Mechanical Fuel Pump Rebuilding

by Bill Bowman

Although some of you might just install a new Brazilian fuel pump on your motors, some of you "original" nuts (like me) might buy a fuel pump rebuild kit and tear down and rebuild your original pump. Usually over the years the diaphram will harden or tear and start leaking gas down into your motor thru the base (contaminating your motor oil - smell your dipstick lately?). Bad, bad news for your motor.

However, the rebuild kits usually don't come with instructions and if you don't know any better, you can still "screw up"the rebuild. The VW factory Workshop manual tells us about an important step in the reassembly process that most of you may not know about. After you've torn down and cleaned up every part of the pump, lay the contents of your rebuild kit and your pump out in front of you. Here is exactly what the Workshop manual for the early 40hp style fuel pump says about the reassembly process:

When installing, the following points should be observed:

  1. Check valves carefully; replace the top half if necessary. (Bill: the "valves" are in the top half of the pump)
  2. Replace the diaphram and rubber gasket if they show signs of hardening. (Bill: the new ones should be in your kit).
  3. Press down (on the) diaphram and spring and insert rocker arm in diaphram. Insert pin (thru rocker arm) and secure by means of the lock rings.


  4. Place the lower half of fuel pump in a vice with the Gage VW 328d inserted. Thus the rocker arm is pressed 14mm (inwards measured from the flange jointing face), bringing the diaphram to the required assembly position. (THIS IS IMPORTANT. Before you place the top half of the pump down onto the diaphram, and insert the six slotted screws and tighten them down, you must FIRST DEPRESS and HOLD the rocker arm inside the lower base exactly 14mm as described above.

    Granted, you'll probably never find the VW special tool - Fuel Pump Gage VW 328d. I was lucky recently and actually found and bought this tool which is for all fuel pumps 40hp and up. At the same time I also bought the other tool VW 328b for the 25 and 36hp fuel pumps also. For your information, the 25-, 36hp tool depresses the push rod 35mm inwards measured from the flange jointing face. Now, don't panic, you can fudge it! Just make a small 14mm tall block or cylinder that will fit inside the lower "base" of your fuel pump to depress the rocker arm. Set this block on a perfectly flat surface like your workbench. Then set the lower "base" down on it so that the point where the fuel pump push rod would touch the rocker arm rests on top of the block. Then when you press down on the "base" of the fuel pump until it is flush with your workbench surface, you have just depressed the rocker arm inside the "base", and hence your new diaphram, the proper 14mm. Somehow clamp the base down in this position and move on to the next step.

  5. Place the top half of the pump in position so that the fuel connections are situated above the inspection cover. Make sure that the diaphram is not creased. (Bill: now with the diaphram properly positioned by step 4 above, insert the six screws and carefully tighten, making sure not to wrinkle up the diaphram as you tighten the top half to the bottom "base".) Insert filter so that the flat side is positioned downward. Do not omit the sealing washer between the pump cover and mounting screw.
  6. Fill the lower half of pump with Universal grease VW - A 052 (anti-freeze). The grease assumes a liquid condition at operating temperature, insuring a proper lubrication of all moving parts. Rocker arm and pushrod which are devoid of grease or oil indicate a leaky diaphram.
  7. Check rocker arm spring for proper seating.
  8. Check inspection cover gasket for wear, replace if necessary.

Again, other than my bracketed comments and clarifications, the above instructions are copied verbatim out of my 1959 VW Workshop manual. Those VW engineers wanted that diaphram pre-tensioned for a reason, and if you don't do it correctly, your fuel pump won't be working as well as it should. I hope this tip helps some of you out!

[Editor's Note: When rebuilding a fuel pump or a carburetor it's recommended practice to soak new diaphragms in gasoline for an hour before installation to acclimate them to their new environment.]

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