GM 12555281 oil pump - high volume?

[Ian G.]

A buddy of mine has some GM Marine engines laying around and gave me one of the oil pumps off of it, since we thought it might come in handy for my 59, thinking it would be a Melling M55. The part number on the oil pump is GM 12555281 and GL214, which seems to have both marine and modern applications and probably proudly made in China. is this a high volume pump? Anyone in the know about this? I've done some internet searches and can't find much information. I'm about to disassemble the Melling M55 out of my car to see if its in good shape, and new ones are about $25 on eBay, so mostly I'm just curious.

[Joe L.]

Re: GM 12555281 oil pump - high volume?

A buddy of mine has some GM Marine engines laying around and gave me one of the oil pumps off of it, since we thought it might come in handy for my 59, thinking it would be a Melling M55. The part number on the oil pump is GM 12555281 and GL214, which seems to have both marine and modern applications and probably proudly made in China. is this a high volume pump? Anyone in the know about this? I've done some internet searches and can't find much information. I'm about to disassemble the Melling M55 out of my car to see if its in good shape, and new ones are about $25 on eBay, so mostly I'm just curious.

Ian------


I can't find anything on that part number. However, it's easy to tell a standard volume pump from a high volume. The high volume gears are longer than a standard volume although I've forgotten the length difference (it might be 1.3" versus 1.5"). In any event, you can discern the difference without even disassembling the pump if you have a known standard volume to compare it to.

Are there GM casting numbers on the body of the pump and on the cover of the pump?

[Ian G.]

Re: GM 12555281 oil pump - high volume?

See below. I see casting numbers 12555281 and GL214. The pumps look almost identical externally. The gears are the same height as well. I can see a slightly different configuration of the two pumps internally though. The M55 has a a hollow-cast mounting plate, while the 12555281 does not. The M55 has an oil channel on the underside of the lid leading to the non-drive gear, while the 12555281 does not. The 12555281 drive gear rod is lower in than the M55. Can you think why the rod would need to be lower creating a hollow in the drive gear? GM on left, M55 on the right. Oh, well the M55 is nearly new inside and I'll be using it on my car.

[Joe L.]

Re: GM 12555281 oil pump - high volume?

See below. I see casting numbers 12555281 and GL214. The pumps look almost identical externally. The gears are the same height as well. I can see a slightly different configuration of the two pumps internally though. The M55 has a a hollow-cast mounting plate, while the 12555281 does not. The M55 has an oil channel on the underside of the lid leading to the non-drive gear, while the 12555281 does not. The 12555281 drive gear rod is lower in than the M55. Can you think why the rod would need to be lower creating a hollow in the drive gear? GM on left, M55 on the right. Oh, well the M55 is nearly new inside and I'll be using it on my car.

Ian------


When you first mentioned the 12555281 I thought you were referring to an assembly part number. That's why I couldn't find it. It looks like 12555281 is a CASTING number for the oil pump body casting, not a finished part number for an oil pump assembly. I strongly suspect that this pump is a GM #12555284, a standard volume, standard pressure pump, or GM #12555884, a standard volume, high pressure pump. The only difference between the two would be the installed pressure relief spring. The 12555284 pump would be applicable to all 1959 to 1992 Corvette small blocks originally using a standard pressure pump. This includes the VAST majority of 1959-92 Corvette small blocks.

I can't clearly see casting number on the outside surface of the cover of the GM pump. That's important because some of the pumps using the 12555281 main body casting used a cover designed for a larger oil pick-up tube. The 1959-92 pumps and some 1993-96 pumps used a 0.620" orifice while SOME of the 1993-96 pumps used a 0.742" orifice.

So, why is the GM pump different from the M-55? Well, some folks think that Melling makes all GM oil pumps and the M-55 is the same thing as you can get from GM. Your situation here is an object lesson in how that's just not the case. There's obvious differences between these pumps. Of course, it is possible that Melling does make the pumps for GM. However, if so, they make them different than the ones they sell into the aftermarket under their M-55 part number.

Which of these pumps would I use all things being equal? The GM pump. Every time. Period. That's not to say that the M-55 is not adequate. It's very adequate and undoubtedly millions have been successfully used. But, it's not the pump that I would use unless I had no other choice.

[Ian G.]

Re: GM 12555281 oil pump - high volume?

Hey Joe, The GM pump has GL214 cast in the lid. I've seen this in pics of other pumps of the same casting listed for newer makes as well, so possibly its the bigger orifice as you wonder. The Melling pump seems superior given the extra oil channel though. However, the melling gears seems to have just slightly less clearance against the lid as its leaving spin marks in operation... GM = tighter tolerance? Is there any way to tell by looking at the spring in the GM pump if its the low or high volume pump?

[Joe L.]

Re: GM 12555281 oil pump - high volume?

Hey Joe, The GM pump has GL214 cast in the lid. I've seen this in pics of other pumps of the same casting listed for newer makes as well, so possibly its the bigger orifice as you wonder. The Melling pump seems superior given the extra oil channel though. However, the melling gears seems to have just slightly less clearance against the lid as its leaving spin marks in operation... GM = tighter tolerance? Is there any way to tell by looking at the spring in the GM pump if its the low or high volume pump?

Ian------


It's possible that the same cover casting was used for both orifice sizes. All you need to do is to measure the orifice size to determine if the GM pump has the size you need (i.e. 0.620").

Both the standard and high pressure springs have 21 coils. The high pressure spring has a stripe of white dye. I've forgotten if the standard spring has any dye marking.

[John D.]

