 bar). The main reservoir(s) holds air at 45psi (3 bar) and gives the neccessary volume to ensure a brake application. The air compressor system has a pressure switch fitted that automatically charges the two main reservoirs once a drop in pressure has been sensed. There is also a safety valve fitted to ensure that the air bottle are not over-pressured plus there is a quick release valve to dump the air when disconnecting the 3 air pipes. (this originally had a 4 pipe system, but got changed after several months of operating experience showed that it could work satisfactorely using a 3 pipe system).

Main engine control system

A 4QD Pro-150 digital controller has been fitted (was a Pro-120) to control this engine and give the best performance possible using Marine type deep cycle batteries. The original 100 amp batteries lasted 5 years and have now been replace with 2 sets of 85 A/hr deep cycle batteries which are changed over once the battery voltage has been depleated.

Phoenix Super Sound

For engine sounds and control authenticity a “Phoenix Super Sound” control desk system is installed, this gives real engine sequence and noise sounds, plus the horns and the guards whistle.There still remains many fiddly bits to be done to finish it, some small adjustments need to be made before the final colour scheme is applied. This will probably mean that the bonnet / body colour scheme will not be finished until about the middle of May 2008, much depends upon some warm sunny weather for the final painting to take place.

M7 #803 – Commissioned Operational.

M7 number 803 became operational for passenger services as from Sunday the 13th of April 2008. Although this engine is passed “fit for duty” there are still several minor bits to make and fit that will improve its appearance and help give the engine its unique character. Driver training and familiarisation sessions have already started so that other club members may take over the controls. A Driving experience on this engine gives you a very similar if not the same one as being on a real engine. If you don’t do things in the correct sequence it does not go, take your foot of the deadmans pedal and it stops! A reset sequence then has to be carried out in order to re-start the engine.

Construction of Class M7

1981 “BRUSH” diesel engine of Sri Lanka.


This page show the start of the M7 project with the selection and acquisition of materials for manufacturing this 7¼ inch scale engine. Having obtained 24, A3 size works drawings the basic engine dimensions were inserted into an Excel data base so as to arrive at the real working engine sizes. Both drawing sizes and actual sizes have been used (or adjusted) to make this modelThe size data then allowed the purchase of materials needed to start construction.Some of the A3 drawings were enlarged to ensure that the scale data would actually produce an engine that would be large enough to represent the full model size which would then permit assessment of the component sizes and fitting of standard available components.One of the major problems with making such a model from scratch is that delivering a real working model is often a compromise between having a nice looking model and one that actually looks about right and performs within the models design parameters that have been worked out initially. Getting the balance right is often more important than exact scale.Works designs can change during construction of a new engine, liaison between the workshop and the office may well take place in order to accommodate changes needed that may affect other parts of the working components when the engine is tested, or may arise during track trials. Having lots of actual detail pictures helps sort this out.

A picture can do what a thousand words cannot, it provides the builder with small details that could otherwise be overlooked. It may also be necessary to modify your own sketches to take account of any anomalies seen and allow you to arrive at a workable compromise thus ensuring that you end up with a good working model. What might work on the full size engine may not on a scale or semi-scale model.

Detailed panels or parts can be flimsy and fragile and susceptible to damage when being demonstrated and operated, for these reasons it might be prudent to avoid (making or) fitting such parts to the engine when being operated.

01plan of chassis

02chassis deck plate

03main frames added to deck plate

04buffer beam plate work (upside down).


In order to ensure that the model would be square, flat and true it was mounted and constructed on two heavy box section steel parallel bars bolted together to form a rigid construction base.

Construction of Class M7

1981 “BRUSH” diesel engine of Sri Lanka.


This page shows the engine body construction of the M7 project.The engine body comprises several sections starting from the nose end section at the front. Next is the radiator and pump compartment or section. Next is the bit that provides the basic power, the engine compartment. Adjacent to this is the electrical compartment that houses the equipment that generates the electrical power to the motors. Inserted between this and the cab is the dynamic brake compartment.The cab has two driving positions, on the left-side looking forward along the bonnet and directly in the opposite (right) corner looking over the battery box compartment.Our engine however will be a “battery electric” having two 24 volt Bosch, 750 watt x 1 hp motors, giving it a total traction output of 2 horse power. This should be capable of hauling a load of 2 tons at up to 12 mph, it will have the necessary torque to do this but you then need to have the necessary means to STOP it safely.I have already purchased some miniature air compressors that operate from a 12 volt dc supply, the idea is that the engine will have one and the driving truck will also have another of these fitted over the manual brake system. A low pressure air cylinder will be fitted so that it can work independent of the manual braking system. Once the manual system has been fitted checked and tested then the air system will be added and be the regular means of brake application. Should something fail then this still leaves a mechanical hand lever for operating the brakes.

