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MG MGB Technical - Austin 1800 block

Anyone know if there are any differences in the Austin 1800 18V block and the MGB block, apart from the fuel pump hole?


Colin Parkinson

Engine to gearbox on the 1800 bolt size cant remember if they are 5/16 or 3/8 mgb sump bolts 1/4"
Andy Tilney

The pilot bushing, in the end of the crankshaft, is slightly smaller than the 18V. RAY
rjm RAY

Ray, crank is knackered, it is the block I was asking about.

Colin Parkinson

I ran a 1800 block in my midget hillclimber a few years back
The differences are----
Different bolt pattern for the sump-but the flange on the block is the same and can be redrilled and tapped to suit the rwd sump/oil pan
There is an extra hole in the block where the oil pump bolts on, as the fwd 1800 gets it's oil supply through this hole from the gearbox-- It just needs blocking off with a little welsche plug or threaded and plugged. There isn't any pressure there so a tap in plug is all that's needed
The fwd oil pump is different as it normally pulls oil from this hole-- A good trick is to plug the hole in the block then use the fwd oil pump but with a rwd end plte/pickup--you get a better feed to the oil pump as it feeds both ends because of the drilling through the pump housing
Not sure about the bolt sizes for the rear plate, but I can't remember an issue there
William Revit

I've done a few, but it's a while...
adding to the above:
I think there was one different bolt position on the backplate.
The dipstick is in a different position
And there is a difference around the rear main beaing cap, oil drain holes which need plugging so the gasket has something to land on.

Willy, that's right about the oil pump pick-up, but you can't be too careful about blocking off the 1800 intake hole. I inherited a block which had a history of run bearings, turned out that blanking plug wasn't a good enough fit and was leaking a smidgeon of air.

Ray, all the 1800 spigots I have seen were the same size as the B. The Marina had a smaller one. In fact the 1800 gearbox is basically the same as the 4 sync B with enough detail differences to make many parts not interchangeable. And it was in the sump via drop gears of cours.
Paul Walbran

Ray be thinking Marina, as in the US it was badged as an Austin.

Dave O'Neill 2

Paul, Dave, you're both right. We got different names for everything over here. Keeps us on our toes. RAY
rjm RAY

From an article I put together regarding transplanting a B series 1800 engine into an MGA. (I used an 18V engine, so the information is from someone I know who used an Austin 1800 engine for this purpose)

Austin 1800 Engine

This is a commonly employed engine in this country for an MGA 1800 conversion. The Austin 1800 was quite a large selling model in Australia in its time, but is not highly sought after today by collectors. Hence the engines are plentiful and cheap. However the work required to adapt a (front wheel drive) Austin 1800 engine is considerable, far more involved than with an MGB sourced engine. Expert engineering assistance is likely to be required in modifying the engine to make it suitable for use in a rear wheel drive MGA.
Additionally one needs to be aware that the Austin 1800 crankshaft was cast and not forged and the camshaft is ground with a milder valve timing and will need to be changed for an MGB camshaft.

Inlet and Exhaust Manifolds:
These are of course completely different, and the MGA items will need to be substituted.

Apart from the valve timing being different to the MGA/B in the Austin 1800, the Austin also has a lobe near its rear end to drive the Austin's mechanical fuel pump. Simply substitute an MGB camshaft.

Fuel Pump:
The MGA and MGB utilise a remote electric fuel pump. The mechanical fuel pump on the Austin 1800 will foul the exhaust. The pump and its actuating pushrod needs to be removed, and its hole in the block blanked off.

Left Front Engine Mount:
On the Austin block, the left front engine mount support bracket has provision for only two rather than three mounting bolts. To drill and tap for a third bolt carries serious risk of entering the nearby oil gallery. The safer option is to use just the two bolts and to modify the bracket by cutting off the redundant part.

Dip Stick:
The engine oil dip stick is in the wrong place to suit the MG sump. A new angled hole has to be drilled through the side of the crank case and the old hole has to be blocked off.

The front wheel drive Austin 1800 had its transmission bolted to the flange of the underside of the block. This flange is more substantial in the Austin for this reason, and also has larger bolt holes. Either the MG sump bolt holes will need to be enlarged to the larger size bolts, or the Austin engine bolt holes will need to be helicoiled to take the MG size bolts.

Oil Galleries:
Two 5/8” steel plugs will require placement at the rear of the engine block to close off the open oil galleries at this site.

Engine Back Plate:
If you thought things were fiddly so far, “you ain't seen nothing' yet”!
The Austin 1800 does not employ a back plate. There are several MG back plates depending on the model. It would probably be easiest, if using an Austin 1800 donor engine, to use the original back plate for your MGA. The machining of the back plate to fit the MGB seal needs to be very accurately performed, probably best performed by a suitable engineering works. But even after this there are potential pitfalls. If the backplate and seal are not accurately centred on the crankshaft centreline the seal will certainly leak. The back of the Austin block lacks the locating dowels of the MGB block making such accurate location of the plate and seal very difficult.
Ian Cowan who provided the information regarding the Austin 1800 transplant suggests a good way to centralise the plate to the engine, is to have a collar turned up with its O/D equalling the O/D of the seal's hole in the engine plate.
The I/D then has to be a neat fit on the crank shaft where the seal runs.
With the collar in place--seal up the oil joint between the plate and the engine block, to prevent any oil leaks.
The rear plates bolted up position to the engine block can be preserved, by drilling 3 holes through the plate and into a suitable location at the back of the engine. These holes must me perfectly true.
Suitable sized roll pins can be inserted into the deep holes. These will serve to locate the plate in the correct position, so assisting its accurate re-positioning, when it is removed next time.
T Aczel

Thanks everyone.

Got all my answers there.

I hope!

Colin Parkinson

This thread was discussed between 02/11/2016 and 10/11/2016

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