Oil Pumps: Melling M328 HV vs Stock | Ford Explorer Forums - Serious Explorations

  • Register Today It's free!

Oil Pumps: Melling M328 HV vs Stock


Explorer Addict
April 6, 2008
Reaction score
Year, Model & Trim Level
1994 Explorer 2dr 4x4
This thread will compare a stock 1994 4.0 oil pump and a new Melling M328 high volume replacement pump.

Melling pump info: http://omnitek.co/mellingcatalog/#ag/part/M328

I purchased the M328 for approximately $140 through a site that shares its name with a river. The stock pump we are comparing is the original from my '94 Explorer with 197k miles on it. The motor had solid oil pressure (gauge was modified).

First, initial comparison has both pumps showing approximately the same external dimensions. The Ford pump's finish is smooth and the Melling's is rough. I note the spot where the drive shaft enters the pump appears to have less engagement surface on the Melling, but hopefully the unit sits farther up on the shaft.


I note that there is a spot which is covered by the gasket on the old one; pulling off the gasket (and comparing the new gasket) indicates that both units are supposed to have the gasket covering it even though there are threads. It's too difficult to get a picture inside this chamber but it appears ribbed or to have a spring. It's visible in further pictures and seems to be a pressure release valve.


The new Melling comes with a note specifying it requires a new Melling oil pickup. I'm guessing this may be due to the warranty requirements rather than a difference in parts.

Join the Elite Explorers for $20 each year.
Elite Explorer members see no advertisements, no banner ads, no double underlined links,.
Add an avatar, upload photo attachments, and more!

The Ford pump disassembles with T-30 Torx head bolts. They require some serious torque to budge; I am using a high quality Wiha set of bits, as I'm tired off twisting off Chinese cheapos.

Once the cover is removed, the internals are visible as a standard "gerotor" pump (https://en.wikipedia.org/wiki/Gerotor). This should be the low pressure pick-up side, though there appears to be some positive return area to lubricate the shaft. I'm not sure the point of the channel, perhaps a sealing effect?

The pump internal and external rotors drop out.

The pump housing.

Another view in the housing. I am thinking that the assembly in there is a one-way pressure valve, keeping the pump from draining oil back through it, but that's just a guess.

The oil pickup attaches here. I believe the chamber on the right (which the gasket seals over) is part of the valve I mentioned.

Here's where the inner rotor is driven by the oil pump drive shaft.

Now I'm disassembling the Melling pump for comparison next to the Ford.

The end covers come off the same, although the Melling uses hex head bolts with lock washers.

The Melling end cover.

The Ford end cover. Note the quality of machining - I prefer the Ford even though neither are perfect.

Both pumps have the same design, but it's immediately obvious the Melling's rotors are sticking out of the pump housing while the Ford's sit flush. I'm not sure how I like Melling's dependence on the sealing between the cover and housing.

Like the Ford, the rotors drop right out.

The Ford rotor is obviously not as deep, but otherwise there is no difference in design.

Note the machining on the surfaces. While the Ford appears to have surface imperfections, it is actually a very smooth surface. The Melling's surfaces are rougher than they look. Perhaps the difference is mileage? I wonder how tolerances compare.

Melling's pump housing cavity. Note the surface finish.

Ford's pump housing cavity. Again note the finish and visible evidence of wear.

For fun I tried inserting the Melling rotors into the Ford housing with the Melling end cap, and the Ford gears into the Melling housing with the Ford end cap. Both sets of rotors slide in easily. Both end caps are the right shape, but the locating pins are different so they can't be tightened down flush.

That's all I've got to share; hopefully someone learned something. These oil pump inner rotor sets make a nice desk toy but they will eventually develop surface rust.

I can see the evidence of minor wear on the stock Ford pump but I can also see evidence of its high quality. I have no doubt it would be reliable for the life of the engine block. The Melling design gives me some doubts about its design and machining quality. Is it worth $140? It appears that a stock Ford XL2Z6600AA pump is available for approximately $100. If that is truly equivalent to the factory unit, I think I would seriously be looking at it instead in terms of quality. The Melling might give higher oil volume, but I haven't seen any issues with oil starvation in my engine during the rebuild other than cylinder wall scuffing which might be caused by something else.