Two heavy duty semi-traction batteries will provide the energy / power source to operate this engine. Each are 12 volts, connected in series to provide the 24 volt dc supply to the controller unit that will manage the heavy currents etc during operation. It will have regenerative features as well as providing braking on the motors.

These two batteries are situated low in the centre section of the main body area and help with the weight adhesion properties.

Our engine will also have two driving positions, to avoid having to turn it round at any track site, a plug in hand controller unit can be operated from either end of the locomotive. Only one hand controller will be required although there will be a master and slave connection fitted to allow this kind of working.

A fuel tank sits underneath the centre body section, however this is a dry container and will house some of the electronics systems that will provide sound effects etc.

05side view plan of body

06cab – battery compartment end

07engine bonnet side panel construction

08nose end and radiator section

Note: Once the engine has undergone its test trials and has been satisfactorily performed under loaded conditions, then it can be finished to the correct colour scheme.

Construction of Class M7

1981 “BRUSH” diesel engine of Sri Lanka.


This page shows the cab construction of the M7 project.The full size cab has two driving positions, on the left-side looking forward along the bonnet and directly in the opposite (right) corner looking over the battery box compartment. The cab body also incorporates the dynamic brake section in the recessed bit just in front.It is proposed to install a plug in control point on both buffer beams so that this engine can be driven from either end without the need for using a turntable. Just like the real M7 it can run round the carriages and hook onto the other end of the train.There are no plans at present to do anything inside the cab. This floor area sits directly above one of the main stretchers that carry the bogie mounting pivots. Once one bogie has been made and fitted with its drive system and motor only then will it be possible to envisage what clearances will dictate and any obstructions or unused space be known.Each bogie has to be able to pivot and move without fouling the cab floor or adjacent area’s. The frame width inside has been determined from measurements taken from sketches and should allow enough clearance where required.

The hood of the battery box compartment is removable so that a heavy duty electrical isolating switch can be fitted to allow quick disconnection to occur if need be.

Working lights and horns will be added after the engine has been track tested and deemed fit for duty.

09plan of cab

06cab – battery compartment end

11forward driving window

12doorway this side facing forward

Construction of Class M7

1981 “BRUSH” diesel engine of Sri Lanka.


This page shows the bogies construction of the M7 project.If, looking at the previous pages you thought that things might be a little complicated or perhaps difficult then now you arrive at the real test! Making a pair of powered working bogies will require hundreds of parts making from scratch. Some parts can be purchased such as the 2 motors, the 8 double row bearings, the gearing, a combination of 4 gears per drive unit a slight modification will be required to at least one gear wheel that fits directly onto one of the wheels.. The 2 cogs and drive chain for powering the second axle of each bogie. The 8 CNC cut wheels, although some slight modification will be necessary to these.Finding a batch of heavy springs that matched my needs was a problem but on a visit to a Exhibition I spotted a box under a bench with loads of odd springs in it, I needed 8 and found 12 that could have been specially made for me! That was a stroke of luck and I think they only cost me £10 for the lot.Generally speaking you need to make at least 4 parts for most things, some need 8 whilst others require 16. Some might require left handed and right handed parts, so churning out 16 exactly the same could be problematic and a waste of time and material! Finding material that matches your requirements exactly may not be possible from local sources, however a few minutes spent carefully searching through a local engineering works scrap bin could deliver an off-cut that has been discarded. This could be used and cut down to obtain your special requirements and at minimal cost. There is however a down side in that it takes a lot of time cutting and machining off the surplus material!Precision machining of parts with close attention to clearances and tolerances becomes a must, parts must fit squarely and not foul on an adjacent moving component thus inhibiting its movement or ability to function correctly. Parts should slide up and down easily or tilt where necessary without restriction to satisfy the design features that make it work. Parts might be interchangeable if very strict sizes are maintained during manufacture but quality does take more time to produce.

