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MG MGA - 3 main 1800 - What to do ?

After many years with fouled water / oilways in the engine block, I've decided to replace the block on my Mk II. The story of how it got that way is a sad one for another day.

With kids done with school and nothing else to spend my money on, I laid hands on a 3-main 1800 block with crank and pistons (nothing else). Lots of good info on this conversion in the archives, but I would be interested in what you folks think I should put on my list of things to do.

I have access to a couple of good engine rebuilding shops in the vicinity, so I'll have them go over the block and crank for any issues. If all looks well, I'm ready to proceed on an engine building project. I've pulled engines and replaced the easy bits a time or two, but this will be my first try at a full rebuild.

So, I'd like to see what you think I might do with a couple versions of this project:

Option 1: Pragmatic, re-use as much as possible

Option 2: Extravagent, high performance daily driver

A couple pictures of the new block attached.


Chuck Mosher

As I understand it, the 1800 is quite peppy for the MGA, and you wouldn't need to do too much to give it some real ummph. I would think that lightening the flywheel a tad and larger carbs would do something quite satisfactory. Maybe a larger exhaust to compensate for the increased volumn.
My two cents.
Mike Parker


It seems that you intend to use the 1600 M11 cylinder head on the 1800 engine.
A quick calculation using the MGA 1600 Mk11 combustion chamber of 43cc with the MGB 1800 bore of 80.26mm would take your compression ratio from 8.9 to 10.6 (the stroke is the same).
Calculation done assuming a flat top piston in both cases, I do not have any piston crown volumes.
A bit high for standard combustion chamber shape and pump fuel.

M F Anderson

Hi Chuck. The 5 main MGB engine is significantly better for high performance mods than the 3 main engine, however the 3 main engine bolts right into the MGA with few changes. I would recommend only light modifications for your 3 main engine so increase reliability and slightly enhance performance. Light porting and polishing of the head, silicon bronze valve guides, stellite valves and valve seats, new valve springs and proper valve guide oil seals, lightened flywheel, and perhaps a slightly hotter camshaft. The H4 carbs should probably be rejetted for the extra engine CC's The engine is pretty easy to rebuild. Just my 2 cents worth, Glenn
Glenn Hedrich

I did a replacement of my 1500 with a 3 main 1800 like you are anticipating. I used a lot of the 1500 parts to rebuild the 1800. Same manifolds, carbs, distributor, etc. Except for the machine shop work I did it all myself for the first time.

The engine rebuild is something that you can do without a problem with a good manual and advise from this forum. A machine shop can fix the block and crank as needed and a proper head can be obtained.

Go for an improved driver not a race car--if that's what you want to drive. Make it like you are going to drive it.


Jim Ferguson

Per MGBMGA./com, the MKII head is exactly the same volume as the early MGB heads.

Fletcher R Millmore


Yes, 43cc for the MGA 1600 MK11 and 42.5cc for the early MGB (as per both Workshop Manuals).
So the difference must be in the shape of the piston crown. That is the only way the bigger bore of the 1800 could give a similar (lower) compression ratio, 8.9 for the MGA and 8.8 for the MGB.
As I said, both crown volumes were the only values I do not have. Does anyone have those figures?

M F Anderson

I believe the 1622 was flat topped, and the HC MGB was 6.2cc. What the deck clearance was, or was it different, I don't know. Lot of detail on mgbmga.

Fletcher R Millmore

Thanks all for the comments !

I think Mitch and Fletcher are right about the head, I'm planning on a new head and pistons.

Jim - Thanks for the encouragement, a good driver is what I'm looking for.

I'll ask for advice again when I get closer to starting the work.

Best regards,
Chuck Mosher


You will need to use the front plate from your MGA engine. The MGB plate is a slightly different shape and the MGA engine bearers do not line up with all the holes.

Steve Gyles


An interesting upgrade on these engines, modify the rear plate to install a correct rear journal sealing as explained on MGA guru. And one more mod. to carry on this 3 main 1800: a dynamic cranckshaft balancing would be worth it.
And from my own, I would install a 18V larger intake valves head, ( 39cc volume chamber ) taking care to mill valves indents at cylinder top.

The 1622 head is identical to the MGB head except for the number cast on it. The difference is indeed in the pistons and the compression is the same.

I'm with Glen - don't muck with it, just rebuild it and bolt it in.

I do use the MGB diaphragm clutch in mine (easy to do).
Bill Spohn

Steve: Could you elaborate on your comment about needing to use the MGA front plate when installing an 1800. The info I have is the mounts were exactly the same distance apart and the same angle also (all MGAs and chrome bumper MGBs).
Dave McCann


You may be right from what others have said above about the 3-bearing 1800 engine. My engine is the later 5-bearing vwesion and the front plate is slightly differently shaped at the engine bearer hole locations. This results in the MGA engine bearer brackets not lining up. In fact, if my memory serves me right, one of the bracket holes is almost outside the periphery of the plate. I had made the (wrong) assumption that all MGB front plates were the same.

Steve Gyles

You want the MGA rear engine plate so that it matches the starter holes in the MGA transmission.

It bolts onto a 3 main B engine but if you use a 5 Main B, you need to bore it for a rear main seal.
Bill Spohn

Chuck, I thought that you had a Judson powered car. Are you going to use the Judson on the 1800?
James Johanski


My weakness is fiddling with the engines on these cars. To support my habit, I've decided to have two engines - one as original as I can make it (the 1622 with Judson), and the 1800 3-main. I'll customize the 1800 a bit, with a 5-speed conversion, street cam, and the Moss SuperCharger. I'll book some time on a dyno at some point and see if I can get some comparisons on some of the combinations.

I know, it's an affliction ;-)


Chuck Mosher

Chuck, if your idea is to put a Moss supercharger on the 1800, you'll want to do some research before rebuilding the engine. If I remeber correctly, Moss doesn't sell a supercharger kit for the 3 main 1800 due to fitment issues with the standard water pump.
Andy Bounsall

If using a 3 main engine always crack test the crank. DO NOT supercharge the 3 main - you will only break the crank. I have a better mod for a seal on the 3 brg motor than I have seen elsewhere. Early MGA's had a 28lb flywheel & this needs lightening to 22 lbs or less if desired. Be aware that some early back plates are cast iron & will break easily. Usually only fitted to Austin etc. If using a diaphragm clutch then you will need an MGB 3 syncro gearbox front cover.

Garth Bagnall

This thread was discussed between 20/03/2010 and 07/04/2010

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