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A semi-scratch building project by
Alan Swartz
Originally posted on SlotForum
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 The combination of an apparent high
level of interest in Mac Pinches Pre-Add bodies as described in "News"
and the recent arrival of some of Mac's new laser cut chassis has
prompted me to start another project along side the W154/Patto chassis
that is ongoing.
Thinking that there might be some interest in this project, I am going
to document it in photos and (hopefully) brief text as it progress.
These will be a series of "real time" progress reports, not a "how to"
since what I describe in one post may well be reversed in the next when
I discover that it didn't work! I'll try to keep a rough log of time and
cost.
The
bits and pieces:
 In no particular order: the body molding, driver, vac-formed screen and
metal castings for the steering wheel, exhausts and filler cap (all part
of the body kit) There are actually two chassis in the picture - on the
lower right the laser cut chassis as supplied and spread about the
middle, the parts after cutting and removal of the webs. The rear axle
carriers/rear motor mount are "stuck" together. The fit of the parts is
so good that the assembly is stable without soldering! Also in the
picture is a guide and guide shaft bearing. One of the first things that
I do with any commercial chassis is to try and figure out what guide
shaft fits the hole. This is of particular interest for laser cut steel
chassis because my experience is that these holes can only be enlarged
with some difficulty, the steel being very hard. In this case, the TSRF
guide made for conversion of plastic chassis is a perfect fit and, on
preliminary examination, to be short enough so that it will not extend
beyond the front of the car (important for some rules)
Now let's do the numbers: A quick Google search yielded:
http://8w.forix.com/brmp15.html
LOA: 158"
Width 56"
Height 35"
WB 98"
Wheels and tires: 5.25 X 18 front, 7.00 X 17 rear
If I believe my digital caliper, the wheelbase is spot on and the length
is very close (perhaps 0.10" too short, but, the body that Mac has
modeled differs from the one pictured on the site so I'm going to call
it perfect!
A listing of tire dimensions for Dunlop Vintage Series tires yields:
7.00 X 17 30.2" diameter
5.50 X 18 30.1" and 5.00 X 19 29.6" so I'm calling 5.25 X 18 as 29.7"
This translates into 0.94" rear tires and , rounded to /100s, the same
at the front.
Quick dimension check: The distance from the center of the rear axle
carrier to the bottom of the chassis is about 0.288" If we subtract that
from the radius of the proposed rear tire, 0.47", we are left with
0.18", plenty of room to add a weight pan and still meet a 0.125" ground
clearance rule.
Second check - height is listed @ 35" or 1.1" in 1/32 scale. Knowing the
axle height we can then check the body to see where the axle would be to
give us that height in the finished car - and it turns out to be exactly
where Mac has placed the top of the rear axle slots!
Little worry (postponed until the initial chassis assembly is done and a
motor fitted) - the fit between the top of the motor and the dash is the
limiting factor for positioning the body on the chassis and, at first
glance, it is a problem. The decision will be: can enough be ground off
the bottom of the dash to accommodate the motor or will the dimensions
dictate a front motor placement (and, in the latter case, isn't it nice
that the chassis kit includes two additional motor mounts!)
An issue which has arisen long before its time: decoration: I am not
really enamored of the peculiar diseased liver green that was chosen for
these cars! (I recognize that there may have been budget constraints but
did they really have to use paint left over from the redecoration of the
railway station loos?) Any suggestions as to alternate correct liveries
will be appreciated. I reserve the right to assign the car to Ecurie
Martini and since this organization (long predating the red, white and
blue "Martini" sponsorships) is a Belgian/American consortium, I'll
paint it yellow!
Time so far, hands on and on the net, about 2 hours - parts @ current
exchange, about $50
Next steps: chassis assembly, motor, axles & gears ... |
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PART II
Warm up the soldering iron. It's a real treat to work with well made
parts. Everything fits together perfectly - no hammering, bending or
colorful language! The various pieces have reasonably large fastening
surfaces so I decided that a high strength soft solder would be
sufficient. I like "Tix" - it has a low melting point but is very
strong. Stay-brite is also good. Contrary to some opinions, i think that
steel is easier to solder than brass with the right solder and flux
(acid). It is a poor conductor of heat so the heat stays were you put
it, the joint region heats up fast and neighboring joints are not
disturbed. I also believe is big irons. Most people use 60-75 watt
pencil irons. Unless the space is too small to accommodate it, I use a
150 watt truncheon with a pyramid tip - when I want hot, Ii get hot!
here's the result - still needs a good scrubbing and sanding:
 Dead
easy - just tin one of the surfaces, apply flux, put the parts togethr
and heat. the parts fit so well that you don't need 4 arms to keep
everything in alignment.