Each bogie will have to be capable of supporting about 50kilo of weight plus it will have to withstand the rigors of operating conditions on different tracks when used at different locations.

These have to perform faultlessly under arduous conditions as may be imposed by variable conditions and at the hands of the driver, experienced or a novice!

13plan of bogies

148 of each item required

12 1516 of these L shaped axle horns

168 equalising bars plus all the bits

note: Assembling the parts just to see what fits etc can be frustrating especially when trying to hold heavy springs in place is problematic!

Construction of Class M7

1981 “BRUSH” diesel engine of Sri Lanka.


This page shows the proposed mechanical drive system of the M7 project.Precision machining of parts with close attention to clearances and tolerances becomes a must. Parts should fit into and mesh without grinding (or being too loose) to work properly. Parts should be interchangeable if very strict sizes are maintained during manufacture but quality does take more time to produce.Each bogie will carry a motor plus the gear driver system directly onto the axles, the support members will have to be precision made and jig drilled in order to ensure that they all mesh correctly.These have to perform faultlessly under arduous conditions as may be imposed by variable track conditions.To determine what speeds are possible some simple mechanical calculations are required, you need to know the speed of the motor in rpm, multiply this by the driver gear teeth divided by the driven gear teeth. This will give you the axle speed in rpm, you then need the wheel tread diameter to be able to work out the distance travelled in one revolution, after that it is a few more simple calculations. The circumference is found by knowing the wheel diameter and multiplying it by 3.142, it is then possible to determine what mph can be obtained from a given gearing set as fitted to the drive system.

A set of 8, CNC turned wheels type, WH522SS have been obtained that are considered an almost exact match in size for those needed to fit this engine. A minor amount of machining should give a finished profile that is near enough compatible to that required with an average tread diameter of 119mm this model.

The set of spur gears to be used is of type G1.5 using 20t driving a 60t and a 20t driving a 50t gearing set up onto the axle.

The Bosch motor is rated at 3300 rpm, so should deliver a maximum speed of about 9.8 mph on a flat track pulling 4 to 5 carriages.

Once trials have been done then any need for adjustment (or redesign) can be considered. In essence, the general operating speed whilst carrying public passengers is about 6mph. At this speed the driver should have adequate braking control of the train.

The ability to stop safely must always be the overriding criteria.

17bogies mechanical drive

18top gear motor – bottom on axle

19gear drive plate assembly being made


drive gear wheel mounted on one axle

Note: The front and rear axles of each bogie will be connected by a heavy duty chain, a 17t cog wheel will be fitted onto each axle. This will ensure that the best possible traction and adhesion is obtained at the rail.

Construction of Class M7

1981 “BRUSH” diesel engine of Sri Lanka.


This page shows the electronic power and traction system for the M7 project.What am I looking for?

  • 2 x 12 volt semi-traction (minimum) 85Ahr batteries, (connected in series to provide 24 volts). There is a wide choice of types available that will provide the current to operate the motors, however factors such as cost, type, size, output and voltage are variables which need to be considered. A deep cycle type battery will give the best performance to ensure that the life and capacity duration is able to meet the demands placed upon the engine when operating. There are high tech. batteries available but these are at a premium price.
  • 2 – 24 volt “Bosch GPA 750 watt bi-directional motors part no. 0 130 302 014. (provides a total of 2 hp of direct traction) Proven means of B.E. traction on past engines.
  • A 4QD Pro 120 Controller unit has been installed and is eminently suitable for this kind of operation.
  • Plug in connections on both buffer beams for a hand controller to plug into. This will allow the engine to be operated from either end without having to turn the engine on a turntable when in service.
  • A audible sound system of horns.

this is now done by the Phoenix master control desk unit.

  • A working compressed air system for operating the engine brakes and the driving trolley is now installed.
  • An electronic engine sound system that emulates the engine noise and sounds under operational use has been installed to give life like engine qualities.
  • Suitable batteries to provide the power to operate the auxiliary electrical / electronic services have also been purchsed.
  • Power braking control: A Miniature air compressor has been purchased, duplex servo-brake low pressure cylinders have been purchased and fitted. A manual operated brake is also needed to back up in case of air failure. Both the engine and the driving truck has air braking systems fitted. Carriages can differ at various railways with type of braking systems in use. Vacuum and or air are in use but may also differ in design, so advance information is required before visiting a different railway to ensure compatibility.
21Bogies- initial electric motor test
22 Battery space / location data
23 4QD Pro 120 Controller unit

24Miniature air compressors for the air brakes system.

