ZB Magnette Engine/Transmission Swap and…
By Allen
Bachelder
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A number of Magnettes have been converted to MGB 1800 engines with vast improvements in performance. Similarly, vast improvements in drivability have been accomplished by switching rear end ratios from the original 4.55:1 to either 4.3:1 or 3.9:1. Providing an overdrive can make further improvements.
The rear end swaps are very
easy and can be accomplished in about two hours. My preference has been for the 3.9 – as found on the
“banjo axle” MGBs built prior to 1968. One need only to remember to retain the spider gears from
the ZB rear end as the half-shaft splines on the ZB are not compatible with the
MGB ones.
A fair amount has already
been written about 1800 engine swaps. However, not all the problems I
encountered have been addressed elsewhere. I will address these problems here.
In considering the
alternative configurations, one must consider goals. Considering the ZB’s comfortable ride, and excellent
handling and braking, I chose to upgrade the car so that its comfort level
could hopefully be sustained up to 70-75 mph. The performance of overdrive-equipped MGBs was our
model. We therefore chose a 5-main
1800 MGB engine, together with a Rivergate conversion kit, designed to mate a
Datsun 280Z 5-speed transmission specifically to the MGB 5-main engine, and of
course the 3.9 rear end. Three
main MGB engines offer the same power and much simpler installation, but the
five-main engines are extremely durable, therefore worth the additional
problems encountered. Because
comfort was our main goal, we also installed seats with more back support, and
air conditioning. These topics will be covered in Part
Three.
Why not an MGB overdrive
transmission? It’s too big
and thus requires substantial alteration to the transmission tunnel. The Nissan box is more compact, sturdy
enough to handle the 6-cylinder 2.8-liter Datsun engine, and it is readily
found in the US. Some owners have
used the MGB overdrive transmission and only they can speak to the problems
encountered in doing that conversion.
Choices for the five-speed conversion are the Ford Sierra conversion
marketed in the US by Butch White, the Nissan 280Z conversion kit made by Novus
in California, and the kit I used made by Rivergate Restoration Products in
Soddy-Daisy Tennessee.
My rationale for choosing the
Rivergate kit ran as follows.
First, either the Sierra kit was not yet available in the US or I
didn’t know of it. I had
read the account of a Sierra conversion in the UK on the MGCC bulletin
board. It rather intimidated me at
the time, and I knew nothing of the Sierra box itself. Second, and this is
rather fortuitous, Soddy-Daisy is only about a half-day’s drive from my
home and I could go down there and meet the people who build the kit and view
the components firsthand. I did
that, and I was so impressed that I chose the kit on the spot. In fairness to others, this is in no
way a criticism of the other kits; it is simply my account of my choices and my
experiences.
Having said that, I was
impressed with the people behind the Rivergate kit. Bill Perry and his son are MG folks –having raced
Midgets for a number of years.
Their shop is incredibly clean and well organized. Their kit is beautifully executed. I came to regret hiding their backplate
between the engine and transmission; I would rather hang it over the fireplace
as an objet d’art – right
next to the refinished ZB dashboard I received from Lou Shorten. They were extremely cooperative and
anxious to see their kit employed in a Magnette. I felt I couldn’t go wrong with that kind of integrity
and enthusiasm behind me.
Experience has since shown that these feelings were right on
the mark.
Part One: The Engine
Problem number one is that
the well in the Magnette engine oil sump is in the forward part of the sump
rather than the rearward part as in the MGB. This is due to the relative locations of the steering cross
members of the two cars. It runs
under the forward part of the engine in the B, but under the rearward part in
the Magnette. Therefore, the 1800
engine will have to use an original Magnette oil sump. Now mind you, this is
not just one problem. You
don’t just swap the two sumps. The oil pump feeds from the bottom of the
well in both engines, so the Magnette oil pickup and strainer need to be
employed also. Furthermore, the
dipstick must read the amount of oil in the well, so the rearward dipstick on
the MGB 1800s is now useless. It
has to be replaced with one under the dynamo that does drop into the Magnette
1500 well. I had been told that
the three-main 1800 as found on very early pull-handle MGBs (up through early
1965) yields the simplest solution to this problem, because first, it utilizes
the same bolt pattern for the sump, making the sumps interchangeable; second,
because the block retains the threaded mounting holes for the Magnette oil
pick-up; and third, because the hole for the forward dipstick is already
drilled – one needs only to remove the plug.
