REPLACEMENT OIL PUMPS
As we know, the 51-53 331 pump is a one-only pump design. Although the pump ‘looks’ like the Early DeSoto they do not interchange due to the slight change in the mounting bolt locations. Additionally, there is a minor change in the length of the rotor/shaft assembly. There are just two ‘foot’ patterns for the several different pump body designs so we can modify a 51-53 331 main cap to accept the ‘other’ pattern which is common to the 392 pump and which is, as of this date, in production from Melling: the M-50.
Because, for this example, we are focused only with the pump and main cap, do not forget that you will also need to deal with the oil pickup tube and screen assembly for the particular oil pan configuration that you will use.
The first step is to set a piece of soft all-thread in the cap. We used Locktite 242 in this job. Be cautious about using a bolt that could be harder than the cap material because we will be drilling through both materials and a hard bolt could force the drill bit to drift into the softer cast. If you use a bolt you should anneal it first.
The next step is to make things flat:
While the cap is positioned in the vise we will also enlarge the oil transfer slot just a wee bit:
Now back to the bolt hole… We located the new hole center with a transfer punch through a 392 pump body. Now it is time to drill and tap the new hole:
We now have a 51-53 331 Chrysler main cap that will accept the Melling M-50 pump without any other modifications. No adapters, no questionable shafts, just stock parts.
If you want to use an A-LA distributor you only need a new intermediate shaft assembly just the same as if the original early pump was in place.
Could this be done by a handy hot-rodder with a good drill press? Possibly, since the oil transfer slot is not critical but rather ‘just nice to do’. The big hurdle will be to make sure that the new plug is dead flat to the mounting surface when you are done.
Can it be done at home? Yes
Should it be done at home? Maybe not.
Installation of the Melling M72Hv oil pump on the 51-53 331 Chrysler main cap.
Although the M50 pump is most adequate for all engines and is a direct bolt-on to all except for the early 331, there is continued interest in using the M72HV pump because of the lower initial cost. The lower initial cost is obviously off-set by the cost of the needed machine work. Additionally, the M72 pump then requires the use of the LA intermediate shaft and gear assembly. Although a series of new ‘tall’ shafts is being designed, currently, the stock shaft is all that is available so using an LA distributor is problematic.
This procedure for modifying the main cap will be the same for all of the Hemi engines, with either mounting pattern.
In this photo we place the M72 pump on the main cap using the existing registration.
In this photo we will use a transfer punch to locate the one bolt hole that lands on the cap.
Now, back into the mill to drill and tap the new hole.
And now we have a new bolt hole for the M72 pump.
Also note that in the last three photos there is a slight chamfer on the edge of the mounting surface. This is required to provide clearance to the pump body.
Next step is to go back into the mill so that we can clean off the top of the cap for the main bolt. Some caps will show alot of galling in this area and we need a surface that is very clean and flat.
Next, we will shift our attention to the fabricated part that will support the second pump attaching bolt. With the new part in the lathe we will square each end.
With the ends squared up we will bolt it in place on the main cap. This will be its final location. Once secured we will take one or two generous cuts to get us close, then start making 0.0005″ passes to get to the cap surface.
The final pass will be a cut across the new part and the main cap.
During the installation of the main cap a strap is attached to the pump body and the fabricated part to hold the part from rotating when the bolt is torqued down. This strap will also keep the new part from becoming separated from the cap prior to installation. These are now precision matched parts and cannot be replaced without putting the main cap back in the mill.
It is now necessary to replace one of the pump bolts with a stud due to the location of the new hole.
The final requirement for this swap is to insure that the pump clears the main cap and new fabricated part so that it has a perfect fit. On this particular pump body some minor grinding was needed to remove excess flashing. The areas marked in yellow were cleaned up.
This is not a do-it-yourself kinda job, unless you have a mill and a lathe. If you have the equipment go ahead and use the photos as a guide to do your change-over.
If you really want to use an M72HV pump and need help you can send your main cap and pump to us and we will do the work for you. We can also supply the pump.
OEM Dodge vs the M50
Although the Dodge pump looks like the M50 (392) pump there is one important difference; the Dodge pump is 0.3125″ shorter:
Note the difference in the above picture. If the M50 is used in a Dodge that is equipped with a rear sump pan then the only fitment issue will be in regards to the pickup tube and screen assembly. If the Dodge has a center sump pan the the pan will likely hit the pump and not seat on the block. This can be cured by placing an appropriate ‘dent’ in the pan.
Also note in this picture how the pump body is machined for clearance to the crankshaft counterweight. The Dodge has much smaller counterweights so the machining needed is minimal, not so with the M50 and the 392. The areas marked in yellow show the machined areas. The Dodge pump should not be installed in a Chrysler engine unless first examined with regards to the clearance needed.