After attaching the rear axle carrier/motor mount, I soldered in a pair
of bronze bushings so that I could get an accurate position for the rear
axle to double check the wheelbase. The Pre-Add chassis offers 6
wheelbases by using 3 different postions on front axle carriers that can
be installed in 2 ways. The BRM wheelbase is 98" or 3.0625" in 1/32. I
found two choices: 2.96" and 3.205" (these are rough measurements made
by eyeballing axle centers and lining up the points of a dial caliper) I
chose to go 0.010" undersize and fitted a length of 1/8" OD brass tubing
to the appropriate holes and soldered in the front axle carriers
followed by the guide plate which fits between them and finally, the
bushing for the guide.
 A
little grinding and the chassis dropped into the body:
  Then
the TSRF guide was "offered up" Ah, well, so much for eyeball estimates
- the guide extends beyond the nose by 1/16" or more and that will get
worse with braids installed. Back to the parts bin...after all, it is
and ordinary 1/8" post - and here's an MRRC one. Now, I have not had the
best of results with these guides but perhaps it is just a matter of
learning - decision pending (do I want to try for the race win or the
concours win?) |
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At this point Prof Fate (aka Rocky Russo)
was able to add a little information regarding the particular paint scheme
that was shared by other British teams during this period.
| "Duck Egg" and "Sky Type
S" are actually the same color. Sky is the official designation,
Duck Egg the fitter's designation because everyone could see the
green tinge. Ex RAF stocks were cheap and a number of cars used
it until about 1960. |
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PART
III
And now
the tale of a couple of 180 degree turns (If anyone went out and ordered
parts on the basis of my last post - I warned you at the outset)
Time to fit a motor: With the guide problem semi-solved, my next concern
was the clearance between the motor and the dash (fascia across the
water) In went a standard FK can (TSRF, Little Ripper, Cheetah, Fox
etc.) and into the body. As I expected it hit the dash. Burr in the
Dremel and grind away - but how much? I made the decision that I would
not cut beyond the point of the preformed hole for the steering wheel.
Cut and try again- the chassis still would not fit all the way into the
body. 3 choices:
-
Cut
more and glue the steering wheel to the top of the motor - not
acceptable
-
Go to a front-mounted motor -
good choice but not for this project - only as a last resort if
there is no alternative
-
 Different
motor - I have an assortment of mini-motors but while poking through
the parts boxes, I took out an SCX motor - it is 0.56" high vs. the
0.6+ of the FK can - enough to make a difference - take a look:
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40/1000" is
just enough! Here is the motor mounted:
 The
installation is not without some problems: The contour of the can is
different from the FK (a bit wider at the flats) requiring that the inner
upper corners of the chassis rails be filed a bit. The shaft-end stamping of
the SCX motor is not exactly a precision part and when mounted with the
single available screw hole (note- drill an additional hole in the next
chassis while it is still in the flat), the bottom is "pulled" and the motor
slopes down towards the front. This cannot be corrected by installing the
front motor mount . the chassis simply flexes. One could solder the motor in
but I was unwilling to do that at this stage. - Solution - a high tech shim
(folded decal backing stock) at the lower edge of the motor mount:
 With
this in place, the front motor mount was set and soldered.
So, the idea of a high revving motor with short gearing has gone by the
board - and, as it turns out, just as well. When I began to fit the gearset,
I immediately realized that the spacing between the axle carriers was too
narrow for any of my 64dp metal gears! They all have an external (opposite
the gear teeth) hub in contrast to the typical plastic crown which has an
internal hub. I fitted a Slot-It 3:1 gearset. These are nice gears, perhaps
the best of the molded plastic but my prejudice is that a well bedded 64dp
metal set is smoother and more durable. Does anyone make metal crown gears
with an internal hub?