I'm open to any comments from members who have used the Melling and can report on its performance, and anyone with input on the metallurgy involved in a high mileage gerotor pump. There is this thread - http://www.explorerforum.com/forums/index.php?threads/melling-m328-high-volume-oil-pump.247048/

Further stuff:

New Melling oil pick up tube: http://omnitek.co/mellingcatalog/#ag/part/345S/from/make%3Dford%252520truck%252CFord%252520Truck%26model%3Dexplorer%252CExplorer%26year%3D1994%252C1994/1
$54, more than I want to pay for a non-moving part. Looks identical to stock.

Pump drive shaft: http://omnitek.co/mellingcatalog/#a...&model=explorer%2CExplorer&year=1994%2C1994/1
About $31. Wonder if, like they say, the stock shaft is fatigued.

Lifters: http://omnitek.co/mellingcatalog/#a...&model=explorer%2CExplorer&year=1994%2C1994/1
These are a set of 4; at $44.82 I'm interested.

Another observation - the body of my Melling pump is stamped M128. This corresponds to their stock replacement pump. One of the cover bolts has a metal tag marked MM328. I think the M128 body is functionally identical to stock, and the M328 version only changes the rotors and cover.

Very interesting, thanks for sharing the pictures. I was tempted to go for one on my engine but couldn't eat the cost for what it is.

I'd venture to guess that the valve is a bypass valve. I remember reading in the FSM that the stock pump bypasses at 60 PSI so it will never build more than that. This means if your oil is thick or super cold, your top end may not get oil for a while. The pump won't force oil up as hard as it can, there's a built in limit. I'd wonder what the Melling bypass is. Obviously, the Melling will help more at temp when the oil thins out, it should maintain pressure (through higher volume). Hands down, this is a must have for anyone doing a serious rebuild (heads-off, pistons out).

I own a Mellings M328 oil pump, but I have not installed it in an engine yet. Overall, I am pretty well impressed. The machining looks great to me. If you compare the stock cover to the Mellings, the machining looks similar. Its just a big endmill that faces off the part. I have no doubts in its sealing ability. Everything smooth on the OEM is because of break in. Pull the Mellings in that many miles, it will look identical. The outside rougher surface was a choice made by engineers for the die cast mold. My guess is it releases better or forms a better part, but I don't know the true answer. To the consumer, it doesn't make a difference. A bad casting will have pinholes in corners, cracks, that sort of thing.

From Mellings, a stock oil pump bypass is 54psi, and the M328 bypass is 63psi. It also boasts 20% more volume than stock. This is a much needed upgrade IMO due to the sheer number of people complaining about valve train noise, even with relatively low mile engines.

The problem I see is the oil pickup tube. I have not tried the stock pick up, nor have I purchased the Mellings one. First of all, just design the thing to use the stock tube. Its a metal tube with a screen, there is no reason it can't be reused. Second, if you need some special pickup tube, send it with the oil pump. As for the splines for the oil pump shaft, I don't see why they did what they did. The stock Ford pump has full splines, while mellings decided to skip the top 1/8" or so. My opinion is that it won't cause any problems, but I don't see why it couldn't be splined fully. When I insert the stock oil pump shaft (which I plan to reuse) it seems like a good fit. No slop in the splines or anything like that.

I see it as a necessary upgrade, but like you said, with the Mellings oil pickup it comes to about $190, vs OEM which is about $100 and can reuse the pickup tube. There are also a host of aftermarket pumps for cheaper, including the Mellings M128, which if it also needs the new pickup tube, is a rip off. I say either M328 or OEM.

The M328 is the only upgrade oil pump I am aware of for these engines, so it's kind of sad to see Mellings kind of taking advantage of the situation.

The m328 doesnt need a special pick up..ive used them in about every build i have built and i do recommend them..the 4.0 OHV suffers from bad oiling to the valve train,the lifters are dead ends..so the more oil you can get up to the rockers and out them the better..

Fyi from what I've researched the stock ford unit IS a mellings m128

Interesting, I'll try my stock oil pickup tube. They send a yellow tag with stating you need a Mellings one, but I don't see why. If you don't have to buy the pickup tube, the M328 is a no brainer.

Interesting, I'll try my stock oil pickup tube. They send a yellow tag with stating you need a Mellings one, but I don't see why. If you don't have to buy the pickup tube, the M328 is a no brainer.
Ive used the stock one on all my builds..only time a different pick up is needed is if you put the SOHC pan on the OHV,then you use the SOHC 2 piece pick up tube which fits it also..