I have owned a lovely little 1934 MG PA that I purchased from Barry Walker in 2016. The car had previously been lovingly restored around 25 years previously with workmanship of the highest quality. I have driven the car for the past 4 years and enjoyed every minute of it. However, in spite of having twin SU carburettors, a 4-speed gearbox and not being particularly heavy, the little 847cc engine needs to be revved to get the best performance out of the car. In particular, it can struggle on long hills ….of which we have plenty in West Sussex; I probably wouldn’t care if I lived in Norfolk.
Anyway, I decided to put a supercharger on the car and
acquired the important parts from Barry Walker – the Marshall supercharger, a
new SU carburettor, boost pressure gauge, oil feed regulator, front pulley and
jockey tensioner, belts and some bracketry. All of these are nice shiny new
parts ….see picture below which shows all the bits except the SU.
As I have got into the habit of taking plenty of pictures
during my motorcycle projects and adding them to this blog I have repeated that
process here and in some detail. This may or may not be of use to anyone also
wishing to fit a side-mounted Marshall supercharger. Whether I have done this
project in an optimum way I have no way of knowing …this is just documenting my experience.
To avoid this blog turning into a rambling monologue (although
that seems to have happened anyway….) I have divided the Fitting of the Supercharger – Part 1 - into 4 main parts,
namely: Initial Mounting of the Supercharger; Fitting the Carburettor; Fitting
the Front Pulley; Fitting the Front Support Plate and Final Assembly. Part 2
covers Initial Start-up and Performance of the Car.
Mounting
of the Supercharger
The first step was to remove the bonnet, the tie rods
between the radiator and the top of the bulkhead, the apron at the front of the
car and to take the radiator out. It is not strictly necessary to take out all
of these parts at this stage but they need to be removed later so it’s just
easier to make space to work at the beginning.
At the “supercharger end” of things, there are
essentially 4 major components; in order from the engine these are: the pancake
manifold sandwiched between the engine intake and the supercharger; the
supercharger itself; a small right-angle elbow between the supercharger and the
carburettor and, finally, the carburettor.
Attaching the pancake manifold: The standard twin-carb
manifold is held on with 5x 5/16” studs. 3 slightly longer new ones were made
for the inner 3 and the outer studs slightly shortened to fit the new manifold.
A new gasket (flexoid) was made at the same time.
A brief note on materials: for all structural studs/bolts/nuts etc I use high tensile steels or components. In particular, all studs that I make are either EN24T (for 3/8” diameter) or silver steel (not heat treated, ie “as received” condition for 5/16” diameter). Why different steels? I can easily find 3/8” and larger size sections in EN24T but I can’t find 5/16” in small quantities in this steel hence the use of silver steel. Silver steel in this condition is still twice as strong as EN1A and more than equivalent to EN8 ….quite adequate for this purpose.
A gasket was then made for the supercharger-to-pancake
manifold (large hole with 4 studs)
When the pancake manifold was offered-up to the
supercharger it was found that the 2 lower studs fouled the aluminium cam
cover.
These studs were provided with a flat washer, a coil-spring
type shakeproof washer and the nut and there was still plenty of exposed stud. To
avoid contact with the cam cover 0.1” was removed from each stud and the plain
washer and coil-spring shakeproof washer were exchanged for an
externally-serrated shakeproof washer (the latter has a larger area).
The oil feed and drain pipes need to be fitted to the supercharger. The oil feed is taken off the cylinder head adjacent to the supercharger by removing a blanking bolt and connected via a pipe to the oil feed regulator on top of the supercharger.
And the oil drain is fed into the central drain tube by
silver-soldering a 1/8” BSP fitting in the location shown in the next picture.