In fact, I did locate a
three-main 1800 engine for this conversion but ultimately rejected it as I did
a 1622 MGA engine, simply because I wanted the durability and longevity of the
five-main engine. I am sure others
will argue this point with me – that perhaps three-main engines are just
as durable. To those I can only
answer that I went into this project with that preconception and acted
accordingly. I have a lot of
experience with five-main MGBs and I do know that these engines are extremely
rugged. That does not mean the
others are not. In fact, if somebody convinces me otherwise, my resident
three-main engine may perhaps end up in my next Magnette project! However, there is another consideration
here: the Rivergate kit is designed to fit the five-main block with the rear
main seal residing in the backplate.
I had a rebuilt 18GB engine
on hand. These were used in
1965-67 MGBs. Among the five-main
B engines, these would be ideal for use with the Magnette box, by the way, as
they provide the five-main durability with the earlier backplate and starter
configuration. But they do have
the rear main seal in the backplate, so they are compatible with the Rivergate
kit. They are also, in
“stock” high compression configuration, the most powerful of the
five-main engines – at 98 horsepower – a substantial boost from the
ZB’s original 68 horsepower 1500 engine.
Problem number two: the sump
bolt pattern is not compatible with that of the must-use ZB sump. Number three: the block is configured
such that not only are there no holes to attach the must-use ZB oil pickup,
there’s no block material in the neighborhood into which to drill the
holes! Number four: while a boss
is cast into the block where the dipstick should go, it is not drilled, and
that is an intimidating task.
In
Illustration #1 we see the bottom of the ZB 1500 engine from our parts
car. Note how the oil pickup
strainer and brackets bolt to the front of the block
surface.
Illustration #2 shows the
same engine with the oil pickup removed. The boltholes are circled in
“White Out” so as to make them more visible. Keep in mind that this part of the
block surface does not exist in the five–main
blocks.
Illustration #1
The issue, then, is to determine a way to
install the oil pickup in the five-main block. Jumping ahead just a bit, I ran across another problem I had
not seen addressed by anybody else.
To connect the ZB oil pickup to the oil pump, one needs to swap out the
bottom of the 1800 oil pump with that from the 1500. In these illustrations, this bottom piece has
already been removed
from the donor engine.
Illustration #2
But when swapped out, and after the bolt pattern on the ZB
sump has been adapted to the 1800 block, one discovers that the 1800 oil pump
does not clear the shallow end of the ZB sump. It’s close and I figured that with a slightly shorter
outboard mounting stud for the pump, it might just clear. But that solution didn’t rest
comfortably. My solution was to
design a 1/4” thick steel plate shaped very much like a sump gasket but
providing the boltholes for mounting the oil pickup, while moving the sump down
1/4” so as to comfortably clear the bottom 
of the oil pump – even with the
original outboard stud still in place.
This plate is shown in illustration #3. Note the four bolt holes in the
inner “ears” of the plate toward the top of the illustration. To
allow just a little adjustment room, 1/4” hardware (with flat washers)
replaced the original 5/16” hardware for mounting the pickup.
An alteration to the plate
was later found necessary: the dipstick would not clear so it had to be drilled
for it in between the oil pickup mounting holes on the right-hand side of the
engine.
A major concern was whether
the plate would allow sufficient clearance between the sump and the
crossmember. At this point, the original 1500 engine was still in the car, so I
cut scraps of 1/4” and 3/8” plywood and tried to slip them between
sump and crossmember, thus determining the original clearance – which
appeared to be just slightly on the plus side of 3/8”. This is with
engine and transmission mounts of unknown age and deterioration. I simply took a
chance!
Illustration #3
Illustration #4
In fact, the
new arrangement did clear the sump but of course the clearance was
tighter. Illustration #4 shows the
solution to that concern. Some
large fender washers –two inches in diameter - were placed on the front
engine mounts. Two of them on each side brought the engine up 1/8” and
once again the 3/8” plywood scrap slid between sump and crossmember with
a little zone of comfort.
Our steel plate also served
another useful function in providing a handy template for relocating holes in
the sump flange. First, it is
comforting to know that four of the holes do line up – one of them being
the center front hole. Bolting the
plate to the sump flange at these points holds the assembly in proper position
for drilling the additional holes and marking those no longer needed to be
welded shut.
Illustration #5
Illustration #6
In Illustration #5, we see a comparison of
the two sumps. The Magnette sump with the well toward the front is shown on the
left. The MGB one with the
well at the rear is on the right.
This picture was taken after the new bolt pattern had been drilled in
the Magnette sump. Note
that on two
locations: the left front corner (as shown) and the forward bolt hole in the
oil pump “bay” on the far left of the picture, the edge of the
flange had to be trimmed as there is insufficient material to enclose the new
hole. But for aesthetics, this is not really a problem, but I did solve it by
welding in some little extension pieces on the flange. It is shown in
illustration #6. Since the sump is facing the other way here, the
“extensions” appear on the right in the illustration. Note the back three holes marked with
“White Out” (visible at the bottom of the illustration) are the
three original holes in addition to the front center one that line up with both
blocks.