With one exception, the rest of the project should be simple plug and
chug. The exception: wheels and tires.
The P 15 mounted 17" wheels at the rear and 18" wheels at the front.
Nothing in my parts bin nor on any of the web sites I visited had an
acceptable combination of diameter and width. I do have some 60's 1/24
wheels that are close but they are all made for threaded axles and boring
and bushing them is a fiddly task that I have never done right. The simple
answer is to chuck up the aluminum bar and turn away but I am going to try
an alternate approach. A 17" wheel will have an outer rim diameter of 19-20"
or about 0.62" in 1/32 scale. The diameter of the ridge of the BWA wheels is
0.62" and, the really hard work, getting a round OD and a concentric, square
bore, is already done so I shall machine off the outer rim of a wide BWA
wheel, cut a tire depression and drill and tap the inner rim (AKA the brake
drum) for a 1-72 set screw so I can glue the tire on and true is prior to
mounting - like this:
 The
broad dashed line defines the area to be removed, the dotted line the tire
recess and the little bulls-eye the location of the new fixing screw. I
think I can find a suitable Ortmann tire - probably a narrow 1/24 front - to
get me to the desired 0.97" tire diameter.
The fronts are still "under review" I am thinking about simply turning
the whole business out of delrin -light, easy to turn and perhaps the
characteristic clatter of hard wheels on a track will offer a simulation of
bits and pieces falling off, rods exiting crankcase etc. I have also picked
up some O-rings with a 3/16" section and want to see how they will look
mounted with the outer surface ground flat.
The best laid plans...........
EM
About 2 hours of fiddling around |
PART IV
Almost rolling: The
next logical step was wheels and tires - I like
to have those set before I do the body mounts -
measuring and calculating is fine but, in the
end, I want to see what it looks like! I decided
to take a shortcut and modify some BWA wheels I
had on hand.
Here's the before
and after:
 For
those interested in the process: I mount a wheel
on a piece of 3/32 drill blank and chuck it in a
3/32 collet then face off the front back to the
edge of the rib. The depression for the tire is
then machined - I used a 60 degree indexable
carbide tool perpendicular to the work giving me
a 30 degree angle on either side. The wheel
front is then hogged out with a 5/16" end mill
mounted in the tailstock (could go larger but
that is all the chuck will handle. The recess is
then finished with a boring bar to a 0.505" ID
to suit cut-down Ninco wires as inserts. (I run
the boring bar in until I hit the set screw -
this messes up the screw and threaded hole but
this is not a problem since I subsequently
cross-drill the brake drum to take a 1-72 set
screw. This allows me to glue the tire on and
true it without worrying about access to the
mounting screw) The wheel is removed and the
axle pushed thru to allow it to be mounted
backwards. The rear section was turned down to
0.44" - to make a 14" brake drum. there was a
fair amount of back and forth on the dimensions
of the wheel, mounting and unmounting various
tires until I got the "look" that I wanted:
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From
left to right: two rear wheels with Ortman 1/32 tires mounted, one candidate
front with and EJ's #15 tire and at far right, the same wheel with a 3/16"
cross section O-ring mounted and ground down to provide a flat tread
(More about the O-rings later) |
The O rings are an
experiment - perhaps a bit too rounded for anything later than about 1950
but pretty good earlier than that and, I think, better than most choices for
30's and earlier. They are available in a vast range of cross sections and
diameters in fractional inch, decimal inch and metric measurements. I have
no idea if they offer any traction at all - not a concern for fronts - but I
shall see if they can be coated with RTV silicone such as Permatex
Form-A-Gasket to serve as rears as well. In addition to the size range, they
offer another advantage - they are about $6.00 for a bag of 50!
Now that I have wheel dimensions, I can cut the
axles and front bearing tube to length and get things rolling.
