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MG MGA - 3 Main B into A water pump question

I have a 3 Main B 1800 engine I have been planning to install in my 1961 MGA 1600. When it comes time to install the water pump, do I use one for a B or the A pump? I would guess the block would take either, but I would rather ask and be sure. I am keeping everything else stock A 1600.
Cleve Crews

Cleve, As I understand it they are interchangeable.
BM Gannon

Hi Cleve. I think the MGA and MGB pumps look similar but have minor differences that can cause installation problems. I once tried putting an MGB pump on an MGA 1500 block, and there were impeller clearance problems, if memory serves. An MGA pump MIGHT fit on an MGA block with no problem, but Im not sure. If the MGA pump bolts on, and the impeller spins without binding, it should be OK, assuming it moves the correct amount of water through the cooling system. Safest bet would be to bold an MGB pump onto your MGB 1800 engine. Just my 2 cents worth. Good luck and have a Happy New year! Glenn
Glenn Hedrich


Your best friend is the part number. I have looked at the Moss Europe website and they quote different numbers for the two pumps concerned, suggesting that they are different.

That doesn't necessarily mean that they are not interchangeable but I would generally suggest sticking to a pump that is specified for the engine, so you know the impellor is moving enough water and is not too small for its housing. You will probably find that there is little discernible difference in external appearance. If you have an original cast-iron type then it is worth replacing the bearings and seal to give it a new lease on life.


Malcolm Eades

The MGB pump has will not fit an MGA as stated above. The MGA pump will fit an MGB but will not be as efficient as the MGB pump.

The most important difference is the length of the fan shaft and housing. The B pump is about 3/4" longer and may cause the fan to hit the radiator. Many people flip the fan over so the offset is to the rear or shorten the housing and shaft. See image of A and B fans and pump pulleys below. B is on the left, A on the right.

On my B engine I flipped the fan and re-riveted the blades.

Jim Ferguson


I think you will find that the MGB had about 4 water pump types. The one I use with my 1800 in the MGA gives 3cms clearance between the fan and radiator without needing any modifications.

My memory fails me a bit, but I seem to recall putting an MGB pump in my 1500 during the rebuild 12 years ago. I believe I used the short shaft version and it worked ok.

Steve Gyles

Just to correct my last post. From my receipts it seems I had the MGA pump (Moss GWP103)in my 1500. My installed 1800 has the Moss MGB GWP130 pump.

Assuming that it is still the GWP103 pump sitting in the 1500 block on my garage floor, it is about 3cm longer than the GWP130 installed in the car (measurements approximate).

Steve Gyles

Okay, so if the B engine had 4 types of water pumps, then I have 5 choices if I include the A. Of all the 5, do any have issues with the crank pulley, or will they all be the same? I would hate to purchase a pump and find that the belt doesn't line up the crank pulley and the water pump pulley.
Anyone with experience putting 3 main B engines into A's?
Steve, is the version of the pump you have working in your A the one for the early B?
Cleve Crews


My redundant 1500 has a 1H1151 (Moss GWP103) water pump.

The Moss MGB catalogue shows the following:

The early MGB (62-64 18G & 18GA) has the GWP115.
The MGB (64-71 18GB to 18GG) has the GWP114.
The MGB (71-74 up to 18V672 27269) has the GWP117.
The MGB (74 to 76 18V672 27270 onwards) has the GWP130.

I cannot speak for the 3 bearing engine. All I can say is that the 5-bearing 1800 with the GWP130 fits in the MGA using the MGA 6-bladed fan without modification, other than shims between the fan and pulley.

Steve Gyles


I was advised that the MGA water pump doesn't move enough water for the MGB engine; I don't know if it's true, but I went with a "B" pump. The four "B" pumps are not all the same as far as alignment with the crank. I tried several water pump/pulley combinations to get one to align with the crank pulley on my 5-main engine.

k v morton


My installation of a 3 main bearing 1800 motor replacing my 57 MGA's 1500 motor required constructing two simple plywood shims to fit between the radiator and the radiator's mounting brackets. The plywood thickness of the shims can be easily determined to give you the correct offset needed to move the radiator forward and clear the fan. Obviously you need to replace the mounting hardware with longer bolts. When carefully shaped and painted black the shims are not easily noticed. I had enough clearance for the bonnet to clear the radiator cap when latched.

Martin Straka

That sounds like something I would do. Did you use the pump for the 3 main B? I don't mind trying different pumps, but I don't want to order all of them just to see which one fits my car.
I checked the prices of all the pumps, and as luck would have it, the 3 main B pumps are almost double the price of the 5 main pumps.
Cleve Crews


Yes I used the pump for the 1800 3 main motor retaining the original 1800 3 main pulley. I believe that the 1800 pump has a greater pumping capacity than the 1500. I would give you more specific dimensions but my MGA is at my new ex-wife's temporary residence (formally referred to as my house.) Diplomatic negotiations would be needed on my part to cross into this disputed territory.

Martin Straka

OMG! Don't get yourself in a fix!
It looks like with what you and the other have mentioned, the best pump would be the 3 main B pump. I hope I used the crank pulley from the 3 main engine and not the A. I'll have to look through all the old A parts and make sure there is a pulley in the stack.
I got the three main B engine at a shop a few years back for $25. It was sitting in a shop on the floor next to the wall and I noticed it was an MG engine. I asked what it was doing there and the owner told me he had pulled it out of an MGB so he could install a V8. I took the engine to a machine shop and had them clean, bore, turn and check it for cracks, and it was in good shape, so I put it all back together with new parts for my MGA. So far, it hasn't run, but it should be a good little engine with little wear.
Cleve Crews

This thread was discussed between 28/12/2009 and 30/12/2009

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