1/8th Scale 1934 Alfa Romeo Monoposto Type B (P3) Chassis No. 50006

1934 Alfa Romeo
Alfa Romeo documents

Part No.1

After agreeing the level of detail with the client, the first process is to measure up a car and prepare a full set of working drawings. By taking a large number of pictures and sketches with major sizes you can then work out most parts of the car.

Obviously digging up chassis, engine, gearbox and other drawings also help a lot. In the case the car was prepared by Hall & Hall and again I was very privileged to be given access to their extensive archive, but this time I was able to help repay the favour.

The model was to be as it raced at Historic Monaco, so after measuring up the car I went to help out on its final shake down the Saturday before leaving.

Unfortunately or not the diff broke late in the afternoon. It could have been first practice! So I rejoined the lad that ran the car at the workshop at 6 on Sunday morning and we were worked till late, then back again early on Monday morning. With the diff in the middle behind the gearbox it is a long job, but the last nut was tightened in time for it to be loaded as last car that lunch time.

For the body the car is placed on a flat floor and starting at the front a gantry with pegs on 3" centres out from the centre line, drop highs are taken with this process being repeated every 12" back along the car. Being a ship model maker after you join the dots you end up with an up turned boat hull (keel centre line and bulk heads every foot).

Now the fun starts!!!

In this case we started with the chassis. The main frames and cross members are fabricated out of the 30 thou Brass sheet, with the others fabricated out of brass tube or rod. As this chassis has Dubonnet front suspension the from of the side chassis rails are shortened and have machined Brass clamp blocks fitted to hold the ridged suspension cross tube.

alfa romeo parts
Alfa Romeo parts

1/8th Scale
1934 Alfa Romeo Monoposto Type B (P3)
Chassis No. 50006

Part No. 2
Engine, Gearbox, Diff & Rear Suspension

With the differential being mounted directly behind the gearbox, it is more sensible to build the engine, gearbox and diff casing first rather than suspension. As we have access to an original block drawing and the full size gearbox and diff, a set of working drawings can be made. As the engine, gearbox and diff castings are to be made from cold cast Aluminium (Aluminium powder mixed with Polyester resin) knowing the shrink rate we machine the numerous casting patterns over size from Acrylic, Brass and Aluminium.

The patterns are then placed in moulding boxes which are filled with de-aerated two part silicon moulding rubber, this is left for a couple of days to go off. With the over-sized patterns removed, you place in Brass cross tubes to the gearbox mould which will later carry the clutch and brake operating rods.

The moulds are then filled with the Aluminium/resin mix, it is important to get the resin/catalyst ratio correct to stop excessive heat and distortion which can damage both the casting and the mould. After they have cured, the castings will have shrunk to the correct size and can be machined to fit together. Finally the desired Aluminium finish can be achieved by either sand blasting or rubbing with fine wire wool.

The cam covers are machined from solid brass and along with the head studs etc are nickel plated. Having fabricated the mountains, the engine and gearbox can now be fired in to the chassis.

A dividing head on the lathe rear is invaluable when building the rear axle, it enables you to evenly space holes on the flanges and castings so everything fits together correctly.

All the parts of the rear axle are machined from brass. It has two round Bevel Gear Boxes who’s inner flanges are bolted to a joining tube, the two prop shaft tubes are then bolted on at an angle of 75 degrees with the other end to the diff casing forming an A frame. The two hub carriers have collars machined in to them to locate the spring carriers, more of which later.

The outer flange are bolted to the brake back plate which are drilled and have a cooling slots etc milled out. The turned Aluminium drums and hubs are joined to the flanges on the outer end of the half shaft which passes through the carrier with the inner ends flange bolted on.

The hole hub carriers are then bolted to the outer flange on the B.G.B. by putting coil springs inside the additional telescopic dampers the leaf springs can be made out of shim brass, they are clamped at the rear to the chassis and to the bottom half of the swivelling axle clamps and locating the complete axle. Finally the friction dampers are assembled and bolted on.

This car has a Dubonnet from axle fabricated from brass, it consisted of a solid axle tube clamped to the front of the shortened chassis rails, either end of which has an independent suspension unit fitted. This unit consists of the main coil spring inside, that is retained at the front by a threaded plated cone. The casting at right angles with an actuating arm for the spring in the middle and out to the friction damper on the inside.

The front held in place by a collar to the stub axle and a link to the casting which keeps it vertical as the stub axle goes up and down. The castings also have the pick ups for the steering arm and the drag link. Finally the drums with attached hub are fitted.

alfa romeo monoposto parts
alfa romeo monoposto wheel
alfa romeo monoposto

1/8th Scale
1934 Alfa Romeo Monoposto Type B (P3)
Chassis No. 50006

Part No. 3
Wheels and Tyres

The rims and hubs are machined out of Brass, luckily my Brass supplier has 76 mm tube with a 50 mm bore, even so the 15 mm blank weighs 325 g, while the finished rim is only 29 g. Once the rim is turned it has three rows of 24 holes drilled in it for the spokes, another job for the dividing head. The machines hub also has its three rows of spoke holes drilled in it. A perspex jig is turned in which the rim and hub are held in the correct alignment. To work out the three different spoke lengths, a piece of thread is laced through the holes of the three runs from which you can work out the size. As we have two sets of rims for this model this makes 192 spokes of each size (576 in all). Each spoke is cut form 1/32 Brass wire with one end having a 14 BA thread put on it, while the other is formed in to a mushroom head, the final operation is to put the slight angle on it at the hub end before passing them through the hub. Next are the domed tensioning collars with their 14 BA threads which pass through rim and part thread on to each spoke. With all in position in the jig one by one the spokes are fully tensioned. After it is painted a few spokes have short lengths of 5 amp fuse wrapped round them like the period balance weights. The final piece of the jigsaw is the tyre valve which is screwed in to the rim. It is a lot of work but the end result is worth it.

The most asked question about my models is “where do you get your tyres?”. The polite answer is a small workshop north of Glasgow. They do any size, any make, treaded or slicks, and even in the case of P3 one set new and the second part worn. The first thing you do is to machine the carcass out of the resin or perspex then bond a Brass etching of the particular tread pattern around it. For the last thirty odd years all my etching has been produced by P.P.D. of Lochgilphead, when you find someone that does amazing work you stick with them. The correct etched lettering is then put on the side walls. The pattern is then suspended on a post the size of the inside of the tyre in a half casting mould with its centre line level with its top. This is then filled with two part moulding rubber and left to off. After coating the exposed rubber with release the top half of the mould is slid over the top part of the post and the rest of the mould is filled. With the two parts of the mould marked for alignment it is opened and the pattern removed. The mould is coated with release and the two halves aligned around the centre post which has two concentric tubes. The inner and longer one has holes drilled out to the side so the moulding rubber coloured with lamp black can flow into the mould at the bottom, fill the mould then pass through the second set of holes out to the gap between the two tubes. After it has cured, the mould is split and the tyre trimmed off the post with a scalpel, you even get the mould mark round the middle like a new full size tyre or it can be removed with wet and dry to give the worn look.

We now have our wheels and tyres, and to attach them to the model we machine the wheel spinners again out of the Brass. As we cannot create splines in the wheel hubs and I do not have left and right hand taps and dies of the correct size all the hubs and spinners are threaded the same way. The only difference is the Brass etching in the centre of the spinners which along with the appropriate writing has the correct on off directional arrow. Finally, as I said we have two sets of wheels and tyres so we have front and back quick lift jacks and yes, even a copper hammer!!!!

alfa romeo monoposto chasis

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