I know this is probably old news but...
The Melling casting numbers on the pumps is not necessarily the part number. Melling somtimes uses their older casting mold designation when casting new oil pumps. .If the description says it is for a "SB Chevy" and nothing else, it is generally understood to have a 5/8" inlet. The numerical prefix "1" indicates the SB Chevy pump has a 3/4" inlet. Just because it is a "high volume" pump does not mean it will have a 3/4" inlet and just because it has a 3/4" inlet does not mean it is a high volume oil pump. The "high volume" oil pumps have spur gears that are longer than standard oil pumps and they have a 0.587" longer pump housing. The pumps that are "high pressure" are equipped with a stronger by-pass spring.
Caution: High volume and high pressure pumps in a engine with standard bearing clearances usually accelerate distributor and cam gear wear. Always machine, grind or file 0.030" wide x .030" deep oil groove from the bottom oil passage on the exterior of the distributor shaft housing to the distributor gear for improved oiling. Always replace the nylon sleeve type drive shaft with a ARP,Melling, Moroso, etc., aftermarket drives shaft that has a steel sleeve, whatever pump you choose.
Melling M55
Std. pump, std. pressure, 5/8" inlet.
Melling M55HV
High volume, high pressure, 5/8" inlet.
Melling M155
Std. pump, std. pressure, 3/4" inlet.
Melling M155HV
High volume, std. pressure, 3/4" inlet
You can increase the oil "pressure" by replacing the standard pressure by-pass spring with the "high pressure" relief spring that comes in the box with the pump or purchase one separately and replace the standard oil pressure by-pass spring. Doing that will allow the pump keep your engine oil under pressure at a higher RPM before it by-passes. (when I was a kid of 15, and could not afford a tachometer, I used the oil pressure guage to tell me when to shift). High pressure pumps require more frequent oil changes because dirty oil and a high pressure pump is just like sandblasting the bearings when spinning the engine at high RPM. That is called "washing out" the bearings, not wiping out. .
The M155HV pump has longer spur gears for "high volume" which makes the housing 0.587" longer. Therefore you must use a deeper pan and an aftermarket longer pick-up screen assembly. You cannot use a Chevy high volume oil pump in a stock oil pan. That is because the stock oil pump pick-up assembly is as short as it can get due to the location of the oil pump. That is why a high volume oil pump CANNOT suck a stock oil pump dry! However, that is not the case for all engines. For example, Pontiac engines with their "high volume" oil pump CAN suck a stock depth oil pump dry because the Pontiac high volume and standard volume oil pump is the same length which permits the HV pump to be used in a stock Pontiac oil pan. Pontiac so-called "high volume" oil pumps merely use a stronger by-pass spring and a 3/4" inlet, and they are not true HV pumps. Usually high volume and high pressure oil pumps should only be used if yoiur engine has a remote oil filter and oil cooler or the bearing clearnces are loose (.003" or more) as is found in some race engines. It has been proven on the dyno that a high volume oil pump requires 3 to 7 HP to turn it (depending on the RPM) compared to a standard oil pump.
I went through this with my 1991 S10 Blazer 4X4 with a 4.3L engine. The pick-up screen/pump combinations are nearly endless when trying to achieve 0.375" to .500" oil pan to pick up screen clearance. The S10 Blazer 4X4 has a oil pan sump that is 9.250" deep and after several trial fits with a few different oil pump pick-up assemblies, I will use a M155HV pump with standard pressure by-pass spring and a Milodon 18201 extreme duty pick-up screen assembly that is designed for a 8.250" deep oil pan.
The math: (8.250" pick up + 0.587" longer oil pump) = 8.837" pick up depth. Then: 9.250" pan depth - 8.837 pick-up depth = 0.413" pan clearnce......dead nuts on. More than 0.500" clearance between the pick up screen and the oil pan will cause a vortex to form and the resulting cavitation will cause air to be drawn into the oil pump with a loss of oil pressure.
The Moroso and Milodon oil pump pick ups press-in the inlet hole and also have a strut that bolts to the bottom of the oil pump. Using Milodon or Moroso bolt on pick-ups is simple and is more secure than brazing the pick-up tube to the oil pump housing. Brazing is a weak weld to cast iron, especailly not done correctly. .Do not trust the the Melling Select bolt on oil pump pick up screen assemblies because the tube is merely bronze soldered into the inlet bolt plate and does not enter the pump at all. I feel like vibration can break it loose with disasterous results.
Be advised:
1.) Securing the oil pump pick-up in some manner is absolutely necessary if you have a deep pan with a extended oil pump pick-up screen assembly.
2.) You may want to use the more expensive Melling Select oil pumps because the regular production Melling oil pumps, made after 1990 are lighter and have weaker castings, thanks to the bean counters at GM. The new design of the standard production Melling oil pumps have been known to break off the main cap especially when using solid motor mounts. Melling gives that disclaimer with a note in the box. I found my pre-1990 new old stock Melling M155HV oil pump on e-bay, still in the original Melling unopened box.
