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MG MGA - SU Fuel Pump rebuild

Hi All,

I have recently completed a successful rebuild and thought I would acquire a knackered pump to rebuild to carry as a spare for my Mark I 1960 MGA ,and got a bargain on ebay for 6.

My car is positive earthed and I am awaiting delivery of a positive earthed rebuild kit. If the pump has come off a car with a negative earth, do I have to change anything else internally to change the polarity or is it just a case of making sure it's wired externally in the same way as the one already on the car ?

It's an L-type pump but I don't know if it's the low or high pressure variety. I also have no idea whether the one I run on the car at the moment is low or high pressure. All I know is that they look exactly the same including the coil housing which is the same length. Is there any way of identifying whether high or low pressure? Both pumps have the same part number 4080 but the one I got on eBay additionally has a metal plate with AUA66 stamped on it.

Grateful for any help.

Safety fast!

Jason Ogelman

Stock SU fuel pumps were not polarity sensitive so no changes need to be made. Pumps fitted with a diode are polarity sensitive and the diode has to be re-oriented. After you re-build the pump measure the pressure and flow. Then you'll know!
G T Foster

Jason, The 4080 is not the pump number but the part number of the base housing and both the HP and LP pumps have the same housing. The chances are it's a low pressure pump with it being used on more models of the era. To make sure take out the diaphragm spring and check how much force is required to squash it flat by placing on a set of digital kitchen scales. The HP spring takes 6 lbs and the low pressure one just 2 lbs. I converted an LP pump to an HP but it's a real pain of a job. I had to rewind a new coil, make a larger diameter steel coil armature, bore out the coil housing to fit the armature. Let me know if you need any more detail.
m.j. moore

Its not true of them all but a high pressure pump can usually be identified by having a 4BA earthing screw. The low pressure one hass a larger 2BA screw.
Malcolm Asquith

Dear Mike/Malcolm,

Your comments have helped me identify that the spare pump I am about to rebuild is an LP one but the one in situ on the 'A' is an HP one. Do you think it is worth rebuilding as a spare for emergency use? Would it run satisfactorily without going to the lengths Mike did converting to HP or maybe I should rebuild it and put it back on eBay. Maybe I'd come out with a profit to bid for another in the hope it was the HP variety!


Jason Ogelman

Jason, I found that the previous owner of my car had incorrectly replaced the pump with a low pressure type and I only found out when I came to replace the diaphragm: the LP and HP diaphragms are not interchangeable. However I did run the car without any apparent fuel delivery problems for a few thousand miles with the LP pump. You might think that it's worth it for the experience to recondition yours as an LP pump and take the risk that it will get you home in an emergency. If you have a lathe and the patience convert it to a high pressure one.

By the way the Morris Minor used the AUA66 and hence they frequently come up on Ebay. The MGA pump, AZX1331,which came in both negative and positive earth versions, is a rarer type that was used also on the Magnette,Morris Oxford, Riley Pathfinder, AC, and, I think, some Rover models of the 50s/60s. Mike
m.j. moore

This thread was discussed between 13/08/2008 and 14/08/2008

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