I used 3/16” OD copper tubing as ¼” seems unnecessarily
large for the low flow rate of a drip every 5 seconds. There are a number of
1/4” to 1/8” BSP fittings required – the oil feed on the cylinder head and the
2x fittings on the supercharger. All fittings on the pipework are silver-soldered. There is a
¼” to ¼” BSP elbow on top of the regulator to minimise the vertical height. The
length of thread on the oil drain fitting in the supercharger was shortened
slightly to avoid it “screwing into” the gear inside – easily checked because
it locks the supercharger. The height of the reglator and pipework above the supercharger needs to be minimised to avoid it fouling the bonnet.
Fitting
the Carburettor
The next step was to fit the elbow and the carburettor.
Both the carburettor and the elbow “as-delivered” are without any holes or
threads for attaching the two together.
The first step was to set up the elbow on the milling
machine and drill and tap the holes for the studs.
The next step is to strip the carburettor and drill the
corresponding holes and then make appropriate 5/16” studs.
The throttle must now be moved over to the opposite side
so that the throttle linkage is on the inside rather than the outside. There
were various steps involved in this.
The first step was that the cutaway in the carburettor
body for the throttle stop must be replicated on the opposite side
Unfortunately the throttle stop now comes up against the
elbow and so the elbow must be machined to accept the knurled adjuster.
It was now found that the linkage fouled on the back of
the supercharger (needless to say, the throttle spindle on this side has been
trimmed to the correct length) and it was necessary to either put a spacer
between the carburettor and the elbow or between the elbow and the supercharger
to avoid this. I chose the latter and made a spacer, as below, with gaskets on
both sides. I would have preferred to have used a piece of Tufnol but used a piece of stainless steel instead as I had that in stock.
The throttle would now rotate without fouling although the throttle linkage itself….
Having attached the carburettor to the supercharger and
the throttle working properly it was now clear that the excess spindle length on
the other side needed to be trimmed back. Putting a straight-edge along the
bonnet line showed that not only the spindle but also the spring would foul the
bonnet.
Although the throttle already had a return spring on the
pedal-side, this is one area where a “belt-and-braces” solution is desirable;
after all, the original twin-carb installation had a return spring (2 in fact,
one for each carb), the “as-delivered” new carburettor had a return spring and
this is something I didn’t want to dispense with.
There is not a lot of space in which to engineer a spring
on this side and, as there is no space on the other side, it seemed that a
reasonably elegant solution would be to use a similar lever arm as for the
throttle linkage (part # AUE 180 from Burlen Fuel Systems) and to put a spring
between there and the dashpot screw.
Apart from the choke, the carburettor installation is
finished.
The solution for the choke is not immediately obvious for 2 main reasons: firstly, the original choke actuating lever on the carburettor
is on the outside of the installation (ie next to the bonnet …it has already been removed in these
pictures), which is a long way from the choke lever that comes through the
bulkhead and to which it must connect and, secondly, the
“direction-of-pull” for the choke lever on the carburettor is orthogonal to the
direction-of-pull in the car. The direction-of-pull of the choke from the driver’s
perspective is along the axis of the vehicle whereas it is now required to be
across the vehicle.
I therefore decided to make an entirely new
cable-operated choke operating system.
Although it might be stating the obvious, using a cable
allows a “pull” in only one direction and a spring must be used to apply force
in the opposite direction. This is unlike the original lever-only choke
operating system which can apply a force in both directions, ie for both
opening and closing the choke.
The pictures below illustrate the carburettor-end of the
cable operated system I devised. The simple bracket, which is attached to one
(slightly extended) carburettor fixing stud, is prevented from rotating by
abutting against the inner flange of the carburettor. The spring required to
close the choke needs to be quite substantial (it is a from a motorcycle
clutch) and the bracket needs to be similarly substantial. The bracket is made
from 20mm x 6mm stainless steel.
A new clevis pin has been made (EN24T) with an additional
central hole through which the cable is passed before attaching the nipple. An
alternative solution would have been to use a trunnion between the cable and
the bottom of the jet but this would have had the disadvantage of increasing
the length of an otherwise compact arrangement. The disadvantage of this design
is that the cable must be inserted through the clevis pin before soldering the
nipple.
