On Sun, 26 Oct 1997 21:43:08 +0000 john w murphy
<murphys-law@worldnet.att.net> writes:
>I'm planning on putting an 1968 MGB 5 main motor in my 1959 MGA
roadster. I'll be keeping the MGA transmission, but using an MGB clutch.
.....
I've done most of this, although not for myself.
The first thing you need to put a 5-main engine in an MGA is the small
flywheel (10 inch, 120-tooth ring gear) from the 5-main MGB engine that
was mated to the 3-syncro gearbox from 1965-1967 (and perhaps late 1964).
Second, since you want to use the MGB clutch, you need the MGB clutch
pressure plate. This will bolt up directly the aforementioned flywheel,
so nothing special there. For spirited driving, I can personally
recommend the MGB competition pressure plate. I ran one of these (and
ran it hard) for three years and 50,000 miles with just one clutch disk,
and the disk had no noticeable wear (but the lining finally came loose
from the rivets). The competition pressure plate requires more pedal
pressure (about 50% more), but you can get used to it pretty quickly.
And, it's capable of spinning the tires in third gear (with street tires,
not sticky race rubber), which is why the clutch disk doesn't slip and
wear out.
Third, you need to use the MGA clutch disk, as it has to match the
splines on the gearbox input shaft. This is much easier than changing
the input shaft to match the MGB disk. As a special note here, ask any
auto parts store to get you a clutch disk for a Studebaker 6-cylinder,
1926-1960. It had a generic description 8-1-10, which means 8" disk, 1"
hole, and 10 splines. This one drops right in, should be cheaper than
the "real" MGA disk, and is somewhat more robust (nice, nice, and nice).
One more caution. The later MGA 1600-MK-II gearbox uses a 23-spline
input shaft and clutch disk, the same as the MGB.
Fourth, since the MGB pressure plate is thinner than the MGA pressure
plate, you need all the rest of the MGB parts in front of the face of the
gearbox. You will need to obtain from a 3-syncro MGB gearbox, the
gearbox front cover (pivot point is different and has a nice rubber
seal), the MGB clutch release arm & pivot bushing & pivot bolt, and the
MGB clutch release bearing & retaining clips. All of this bolts right up
the MGA gearbox and can be used with the MGA clutch slave cylinder and
slave push rod. However, you may need to file or grind a very small
notch at the front end of the boot opening in the MGA gearbox bell
housing to get the MGB release arm into place. As I recall you need to
slip the release arm into place as the front cover is being installed.
Fifth, the MGB 5-main clutch pilot bushing (spigot bushing) in the rear
end of the engine crankshaft is larger than the bushing in the MGA & MGB
3-main engines, both ID and OD. The MGA and 3-main MGB bushing is 5/8"
ID. I do not recall the OD of the 5-main bushing (anybody?). The
solution here is to go to your local bearing supply house and obtain a
bronze bushing with 5/8" inside diameter and the outside diameter to
match the 5-main bushing. 660 Bronze (bearing bronze) is commonly
available and works nicely with a little grease at installation.
Otherwise you may be able to find a porous bronze Oilite bushing with the
right dimensions. In either case you will likely have to buy a long
bushing and cut it down to the right length (to match the MGB 5-main
bushing length). If you buy the right length to start with, you can end
up with two useable pieces, one for the installation, and one to keep for
a spare.
Sixth, the MGA 1600 has a high starter position similar to the MGB. The
MGA 1500 has a low starter position. You will need to use the engine
rear plate from the MGA to match the MGA gearbox. You may be able to use
the engine rear plate from a 3-main MGB engine with the MGA 1600 gearbox,
just a guess, can anyone else comment on this?
Seventh you may have a bit of trouble fitting the MGB starter motor to
the MGA, as the MGB starter motor is a bit larger. You can use the MGA
starter motor, and it will work well in most cases if you keep the
electrical system in good condition. The MGB starter from 1968 and later
engines, having an attached mechanical solenoid and actuator arm,
probably will not fit with the MGA gearbox (my guess). The 1963-1967
starter should be a direct bolt in, but ..... in some MGAs you may need
to grind the edge of the MGB starter mounting flange a bit to clear
something on the engine block. Then again, this modification may only
apply to an MGB starter with an MGA engine. I haven't actually done this
bit, just repeating things I've heard.
