Fuel Pump Components

- The Ffestiniog and Welsh Highland Railway.

In Early 2022, We were contacted by the Engineering Team at the Ffestiniog and Welsh Highland Railway. The enquiry regarded the reverse engineering and manufacturing of new components for Kerr Stuart 4415, a 1928 diesel locomotive under restoration with the Ffestiniog and Welsh Highland Railway.

KS4415 is a unique locomotive. It is the oldest purpose-built diesel locomotive in the UK. It has a fascinating history with a long working life, both in the UK and in Mauritius, before being repatriated to the UK in 1997.

Components from the original Fuel Pump assembly

The Railway team initially approached us for the manufacturing of new Diesel Atomiser assemblies which we completed in 2022.  In 2023, we were approached for the next batch of components to be manufactured – Fuel Pump assemblies and a Fuel Pump rack.

Fortunately, the Ffestiniog Team had been able to find the original drawings for these components. These provided technical dimensions and tolerances but also advisories on assembly and slide fits. An section drawing can be seen below.

Furthermore, they had one of the original Fuel Pumps available for reference. Some of these components can be seen in the image to the left.   

Original Section Drawing of the Fuel Pump Body

Recreating Forged Components

Some of the components were straight forward to reproduce, such as threaded plugs, casings and other turned parts. However, other components required a lot of planning. For example, the main Fuel Pump body is an unusual shape and would have originally been forged. Without access to any of the original patterns, it was up to Us find another way to manufacture a new Fuel Pump body.

Autodesk Inventor, our 3D CAD Package, was invaluable during this stage, allowing us to view and section the component in order to plan the various stages of machining.

We began by turning and boring a large, round blank of EN8 steel on our high precision DMT CNC Lathe. We then progressed the components onto our Hermle CNC milling machine. Held in a round chuck, we begin milling away access material to match the original profile of the Fuel Pump bodies. 

A New Fuel Pump body After A CNC Milling Operation

Cross Drilling & Welding

Another slightly tricky element of this component was an angled hole that needed to be drilled to connect the two internal chambers of the Fuel Pump body. However, the entrance hole would need to be blanked off/covered once machined to ensure correct operation.


We designed and manufactured a specialised jig to hold the Fuel Pump body securely at the correct angle. Once the drilling operation was completed, a bespoke plug was turned and welded in place. This created a hole/passageway internally whilst still sealing the fuel pump body externally. Once welded and polished, the witness mark left is hardly visible.

Fuel Pump Body after Cross Drilling and Welding

Matched Components

The Fuel Pump assembly contains multiple metal-on-metal moving/sliding parts. Therefore, it is crucial that they are precisely matched to each other. The collection of components contains the Fuel Pump Body, Plunger and Body Plug. Both the Plunger and Body plug need to be a stiff sliding fit within the main bore of the Pump Body. 

We turned the Plunger and Body Plug from EN36 Steel. It was then heat treated and ground. Therefore, the finished components are both parallel to 0.002-0.003mm and have a very fine surface finish.

Once both sets of components had been ground, inspected and measured, we were able to determine the required diameter of the mating bore in the Fuel Pump body.

We then began honing the hole in the Fuel Pump Body. Honing removes minute amounts of material from the component to slowly creep up to the desired diameter with a very high standard of surface finish. 

As the honing process is a slow and repeatable process, it required a lot of pausing, measuring and “offering up” the mating parts to inspect the feel, interference and slide until the stiff sliding fit was achieved. Once completed, the Fuel Pump Body, Plunger and Body Plug were marked up and labelled as a matched sets, ready for installation.

The Final Product

It was a pleasure to receive the following feedback from Rob, one of the Engineers at the Ffestiniog and Welsh Highland Railway:

“I got to Boston Lodge today and opened up the boxes you sent.  There’s only one word really – wow!  The finish on the parts, particularly the fuel pump bodies, is amazing, and the plungers feel like a great fit in the pump body.  Like the injectors, it’s going to be a while before we’ll be able to set any of these parts up and test them, but I’m confident they will operate as intended.

Best regards,