Construction of Class M7

1981 “BRUSH” diesel engine of Sri Lanka.


This page shows the process of testing both static and on the track.Static tests

  • 2 – 24 volt “Bosch GPA 750 watt bi-directional motors provides a total of 2 hp of direct traction using a gear drive made of mod.1.5 gears. A 20t wheel is fitted onto the motor shaft revolves at 3300 rpm. This drives a 60t wheel that meshes with another 20t gear that finally drives a 50t gear wheel that is mounted onto the side of the driving wheel of the bogie. This was all set up to spin freely without undue tightness or slackness.
  • The bogies were checked individually for rotation so as to ensure that they both drive the wheels in the same direction, before they were both connected into the 4QD Pro 120 (Mk2) controller unit.

Track tests

  • A simple test of one of the bogies took place in September, this involved placing it on the track and giving it a gentle push to see how it went. It glided smoothly round the track and did not show up any problems going through points or on the curves.
  • The first engine track test was carried out on Wednesday the 24th of October 2007, this test exceeded all expectations with the locomotive operating perfectly, in fact it had one slight problem. It wanted to go too fast! This required changing the 40t gear wheel to a 50t gear so as to slow it down. This new gear was ordered and changed ready to do another test the following week.
  • The second engine track test was carried out on Wednesday the 31st of October 2007. the engine was checked out to now have a top speed of 11.2mph, which is at the very top end of speed range for this size of miniature engine. (12mph is the max.).
  • During the testing 7 drivers have now took the controls and have been delighted with its handling and track performance. The controller has regenerative braking so that when you release the throttle lever it automatically slows and stops. This lever controller also acts as a “deadmans handle”. The controller has dual stage ramping which allows gradual acceleration and hpl, which prevents the engine from moving when the hand controller is first plugged into the engine controls.
  • The final testing will be with working air brakes and pulling 5 fully loaded passenger trolleys in January 2008.
21Bogies- initial electric motor test
26 4QD Pro 120 Controller unit
27Track test 1

28 track test 2

Construction of Class M7

1981 “BRUSH” diesel engine of Sri Lanka.


This page shows the latter process of painting and finishing the model.Selection of Paint

  • For initial undercoating I used Japlac enamel paint which was readily available locally and came at a reasonable cost. It was decided to fill in any problem areas with a suitable filler and then apply about 4 grey undercoats of paint to protect the metal and try to exclude any rusting of the bodywork etc.

Preparations and finishing

  • For the intermediate painting stage it was decided to use Humbrol enamels to give a good quality finish but for this stage to mix a gloss Grey using White (22) and a small amount of Black (21) purchased in 120ml tins.
  • To apply 3 or 4 coats of Grey gloss leaving it to dry hard prior to taking it for showing at an Exhibition, this would then be left to harden off and be displayed in this condition despite it not being prepared and finished properly.
  • Once the show is over the engine will then be prepared for the final colours of paint work using selected Humbrol enamel colours. The actual colour tint will be as that supplied, with about 50% thinners added to the colour for good drying and colour matching.

Final painting in Intercity Sri Lankan Colours

  • Humbrol enamel paints were obtained from “modelsforsale” to complete the final finishing. A fast thinners was also purchased locally so that spraying / finish quality can be assured.
  • The next step will be to fill in any blemishes using model paste, left to dry and harden before being rubbed down to acquire a good clean surface finish.
  • Black no. 21 underside/bogies
  • White no. 22
  • Red no. 19 buffer beams
  • Green no. 2 lower body panels
  • Yellow. no. 69 waist band line
  • Matt black. no. 33
  • Silver. no. 11 roof sections
  • Grey. (my own mix). Lower cow-catcher
  • Pale green (nearest is no. 90) Inside of cab
  • Azure blue engine compartment (my own mix)
29Loco- in grey undercoat during trials

Internal body undercoat primer done


Underside undercoat being applied


3rd Gloss coat of Grey completed

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