A brief note on attaching nipples to cables: For many
years I have used the time-honoured method of splaying the cable in the nipple
and then soft-soldering (plumbers solder) the splayed cable to prevent it
pulling through. This works without a problem on motorcycle throttle, air, ignition
and clutch cables. However, there can be problems with the front brake cable on
vintage motorcycles. As an example, one of my bikes is a 1923 500cc Model 18
Norton that weighs next-to-nothing and goes like stink – it’s good for 70+ mph
but has a 5” front brake and a rear brake that is essentially a lump of rubber
pushed into a dummy belt rim. This gives the bike stopping characteristics
normally associated with a fully-laden oil tanker and I therefore try to avoid
situations that require emergency braking. However, on those occasions when
substantial braking is required, it is just a natural behaviour to keep yanking
on the brake lever in the misguided belief that extra pull equates to more
braking. It doesn’t. And the most likely outcome is that something will break and that is invariably the nipple that is
pulled off the end of the cable in spite of being properly fixed in the first
place.
For this reason, I have taken to silver-soldering the
nipple onto these cables and the same has been done here because of the
strength of the spring that is used/required to close the choke.
Great care has to be used in silver-soldering nipples to
avoid heat damaging the cable and destroying the temper. I clamp the cable
(sufficiently to hold it but loose enough not to squash it) in aluminium vice
clamps (to conduct away heat better) with just the nipple and the required
amount of cable within the hole visible. The cable is splayed although this is
not as critical as when soft-soldering. Oxy-acetylene with a #1 nozzle and the
smallest flame possible is then directed very carefully at the nipple (trying
to avoid the exposed wire within the nipple hole), having previously fluxed the
hole, and the silver solder is then applied.
If carried out correctly, the resulting bond is much
stronger than a soft soldered nipple and I have yet to have one of these break
or pull through.
The final part of the choke operating mechanism is at the
other end of the cable. Here, I made a “top-hat” arrangement by TIG welding 5
pieces of stainless steel together, a cable adjuster and making a trunnion to
take the cable to fit on the driver-side of the bulkhead and attached to the
existing choke mechanism. It is important that the trunnion is made with a slot
to take the cable otherwise it could not be assembled by passing the cable and
the adjuster (minus the locking nut) through the bulkhead without drilling an unnecessarily large hole..
Here, I have drilled a second hole in the original choke
operating lever closer to the centre of rotation to get more leverage as the
leverage that would have been obtained from the choke lever attached to the
carburettor is now absent with the straight-pull cable.
The
Front Pulley
The first step was to expose the end of the crankshaft by
removing the Front Engine Support/Radiator Support and the End Cover.
First, the engine needs to be supported under the sump
with a block of wood and a jack to avoid it just dropping by pivoting about the
rear engine mount.
The Front Engine Support is fixed to the chassis by 2
bolts and to the End Cover by 4 bolts. The picture below shows it after
removal.
The next step is to remove the End Cover, shown below, by
first removing the crankshaft end nut and the existing pulley.
The end of the crankshaft is then exposed.
There are 2 machining modifications that need to be
undertaken on the end cover: the 4 fixing holes need to be countersunk to
enable 4 new countersunk screws to be used (this is necessary because the
pulley would otherwise foul the bolt heads) and the inner diameter of the end
cover needs to be machined to take an oil seal. It turned out that the second
of these modifications had already been done on my engine although the oil seal
needed to be replaced with one with a slightly larger ID. The new seal was SKF CR35x45x7HMS5RG. If
the machining for the oil seal had not already been done the end cover would
need mounting in the 4-jaw chuck and carefully set up to machine the recess.
The next step was to fit the pulley and here a problem
emerged. The new pulley is shown in the 2 pictures below.
The problem I encountered was the pulley would not go onto
the end of the crankshaft because there is insufficient space between the shaft
and the chassis cross member immediately in front. The pulley was simply too long to go over the nose of the crankshaft without fouling the cross member.