In
Illustrations 7 and 8, we see how the steel plate functions to mount the
Magnette oil pick up to the five-main 1800 block. And finally, illustration #9
shows the completed steel plate “sandwich. I used two sump gaskets and clear RTV on both
joints.
Illustration #7
Illustration #8
Drilling the hole for the new
dipstick location is scary. Note,
by the way, that the dipstick tube does thread into the block. Those two grooves at the top of the
tube are there for a purpose: placing a large screwdriver in them allows one to
turn the tube out of the hole. It
is a 1/8” BPT, which, courtesy of Mike Asch, I learned is “British
Pipe Thread” and an American 27 TPI tap is so close that once threaded,
the tube will snug several revolutions into the hole before fighting back. It is all too tempting to drill the new
hole perpendicular to the face of the boss, but don’t do
it! I made
an adjustable drill guide from a Midget rocker platform on a bracket that I
made to bolt into two of the sump boltholes
from the topside.
It did hold
the drill true. But the trick is
to set it correctly in the first place.
It is quite critical. If
the angle is too shallow, the dipstick will foul the pickup strainer. A little to steep and it will come
through the sump-mating surface at the bottom. All I can say is I got very
lucky. This is one part of the job
you might want to entrust to a machine shop. Also, this is best done while the
engine is being rebuilt – before bottom end is assembled. That way, one can be sure of keeping
metal shards (from the drilling) out of the bearings, etc! As it was, I dealt
with an engine that had been previously rebuilt. Needless to say, I masked the area in plastic and duct tape
very thoroughly. And I smeared a lot of grease around the area of anticipated
drill emergence – to entrap the shards. It is amazing how easily the cast iron can be drilled. I always oil my bits when drilling in
metal, but here it was particularly important as the oil also kept shards from
flying.
Illustration #9
Even though I
was lucky, it was necessary to extend my dipstick rest platform (in the bottom
of the sump well), as my relocated stick did not quite line up with the spot
where the factory wanted it. Illustration #10 shows my extended platform. Note
the imprint of the pickup strainer on the bottom of the well, and how close it
is to the dipstick rest. This illustration also clearly shows my homemade sump
flange extensions on the right side of the picture.
These are the major engine problems, but there are a few smaller ones that one should be prepared to meet. First, the 18GB engines used a longer water pump. I’ve read somewhere that the five-main engines are actually longer, but I didn’t notice any difference except in the length of the water pump “nose”. At the time, I was anticipating needing an extra pulley to drive an A/C compressor, so I switched over to a pump designed for an early ‘70s B with the spare pulley for the smog pump. It turned out I did not need the second pulley, but I did need the shorter length of those pumps so equipped. Yes – two pulleys but shorter pump! In other accounts of this conversion, I have read that in no case will the original 1500 pump serve the 1800 engine – even though it may physically fit. So don’t even consider a 1500 water pump. It’s also a good idea to use a ‘70s B plastic radiator fan.
Further down the line, there is another small constellation of problems. What kind of exhaust manifold or header? Know right up front that nobody makes a header that will fit the Magnette. If you want one, it will have to be custom built for you. The MGB headers and manifold snag all over the place on Magnette steering gear. Not even close. My solution was to use the original Magnette exhaust manifold. And by the way, the 1800 engine fits all the Magnette ancillaries – manifolds, carbs, intake plenum and air cleaner, generator, distributor, etc. This engine swap can be done so the only way an observer knows you have an MGB engine is by looking at the embossed “1800” on the lower left front of the block. Yes, the engine number would give it away also, but that can be removed or changed. My car, however, had so many obvious modifications, I did not concern myself with deceiving the observer – although I do take some pride in the close resemblance of the engine appearance itself.
Back to the exhaust manifold… While I did replace the entire exhaust system with 1-3/4” pipe – for more than one reason as we shall see – the Magnette manifold allows the 1800 to breathe quite well, and of course the finished job looks more original.
But… the Magnette exhaust manifold will not clear the MGB front tappet cover. Save the front tappet cover from the 1500 and re-use it. This means using the original Magnette crankcase ventilation. The vent pipe is held in place with a bracket to one of the studs holding the blanking plate over the hole for a mechanical fuel pump. Now this hole/blanking plate/stud configuration disappeared on the 18GB block. It returned later on – when I don’t know, but those using 18V blocks will find it just where it was on the 1500. On the 18GB block, I had to make an additional bracket to support the pipe from one of the sump bolts.
