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A semi-scratch building project  by Alan Swartz
Originally posted on SlotForum

BRM V16The 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:

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 ...


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:

TSRF GuideMRRC GuideThen 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?)

  • chassis soldering - 30 minutes

  • body grinding and fitting - 45 minutes
  • pondering the guide problem - 1 hour!
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.


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:

  1. Cut more and glue the steering wheel to the top of the motor - not acceptable

  2. Go to a front-mounted motor - good choice but not for this project - only as a last resort if there is no alternative

  3. 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:

  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...........


About 2 hours of fiddling around


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:

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 difference in width is apparent:

Again, left to right: Ortmann, EJ (rounded), O-ringAnd finally -

AlF in drums, of course! 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.


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.