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Technical Information (MGA 2)

Installing an MGB 1800cc (5 Main Brg) Engine in an MGA with its Original Transmission

Last Modified - 7/7/98

The following article describes a procedure for adapting the 5 Main Brg MGB 1800cc engine (18GB through 18V) to an existing MGA transmission. Although, we have not fully carried out the modification below, we have done all the preliminary work leading up to the installation of the engine into the car.

We see quite a few MGA's passing through our shop that have the MGB 1800cc 3 Main Brg engine fitted and mated to the OEM transmission. This modification is perfectly okay. But why not use the later 5 main bearing engine block? Unlike the 3 main bearing engine block, the 5 main bearing engine block can be increased to as much as 1950cc. (We suggest that you use British Automotive's 1924cc conversion).

MGA 1622cc engine block already has the advantage of being able to rebore to 1800cc. This article would then be more useful to the MGA owner with either the 1500cc or 1600cc engine.

The following conversion is relatively simple:

Retain your original rear engine plate and have the center bored to accept the 5 main rear oil seal part #88G621 (Moss #120-820). This bore diameter is approximately 4.125."

Make 4 spacers 0.750" OD x .225" Depth with 21/64" bolt clearance hole. These spacers will be used when installing the rear oil seal retainer plate, part #12H1547 (Moss #120-830).

If you are using one of the 18V series engine blocks, you may come across a crankshaft having only one flywheel locating dowel. If this should be the case, and since we are fitting the 18GB style flywheel, just use the one locating dowel.

The depth of the crankshaft bore and pilot bushing are different on the 18GB ---> 18V engine crankshafts (1.1255" to 1.126" OD x 1.5" long) than that of the 18V series crankshaft (1.255" to 1.256" x 1" long) Transmission pilot shaft OD .622". However, a custom oilite bushing should be installed for either crankshaft with the following dimensions: 1.1256" OD x 1.125" depth with 0.625" bore. Note that Moss # 330-570 bushing that could be used for this conversion will not fit 18V style crankshafts, unless machined to length. In addition, it is simply one bush fitted inside another bush, with the tendency for the inner bush to fall out and, thus, is not recommended.

Originally, we tried, unsuccessfully, to adapt the MGB diaphragm style clutch cover and throw out bearing assembly to work with the MGA transmission, However, since that time we have received customer feed back pointing out that it is indeed possible. We proceeded as directed and used the front transmission cover from the MGB non synchromesh series transmission along with the matching release bearing arm. We did all the necessary measurements and concluded that the only problem that may arise would be in the amount of release bearing arm travel required to release the clutch, after new clutch component replacement, before the clutch slave cylinder piston exited from the slave cylinder. We measured several MGB¹s for maximum pushrod travel (driver actually depressing the clutch pedal fully to the floor) and found this to be approximately 1/2². If clutch release does becomes a problem, then lengthening the pushrod by approximately (1/8²) should solve this.

To allow for the fitting of the bendix style starter motor we must grind away the excess casting material, which prevents the starter from lining up with the backplate. Also, it may be necessary to grind a small amount of the 3rd, from the rear, oil pan retaining bolt threaded Boss.

Obviously, there is no provision on the 5 main bearing engine block for the tachometer drive, so, use 65-67 MGB electric style tachometer, wire in and modify the bulb holder.

To allow for greater fan blade to radiator clearance, one of the later style water pumps should be used, either part #GWP117 (434-035) or GWP130, (434-045) matched with a used fan pulley part #CHM56 (434-325) and pulley spacer both from the MGB electric fan model (1977--> ). This will require that you drill the pulley to match your original fan blade mounting 4 hole pattern. Alternatively, you could secure the fan blade by only using the 2 additional holes that are provided in the blade. I personally prefer to use the 4 bolt securing method. You could also install plastic 7 blade fan part #12H4744 (434-340) without the spacer, which, should provide sufficient clearance, especially if you move the radiator as far forward as is possible with the use of packing strips part #AHH5144 (2) (296-110). Another alternative would be the 6 blade steel fan part #BHH1604 (434-330) as fitted to the MGB. A longer fan belt will be required such as Dayco part #15365 or Gates part # 7365.

In the event of a front end collision you may consider installing used MGB (68-74) engine mounting recoil brackets part #AHH7890 (413-075). This should prevent the fan blade from burying itself in the radiator if a collision should occur. Make sure that your transmission mount is in good condition i.e. not saturated with oil, as many are. Replace if necessary.

Either the 5² or 6² size crankshaft pulley can be used, just be sure you have the matching front cover so that the timing marks match the crankshaft pulley TDC mark

To resolve exhaust manifold fitment problems, use the LCB exhaust header part #EXL99. You will also benefit with a little more HP over the stock OEM cast iron manifold. Install a free flow exhaust system to take advantage of this installation.

The consequences of installing a 1800cc or larger capacity engine, with its potentially increased BHP and Torque, could result in premature transmission failure. Anytime we significantly increase engine power output the OEM transmission gears have to transmit these same power increases resulting in greater tooth surface and laygear bearing loads.

We are presently working with Bob West of Pontefract England to bring you his MGA 5 speed transmission conversion. However, we do not see this modification being available here in the USA until later this year at the earliest.