Part Two: The
Transmission
If placing another MG engine utilizing the same basic block and ancillaries requires almost 3000 words and ten illustrations address the problems, what is it going to be like to put in that Datsun transmission? Here I will speak only for the kit I used. But Rivergate solves just about all the problems for you. The kit utilizes their own – beautiful – backplate. They also supply a modified five-main flywheel and, of course, a correct pilot bushing. The kit utilizes a late-model MGB starter, a late model MGB clutch cover. You use a cold chisel to knock off the piece on the rear intended for the carbon throw-out bearing, because it uses the Datsun roller-style throw-out bearing (and slave cylinder). Since the kit is designed to work with the late model MGB starter, I took this opportunity to go a step further and use one of the new smaller gear reduction starters. Everything goes together just as easily as assembling an MGB engine to an original transmission.
Illustration #11
Illustration #12
Illustration #11 shows all components installed – ready to accept the transmission. The new starter did create a bit of a problem as it snagged the steering crossmember, but I discovered these could be adjusted rotationally to three different positions and one of them worked perfectly.
Illustration #12 shows the completed assembly. The gear lever is attached just for
amusement! We are now ready to see if this will actually fit in a
Magnette! (Illustration #13)
Well, indeed it does fit very well. There are some small problems. The top rib of the Datsun transmission
comes excruciatingly close to the top of the opening of the transmission tunnel
at the bulkhead. One solution is to file away a bit of that rib, but the actual
clearance problem is 
not the top of the tunnel itself, but rather
the overhang of the bulkhead material into the tunnel opening. I confess I fixed that with a crowbar
– bending the overhanging bulkhead material back into the tunnel.
The other problem concerns
the distance from the bell housing front surface to the shift lever. It is perfect for replacing a 4-syncro
late MGB transmission, but in any MG originally outfitted with the earlier
3-syncro box, the Datsun gear lever is going to come up about 3+ inches further
back. Rivergate made up a gear
lever offset forward about 1-3/4” (see Illustration #12) and that took
care of half the problem as shown in Illustration #14. Illustration #15 then shows the
solution. I trimmed back the
opening in the transmission tunnel – cutting back 1-3/4” by
2” wide. In illustration
#15, the cut appears to be much larger than this, but what is seen in the lower
foreground is the piece of carpet folded back away from the cut. This was a small cut to make on the
car. I actually saved the little
piece so it could be welded back in should anybody wish to take the car back to
original configuration.
Illustration
#13
Illustration
#14
Illustration #15
Beyond this,
everything connected with the engine/transmission swap was totally uneventful.
The original Magnette drive shaft fits with no change in its length.
The front U-joint yoke is
reamed out slightly to accept a Datsun U-joint, which connects to a Datsun
front yoke and transmission spline.
The process is certainly as easy as installing the original parts.
Since the 1800
engine requires
a bigger diameter exhaust system anyway, it is of little concern that the old
exhaust system will not clear the new transmission crossmember that Rivergate
designed and built for me (while I
waited in their shop!). It is flat on the bottom rather than
arched, as the original is, to accommodate the original exhaust
pipe.
I ran into a small problem connecting
Rivergate’s clutch line to my Magnette master cylinder. The adaptor fitting they supplied did
not quite catch the threads in my MC.
In their defense, it appeared that I was dealing with some damaged
threads and I bought another adaptor as a local auto parts store that had a
longer threaded barrel. This
worked fine. The clutch was then
quite easy to bleed and it works very smoothly 
Illustration #16
with modest pedal effort. Illustration #16 shows the adaptor, and #17 shows the connection to the master cylinder.
#Illustration #17
Part Three will later
describe some other changes I have made in this car. They include air conditioning, 1987 Mercedes Benz high-back
front seats with head restraints, and three-point seatbelts. Also, in honor of the electrical
demands of the AC system, I have installed a Delco alternator.
This should not be interpreted as a
rejection of Lucas
electrics – which work fine
– particularly considering their age. It was the 63 amps of power for
around $40 that lured me.
Note that the entire
installation, as well as removal of the original engine and transmission, goes
much more smoothly if one removes the horns and heater blower unit from the
engine bay.
The A/C system has just been
finished, and the car now runs and drives superbly. Much to my driving delight,
it does seem to cruise quietly and effortlessly at 70-75 mph. I am very pleased with all the
components I selected and would strongly recommend them. For those interested, I’m happy
to share more details of my experience to anybody who inquires.
The design of my steel plate
in Illustration #3 has been preserved in AutoCAD so I can make it available at
cost to anybody interested. My
guess is that it will cost somewhere in the neighborhood of $50-75. My
production cost for the first one was about $200.
To lure you on to Part Three,
I’ll leave you with what I hope will be an enticing photograph as the
work was still in progress. Look for Part Three in another couple of
months! First, we have to finish
this car and drive it from Virginia to Oregon for the upcoming North American
MGA Register GT 28.
Allen Bachelder
bachldrs@swva.net
June 30, 2003