Re: GM 12555281 oil pump - high volume?

If you buy a Melling 55 oil pump today it typically comes with two springs. Neither one will put out the desired oil pressure. Neither spring I should say is what you want.
The simple solution is to order the spring from GM.. Readily available as of two months ago. Part# 03814903. Pump pressure with this spring will be about 45 #'s at cruising speed according to one of my customers. John D

[Kenneth B.]

Re: GM 12555281 oil pump - high volume?

Ian------


When you first mentioned the 12555281 I thought you were referring to an assembly part number. That's why I couldn't find it. It looks like 12555281 is a CASTING number for the oil pump body casting, not a finished part number for an oil pump assembly. I strongly suspect that this pump is a GM #12555284, a standard volume, standard pressure pump, or GM #12555884, a standard volume, high pressure pump. The only difference between the two would be the installed pressure relief spring. The 12555284 pump would be applicable to all 1959 to 1992 Corvette small blocks originally using a standard pressure pump. This includes the VAST majority of 1959-92 Corvette small blocks.

I can't clearly see casting number on the outside surface of the cover of the GM pump. That's important because some of the pumps using the 12555281 main body casting used a cover designed for a larger oil pick-up tube. The 1959-92 pumps and some 1993-96 pumps used a 0.620" orifice while SOME of the 1993-96 pumps used a 0.742" orifice.

So, why is the GM pump different from the M-55? Well, some folks think that Melling makes all GM oil pumps and the M-55 is the same thing as you can get from GM. Your situation here is an object lesson in how that's just not the case. There's obvious differences between these pumps. Of course, it is possible that Melling does make the pumps for GM. However, if so, they make them different than the ones they sell into the aftermarket under their M-55 part number.

Which of these pumps would I use all things being equal? The GM pump. Every time. Period. That's not to say that the M-55 is not adequate. It's very adequate and undoubtedly millions have been successfully used. But, it's not the pump that I would use unless I had no other choice.

I made or worked at pattern shops that made a lot patterns for the pump & covers for Melling in Jackson MI. starting in the 70's. Never made a pattern with a GM casting # for melling. Maybe GM furnished them the castings but I doubt it. I believe that Harry Melling's factory was strictly a aftermarket supplier.

[Ian G.]

Re: GM 12555281 oil pump - high volume?

yep, looking at the two side by side, the castings are definitely unique. They both use metric screws to hold on the lid as well, so they are both newer vintage.

[Ian G.]

Re: GM 12555281 oil pump - high volume?

Thanks John, very helpful!

[Stephen C.]

Re: GM 12555281 oil pump - high volume?

Ian: I agree with Joe, the high volume pumps were mainly use on high performance (racing) engines it will work on your engine but it will peg your oil pressure gauge even when hot and at an idle. If you are going after a performance verification award from NCRS this will not pass.

[Ian G.]

Re: GM 12555281 oil pump - high volume?

Oh no, I have no intention to use a high volume pump. I've read lots of threads on that topic. Just trying to figure out if I should use the GM pump or the Melling. Sounds like most people only encounter the Melling or maybe the original pump, not this newer GM casting.

[Joe L.]

Re: GM 12555281 oil pump - high volume?

I made or worked at pattern shops that made a lot patterns for the pump & covers for Melling in Jackson MI. starting in the 70's. Never made a pattern with a GM casting # for melling. Maybe GM furnished them the castings but I doubt it. I believe that Harry Melling's factory was strictly a aftermarket supplier.

Kenneth------


I agree. I don't think that Melling made any of the Chevrolet small block or big block PRODUCTION or GM SERVICE oil pumps. At least, not until very recently. I believe the GM pumps with GM casting numbers were manufactured internally by GM. It's possible that some or all of the castings were manufactured by an outside foundry to GM specifications but, at the very least, I believe the pumps were machined and assembled by GM manufacturing operations. Most likely, this was done at the GM Bay City Michigan engine components plant (which is still in operation today). This includes the following standard pressure pumps: GM #3764547, GM #10055849, and GM #12555284. The differences between the GM pumps and the Melling pumps are obvious enough that there's no way the GM pumps were simply Melling pumps in a GM box.

Unfortunately, things are different now. All of the above GM pump assemblies are discontinued and replaced by GM #93427692. I am pretty well convinced that this is nothing more than a Melling M-55 in a GM box with a GM part number. In fact, "M-55" is prominently displayed on the casting. So, in this case, aftermarket has become OEM with an OEM price to match. I find this very disappointing. Fortunately, it's very unusual. If GM was going to convert aftermarket to OEM you'd think they would have at least gone with the Melling 10553 which is a premium aftermarket pump. But, they didn't. Like I said, very disappointing.

I want to make it clear that I am not saying that Melling oil pumps are bad. They are not. In fact, they're very good and millions have been successfully used. Melling is now also a major supplier of oil pumps to the OEM automotive industry. However, their OEM product lines are NOT the same as their aftermarket. The M-55 is an AFTERMARKET oil pump.

[Ian G.]

Re: GM 12555281 oil pump - high volume?

Would you recommend getting an oil pump drive shaft with the steel coupler on the end, or just stick with the stock one with the nylon coupler?

[Joe L.]

Re: GM 12555281 oil pump - high volume?

Would you recommend getting an oil pump drive shaft with the steel coupler on the end, or just stick with the stock one with the nylon coupler?

Ian------


I like the steel coupler. This type was actually used in PRODUCTION for L-88 and ZL-1. However, the nylon coupler type is completely adequate. Millions have been used and I've never heard of a problem.