Time - about 2 hours to figure out the first
wheel and tire set-up - 20 minutes each to do the rest
Costs: 2 pr BWA wheels @ $6.50/pr, set of Ninco
wheels - about $5.00, Ortmann & EJ's tires - $10 - axles $1.00 (I buy ground
drill blanks in lots of 25 from an industrial supply house)
Looks like it's going to be about a $90-$100
slot car. |
The difference in width is apparent:
Again, left to
right: Ortmann, EJ (rounded), O-ring |
And finally -
AlF in drums, of course! |
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PART V
We're rolling! the
chassis that is:
 The
axles are cut to length (although with a 52"
front track and a 51" rear track, perhaps I
should say that they are cut to "shortness") In
the foreground is the rough-out 0.063" brass
weight plate.
The body mounts used the two centerline holes
provided in the chassis. I have a "formula" body
mount - thusly: pieces of 3/8" hollow plastic
tube are cut well over length to fit between the
chassis and the inside of the body. Knurled
brass 2-56 inserts are pressed into one end of
each tube (I ease them in with a warm soldering
iron - never had one pull out) They are bolted
to the chassis and the body set on top. the over
length tubes are ground and filed down until the
body resets at the correct height. In the end
stages, I try to shape the tops of the tubes to
fit the contours of the inside of the body. When
I am satisfied with the fit and convinced that
the body will rest squarely on the tubes, I put
a dollop of 5 minute epoxy on the top of each
post, position the body and go away for a while.
When the epoxy has set, I carefully undo the
bolts (the post/body joint may be weak) and then
reinforce that joint with gussets of 0.063"
styrene sheet set in place with high viscosity
gap-filling CYA
All that done - the question is now - do we have
the right "look?" - The overall impression from
photographs is of a very low car (35") with rear
tires reaching the base of the headrest fairing
and fronts that are just slightly above the body
line:
 The
TSRF guide, forward protrusion aside, is a near
perfect fit. The MRRC guide will need to be
machined down to get it low enough to fit under
the nose.
Paint shop time - Bowing to tradition, I have
ordered some Duck Egg Blue (Humbrol) with
Floquil Polly-S Sky Type S as a back-up. While
waiting for the paint, and then waiting for the
paint to dry, next steps include finishing the
weight plate, gluing and truing the tires and
fitting the wheel inserts. |
 Well
- we are moving along albeit slowly - there is
this expression: "the Devil is in the details" -
and so it turns out. After looking at photos of
the real car and, in view of the fact that the
"ears" on the chassis offered a perfect mounting
spot., I decided to add the trailing arm front
suspension. the suspension bit was easy - a
little cutting, filing and folding of a short
piece of rectangular brass tubing - 1/8" hole on
the outboard end to fit over the axle tube and a
bit of solder at the inboard end to hold it to
the chassis.
 The
tinned brass was painted with Humbrol
burnishable steel and the axle tube got a coat
of invisibility paint ( aka flat black) AS in
the prototype, a considerable opening in the
body is needed. And now the fun begins - first,
the body seemed a bit flimsy after all that
material had been removed so I elected to
reinforce the upper part of the body/nose with
several layers of kevlar scrim set in epoxy -
glass fibre, carbon fiber, or even old nylon
stocking material (the latter set in airplane
dope) would do as well. That done, I did a trial
assembly at which point it became painfully
obvious that the front body mount was off center
by about 1mm - the plain axles sticking out each
side had been less revealing. No way to fudge
that much so - grind it out and make a new one -
about half an hour + overnight drying time that
was unplanned. Having come this far, there was
nothing to do but add the trailing links for the
rear suspension - easy - a little recess in the
body, two hole for two bits of wire etc. Here's
the result of the third attempt to get the
"little recess in the body" right.
 With,
of course, the requisite body filler
application, setting time, sanding and repriming
following the first and second attempts! Is not
perfect, but it is done! My driver figures have
never been very good, at least in part because
they are always the last (and rushed) thing that
I do so I decided that this one will be
different- some scraping to create the "bare
arm" polo shirt uniform that will go with the
brown leather helmet and a lot of fiddly heating
(butane cigarette lighter) and twisting of arms
and hands got me to here. With the wheel
installed, he is held forward a bit and grips
the rim quite convincingly - and yes, the wheel
really does stick up that far! Tomorrow should
see the first coat of color (then 24 hrs - then
the second coat then 24 hours then the first
gloss coat etc. etc) and while all that drying
time is elapsing, I'll finish the weight pan. |
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