This little story relates to my 1935 Derby Bentley, B 115 CW, which in general is in very good condition so requires little improvement, however one doesn't know what is round the next corner, such as when I suffered severe steering wobble as reported in my last article. It was a hot day and I was returning home from a good run out when I noticed that the SU fuel pump was clicking rather fast, even when I was sitting at the traffic lights. Something was wrong, but at least I got home without failing to proceed. On the Derby Bentley, the RR fitted Autovac was replaced by a double-ended electric fuel pump made by SU ( which stands for Skinner Union for the pub quizzers out there ). This pump is positioned immediately in front of the driver on the front of the main bulkhead, so its regular clicks can be heard when the car is stationary. Rapid clicking can be an indication of petrol vaporisation at the pump, or that air is leaking into the line. It was time for an investigation.


Fig 1 shows the arrangement of components in the vicinity of the petrol pump. Behind the pump can be seen the main/reserve tap. This has two positions and connects either the main supply pipe or the reserve pipe through to the pump. These two pipes come from the petrol tank where the reserve pipe reaches to the bottom of the tank whereas the main sits a couple of gallons higher. This rather expensive arrangement was replaced on later cars with a single petrol feed pipe but the level sender was fitted with an electrical contact to illuminate a warning lamp on the instrument panel when the petrol level was low. The top pipe on the tap goes to the pump. I had always run on 'main' so during my investigation I turned the tap to 'reserve' and got an immediate rapid clicking and no petrol. The reserve gave me no petrol at all, and this state of affairs appears to have been extant since I bought the car! There was something wrong in the vicinity of the tap assembly.


To get good access I first removed the pump and its pipes. I was then able easily to access the two pipes connected to the bottom of the tap. The right hand one is the main, and on touching this, the nut fell off in two pieces! Experience had taught me that these original nuts can become brittle with age and must be treated with the utmost of respect on tightening. The male thread is made from brass, with a spherical sealing face, and the 'bib' soldered onto the pipe has a female conical sealing face, so a perfect seal between the two is easily made without using excessive torque on the nut. Some fitters, unfortunately, do not understand the elegance of this design and over-tighten the nut, with the result that I discovered on my car, a nut holding on by a prayer and giving a small air leak, exacerbated by high temperature. A new main supply pipe was obtained from one of the usual sources, and these now use a stainless steel nut to avoid the cracking problem.


This still left me with the mystery of the permanently leaking reserve supply. On close examination, I discovered, to my horror, that the nut was cross-threaded! This can be seen in Fig 3. The previous idiot fitter has misaligned the nut, on the end of a carelessly bent supply pipe, and tightened it without checking the threading, coming to a stop when it was too tight to proceed. This left a gap between the pipe and tap body, and a massive air leak. How was I to remedy this disgrace, given that these taps are not available from any source, and the proximity of the two parallel threads makes the use of a die set impossible?

This is when I remembered the special tool called the thread file. One is shown in Fig 4. The thread on the pipe connections is a British Cycle Thread 9/16 x 20 tpi. The thread file has eight different pitches on its four sides and two ends, one of which is 20 tpi. Use only the correct pitch section of the file for the thread that needs to be repaired, and place the file firmly over the threads at the correct helical angle. Start at a good section of thread and file carefully around the thread, into the damaged area. When filing forward, always keep the back of the file engaged in a good section of thread in order to remove the cross-threaded section but leaving the correct original thread. With care, the correct thread form can be achieved, albeit with some of the thread peaks being lost due to the cross-threaded nut previously cutting across them. Use the nut to gauge how much metal needs to be removed in order to get a good fit and full engagement.



Once sure that the male thread is back in good form it is important in this application to ensure the pipe runs are correctly bent. This can only be done with the pump removed and the two inlet pipes must be carefully bent until they fit snugly against the bulkhead, are routed around the back of the pump and enter the two threaded connections on the tap at right angles. This job can take quite a time but is essential. The previous idiot fitter had obviously not removed the pump first. Once all the pipes are correctly routed, tighten the nuts, but not too much. The repaired thread is shown in Fig 5. This just goes to show that however good a car looks and runs when you buy it, there will always be something that needs attention. Yet to be repaired is the fuel level sender in the petrol tank and the offside headlamp dipping solenoid. Oh the joys of pre-war motoring!