I have a 63 cadillac and I took it to a mechanic for a motor tune up long story short the fuel pump gave out and hes tried twice to order it and he says it didnt fit the original pump is smaller on the round part then the one they sent him but I dont know why is it bigger ?so when he goes to put it on it hits the motor he wants to go with an electric fuel pump but I dont want electric anyone out there know why we keep getting the wrong pump?

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 Daniel, my best guess is that your mechanic is ordering a fuel pump for a Cadillac 390 engine,  needs to order one for a 1963  390 engine. Normally would not be an issue, but we are talking Cadillacs here.

I'll let him know and I'll post pics of original and new pump I will let him know if hes ordering a 63 pump but I remember him saying the new part did say it would fit a 63 cadillac coupe deville

There are two different 390 engines — a 1962 and 1963 Cadillac 

The 63 is a smaller redesigned 390 engine so that may be part of the problem in getting a  correct 390 fuel pump for yours 

Do stay with a mechanical pump if possible 

If you go with electric be sure you install a shut off valve in case of an accident to shut the pump down 

You may have to open the  new mechanical fuel pump and be sure the valves in the fuel pump are staked ( attached to the fuel pumps base ) as some have not been staked which cause a problem with being very  short lived 

Hope that helps some 


Yeah we are gonna try hard to find the mechanical pump I went to his shop and we even gave them the actual part number of original pump and in there website it shows the pump we need but when we got it its 4" and the one we need is 3 3/8" circumference wich doesnt seem like much but it won't clear the engine,I have pics but this doesnt have the option of putting them up but thnk you for your advice.
And I found the stamps on the engine block do you have any idea how to tell if it's a 390 here are the numbers 1477857

Those numbers are not the engine S/N# (VIN) Here is where to find that:


The VIN will tell you exactly what the motor is but here is a quick dirty method to tell the difference between the old and new 390:

59-62 390 - distributor is in the rear of the engine.

63 390 - distributor is in the front of the engine.

There are 2 versions of fuel pumps used. Both work as advertised. the pump on the botton in the second pic is the original design. There has been problems with the remanufactured original design pumps (new and reman) The valves pop out after a tear to 3 years. 

Fortunately it looks like a new internal design has hit the market.  The valves are installed from a different direction, so the fuel pushes the valves against the seats, instead of trying to push them out. 

First pic shows the new design.  Best way to identify the new design is the screws on the bottom of the pump. The new design has Philips screws. 

The brand of the new design is HERKO 

You can get a rebuild kit for the original pump,  it’s a pretty straightforward task. It took me about an hour and a half.  

I just got one from ebay the problems is that the round part circumference of the one we need is 3 3/8" and it looks like yours a compression fit the one we got is 4" and has screw but it wont clear the surroundings.

I bought the Carter M3993 pump for my 64 429. The 63 390 takes the same pump. It fit and works perfectly. I got mine from Rockauto. They have it in stock. Amazon has it too but Rockauto is cheaper by more than $10, and they ship faster too.

just got my carter pump m3993 and it did not fit back to square one 

Something is really off here, because that is the correct pump. Check the photo of my engine with this pump installed...




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