Part 4 – Bonnet tops and electronics

Quicklinks – OverviewPart 1 Part 2Part 3Part 4


A pair of roof profile hoops were cut to take the piece of plate for the cab roof top, the cut plate being rolled to the curvature and form required. Once done this was then welded onto the pair of roof hoops to form a strong one piece lift off structure.

The engine bonnet bonnet needed a formed section over the area just in front of the cab as a fixture since the engine exhaust stack protection needed a solid base bit. The front engine bonnet top was a bit more tricky to make and form since it had both flat and curved sections which then met in the centre section as parts of the inspection cover lids as per the real engines, except I made mine a one piece lift off unit. Access to the battery and other parts like the fan, horns and lights being essential for maintenance and operating.

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The front bonnet and the rear bonnet have the waist line at different heights, this can be seen from the drawing / plan as above. The bonnet top just in front of the cab was a bit tricky to make since it is angled from the centre down to a radiused edge that seats into the bonnet sides.

Some sorting and adjusting needed to be done within the inside of the bonnet area with the bulkhead profiled partitions to get the visual aspects looking something like the outward appearance of the Sentinels.

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Once the actual positions had been sussed out bits of small section angle iron were cut and used to secure the bulkheads in place using 3mm stainless steel bolts.

Triple air horns are fitted next to the Eaton 7 hyd. unit to give fair warning as and when required by the driver to rail users as per the rules. A separate air compressor is fitted to activate these horns for the desired warning duration, and yes, they are loud!


The front grill has a pair of high intensity white LED lights mounted in the top apex plus the buffer beams have a red and a white shunting operation led lighting unit fitted.

The lighting is fed from a fuse inside the cab to switches fitted into the control desks rear wall plate on the right hand side.

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The front bonnet top was even more of a pig to make as it had a central box section plus the slanted tops went down into a radius that has to precisely sit in the bonnet side plates aperture. Getting it square and parallel was even more of a pig! Heavy clamps had to be used since they were the only thing with a big enough gap which could over-reach the section edge to allow me to spot weld the plate in place. The slope had to be angled at such an accurate degree plus the width had to be an exactness so as to allow a neat tuck fit into the fixed slot or gap between the bonnet sides. (Plus I needed to allow for several coats of paint!)

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The sword (Paper knife) was the nearest thing I could obtain to duplicate the Sentinel one as used on the original design on the engines.


Electrics plus:

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The electrical circuits are fused for; Lights/Fan/Horns/Engine. (Caution do not look directly into the twin high intensity main lights).

Through the radiator mesh can be seen the special Optima Yellow top 12 volt battery which provides the power for all aspects of this engine. It is let into the central void area behind the original front buffer beam so that it does not impair the flow of cooling air through the bonnet.

The plastic box contains a twin, 12 volt relay unit which operates the guards alarm, this employs a simple change-over circuit.

The mechanical control lever on the left of the control desk operates Forward/Reverse mechanism.

The drop handle on the right is for the engine speed control. The small grey lever underneath the air filter box is for the choke control.


Initial track testing took place on the 13th of August 2013 at my home club track at Lynnsport. It was hooked up in front of the duty loco Harlech which had all the clubs set of carriages attached. 3 drivers experienced in driving this type of engine took the controls and all were satisfied with the handling and performance on our small track circuit of 725 feet. The control desk was fitted with temporary levers and controls to enable live testing to take place without having any public riding during this time. Once this live test was considered complete Sentinel came off and Harlech took over the operating.

Any complaints ?, well just the one (or two) those horns are noisy! and when is it going to be my turn to have a drive? (Unfortunately with such short distances of straight track driver training and handling will need to be carried out elsewhere in order to get a real feel for the handling at speed and anticipating the braking capabilities of this engine, also bearing in mind what weight it is capable of pulling and real diverse track conditions).

To be continued…!

Quicklinks – OverviewPart 1 Part 2Part 3Part 4