I dropped the front of the engine further and further in
an attempt to fit the pulley but there was no way the pulley would go on. To
lower the front of the engine as far as possible I ended up: disconnecting the
track rod; removing the seats and the floor to allow access to the rear engine
mounts to slacken off the pinch bolts/remove the locating pin to allow the
engine to pivot about the rear mount,
and then removing the tachometer drive from the rear of
the cam cover to avoid it fouling on the bulkhead, slackening off the bolts
holding the propshaft tunnel sheet metalwork and removing the odometer drive
….but still there was no chance of the pulley going onto the end of the
crankshaft. The only way I could see that this could be done with the engine in
situ would be to remove the chassis cross member and I was not prepared to do
this.
Plan B. I made a new 2-piece pulley with a hub that would
fit onto the crankshaft and a pulley that would then bolt onto the hub.
The new hub is made from EN24T, the internal keyway is spark eroded and the existing pulley was machined to fit the new hub. They are held together with 4x 5/16” BSF high tensile cap screws.
And now it fits!
It is important to check that the back of the pulley does not foul the end cover and that the engine turns freely when everything is bolted up tight.
The belts can now be put in place and the front engine
support reattached.
The
Front Support Plate
The next sub-project is the support plate that is bolted
to the front of the cylinder head with 2x 5/16” BSF studs/nuts. This plate
supports the end of the supercharger and also carries the jockey wheel
tensioner for the belts.
The parts to do this were provided, namely the main
plate, an additional small plate with an offset bend that requires welding to
the main plate, the jockey wheel and its support plate. These can be seen in
the picture at the start of this blog.
However, I decided that instead of using the main plate
and the plate with the offset bend that had been provided I would make a
complete single-piece plate and (luckily) Barry Walker had a drawing for this
that had been made by, I believe, a German enthusiast, Oliver Renzow, that had fitted a similar
supercharger. The one-piece plate is straightforward to make from a 200mm x
200mm x 5mm piece of steel and is shown below before the tensioner was fitted.
A few points worth noting: the studs in the cylinder head
are threaded 3/8” BSF in the head and 5/16” BSF externally and new, slightly
longer, studs need to be made.
A stud supporting the supercharger to the plate needs to be made.
The hole in the supercharger lug was of nominal 5/16” diameter but with about
¼” length of 3/8” BSF thread. Rather than extend the threaded portion (which
would be a viable option), a shouldered stud with 3x threads was made that can be firmly
attached to the supercharger.
And, together with a spacer and a high-tensile half-nut
and serrated spring washer, the supercharger is fixed to the plate and with
adequate clearance from the belt.
The holes for holding the jockey support bracket are
5/16” and 3/8” and it is convenient to tap these into the plate (as it makes
for easier adjustment) with a locknut on the back.
A couple of points to note here: in the picture there are
multiple washer acting as temporary spacers on the bracket holding studs
substituting for the radiator support bracket (coming next) and the length of
the bolt head closest to the inner belt was subsequently trimmed slightly to
avoid possible interference with the belt. There are 2x 1/2" BSF nuts on
both ends of the jockey shaft.
The solution to this is to “cut and shut” the radiator
support bracket and the only really fool proof way to do this is to first cut the
bracket in two, tack weld the 2 parts back together in situ and then remove and complete the weld.
Incidentally, the “nick” that can be seen on the left of the upper part of the bracket in the picture is to avoid the belt and bracket interfering.
The picture below gives an indication of the amount of
overlap required to get the radiator back to its original position.
The final part that needs to be remade is the radiator
tie rod so that it goes around the oil feed regulator and the dashpot on the
carburettor. This is a length of ¼” diameter steel, threaded at both ends for the trunnions and
shaped as shown below.
Installation completed!
I decided to pour a stiff G&T and take the rest of
the day off.
I would like to acknowledge the valuable advice given to
me in a few phone chats with Oliver Richardson.
The 2nd part of this project, namely setting up the carburation and the performance of the car after fitting the supercharger, can be found here.
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