Replacing the petrol pump diaphragm

petrol 4How you know when to replace your petrol pump diaphragm? The probable answer is when the pump ceases to pump petrol on a cold, wet night down some remote country lane!  The smart answer is in your warm dry garage with all your tools and a new diaphragm to hand!  The choice is yours!  Either wait for it to fail or carry out a bit of preventative maintenance before the event.

Assuming that you opt for the latter the following instructions should help you do the job. They describe the type “T” AC pump commonly fitted to the Seven but can be applied to the similar “M” type fitted to earlier cars.

Remove the pump from engine and note the position of the inlet/outlets.

Unscrew and remove the six screws which hold the two halves of the pump together. If the halves don’t come apart readily hold the base and gently tap one of the unions , in an upward direction, with a screw driver handle.

Put the top half of the pump to one side.

The diaphragm which is retained in the lower half of the pump can now be removed as follows;-

Make sure the diaphragm is not stuck to the housing. If it is then you must gently peel it upwards to free it.

petrol 5Grip the centre metal flange of the diaphragm assembly between your thumb and forefinger, press down and rotate the diaphragm through 90 degrees. This should release the diaphragm.

If the diaphragm is unwilling to turn it can help if you push the pump lever away from the body of the pump at the same time as you press down and turn.

Fit the new diaphragm as follows:-

Reposition the large spring in the lower body. Position the new diaphragm so that the flat on rod is in the same plane as the operating arm.

Push the rod, centrally, down through the spring into the pump body. Now for the difficult bit!!

petrol 6Push the operating lever away from the pump body and at the same time grip the diaphragm as before, push down squarely and rotate through 90 degrees (a quarter turn).

Rotation by this amount is most important.

There is usually a small tab on the edge of the diaphragm and a similar tab on the lower body which can be used as reference points. If the diaphragm is NOT turned sufficiently the operating rod will not be locked in the correct position and could release itself. NB. The holes for the screws in the diaphragm should now line up with those in the lower body.

If not repeat the process!

Now take the pump housing that was initially put to one side.

petrol 7Unscrew the hex head bolt which secures the domed pressing and separate the dome from the housing.

Remove and clean the filter. Remove any accumulated sediment from the chambers in the housing. The valves in the housing tend to give little trouble so the best advice would be to leave them alone .

Replace the filter, making sure that it seats properly, and having made sure that the mating surfaces between the dome and the body are clean replace the dome and tighten down. Only tighten sufficiently to effect a good seal; over tightening is likely to distort the dome and/or strip the thread.


Now position the top and bottom halves of the pump together ,making sure the outlets are in the correct position and insert the six screws. DO NOT tighten them yet.

Pull the pump operating arm towards the pump fully and whilst holding it in this position tighten the six screws fully.

If you now operate the pump lever you should hear a croaking noise!

Well done! All that now remains is to refit the pump to the engine. It is important that the lever is positioned on the correct side of the camshaft. i.e. between the camshaft and the side of the crankcase.

petrol 8

This is easier to do than describe! Tilt the pump away from the engine and carefully insert the end of the lever just into the top of the hole in the crankcase. Feed the lever into the engine in an upward direction. (What you are effectively doing is sliding the tip of the lever up the inside of the crankcase.) As you feed it up you must also rotate the pump so that it locates on its fixing studs. Some resistance may be felt as you do this since the lever should now be pressing on the camshaft. Replace the washers and nuts and tighten evenly but not excessively and reconnect the fuel pipes.

It is also advisable to fit a new gasket when refitting the pump.



FINALLY before starting the engine turn it slowly on the starting handle to make sure it turns freely. Make at least three full turns. The reason for doing this is to check that the pump lever is correctly positioned. And turning by hand will show this. If it is not correctly fitted the engine may jam before any lasting damage is caused.


This article, written by Andrew Jarmin, originally appeared in Seven Focus Jan 2001 pp21-26.