Introduction
There are at least two ways to install a MGB 1800 engine in a MGA:
- Fitting a 3-bearing-engine from an early MGB (easier way,but 3-bearing-engines are expensive and hard to find)
- Fitting a 5-bearing-engine from a late MGB (recommended, because the 5-bearing-engine are known as the smoother and physically stronger engine)
Fitting a 3-bearing MGB engine:
- Remove MGA engine
- Change the backing plate from the the MGB engine against a backing plate of a MGA 1500, 1600 or 1622 engine (backplate has to fit to your transmission housing - position of the starter!)
- Change the flywheel of the B engine against the MGA engine flywheel and clutch
- Refit the modified MGB-engine
Fitting a 5-bearing MGB engine:
- Remove MGA engine
- Again the backing plate of a MGA 1500, 1600 or 1622 engine (backplate has to fit to your transmission housing - position of the starter!) is used
- Machining the backplate: Machine the rear crank hole to a diameter of 4.125" to accept the five main rear oil seal (measure the outer diameter of the seal BEFORE the plate is bored!)
- Change the MGB flywheel against a MGB flywheel from 1965 - 1967 (crash starter)
- Making a distance bush for crankshaft bore and pilot bushing (inner diameter 0.625", outer diameter 1.125".)
- Clutch: Fit an additional locating dowel, if the MGA clutch should be used (pay attention to the exact position, otherwise you will have balancing problems); if the MGB clutch should be used, the MGB gearbox front plate and the corresponding fork must be used, too (MGB clutch pieces are known as less expensive and better engineered than the MGA pieces)
- Rev counter: Because there is no mechanical drive fpor the MGA rev counter, use an electronic rev counter from the MGB. Take care of postive or negative ground while installing the rev counter.
- Distributor, carburetters: MGA parts can be used, but I personally would fit HS4 or HIF-carburettors and a 25D-distributor with the matching ignition curve.
If HS4 or HIF carbs with K&N filters are are used, check the needles (use richer needles: #6 for HS4 fixed needle, ABD for HS4 biased needle, AAA for HIF4 biased needle)
- refit modified MGB engine
Fuel for thought
The new engine will not solve rear axle problems - you have more power, but not more relaxed driving. Install a longer rear end (4.1, better3.9) or a 5 speed gearbox (will be added to this site later).
The MGB has a larger radiator. A recored MGA unit will work most of the the time, but if overheating is a problem, use an early B radiator, it will fit.
If you are still at work, you can swap to a stageII or stageIII cylinder head.
Alternatives
A well machined stageII or stageIII head will increase the power output by 25 to 30 percent, so your original MGA 1500 engine will come in the region of a MGB engine.
MGA 1500 or MGA 1600 engine can be replaced by the MGA 1622 engine. This engine is known as powerful (90 bhp) and revs pretty well. No more changes are necessary!
You can rebore the MGA 1622 engine to 1800 cc.
And please notice: in some states it is not legal to drive on public roads with power tuned cars.