Eighth, the MGB 5-main engine does not have a mechanical tachometer
drive. So you need an electric tach. And, to keep appearances in order,
You want to obtain the electric tach from the same model MGB as the
aforementioned flywheel, 1965-1967. Follow the wiring diagram for the
MGB when installing this tach. If you have converted your MGA from
positive earth to negative earth, this tach can also be converted from
positive earth to negative earth, just ask. But don't ask me, I have one
but haven't needed to use it yet.
Ninth, you need to use the water pump from a 5-main engine, and it should
be the short nose pump, I believe also from the 1965-1967 MGB engine, but
someone may jump in here if I err. I think the pulley mounting flange on
this pump shaft sits about 1/4" farther forward than on the MGA pump.
You should then use the MGB pump belt pulley to match so that the belt
lines up with the crankshaft. Rumor has it that you may also have to
shim the radiator forward 1/4" to clear the fan, but I didn't notice this
on the one installation that I helped with. I have seen a description
for a fix for this (rumored) problem. That fix is to machine 1/4" off
from the front face of the MGB water pump front flange, and then use the
MGA water pump pulley. I don't know if this is required, but it bears
investigating.
Tenth, you need to use the water outlet elbow (thermostat cover) from the
MGA or MGB 1956-1967. This also pretty much precludes using an air pump
without some serious custom bracketing, so I'd advise converting any post
1967 engine to pre 1968 emission standards. This can involve changes to
timing cover, tappet cover, valve cover, intake manifold, air cleaner(s),
and other possible parts as well. Given my "druthers", I would rather
start with a 1964-1967 engine.
Eleventh, you can use either the MGA or MGB carburetor setups. The twin
SUs used on the MGA are the same size, 1-1/2" throat (not same part
numbers), as the MGB SUs, and will have the same breathing capacity. If
you use MGA carbs on your 5-main engine, you need to consider the
following paragraph.
Twelfth, emission setup on the MGA and 3-main MGB (1956 to early 1964)
was limited to a breather hose front the valve cover to the front air
cleaner, and a breather pipe on the front tappet cover. The early 5-main
engines (1964-1967) did not have this hose, but connected a hose from the
front tappet cover to a PCV valve attached to the intake manifold.
Starting in 1968, MGB 5-main engines had an air pump and other additional
parts, much of which is not readily adaptable to an MGA. I suggest you
revert to the MGA or MGB 3-main emissions setup for simplicity sake.
Thirteenth, you can use the MGA exhaust manifold and the rest of the MGA
exhaust system in its entirety. Some people say that the MGA exhaust
system un dually restricts the performance of the larger displacement MGB
engine. But, it will run just fine with this setup, the drive ability
will also be just fine, and since you're going from 1500 or 1600 cc to
1800 cc, you will be getting a substantial performance gain, and will
likely not be concerned with the small loss caused by the MGA exhaust
system.
Fourteenth, If you do want to use the MGB exhaust manifold in the MGA, be
aware that it may hit the steering column. The fix for this is to mill a
slight angle on the face of the manifold where it mounts to the cylinder
head, something like 1-1/2 degrees as I recall (but that may not be quite
right), in order to move the bottom end of the exhaust manifold a little
towards the engine. You will also need to have a new front pipe formed
to adapt the MGB header to the MGA exhaust.
Fifteenth, of course this is also a good opportunity to install a
complete high performance exhaust system. And as a last note, and others
may chime in here, think "long center runner" for the exhaust manifold or
exhaust header.
Now listers, did I miss anything? Or would you perhaps like to add some
more steps to make his venture more complicated?
Barney Gaylord
1958 MGA with an attitude (and still with 1500 engine and stock exhaust)
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