Building The Car
This is the story of Tavenor
Getting Started
I had always felt that the front of the Marcos was rather taller than necessary, so with the body off here was a chance to chop the scuttle area down – after which the body was not going to fit any more – so I sold the bonnet to a chap who wanted to improve the appearance of his ugly duckling (I think his name may have been Benjamin Orza - sorry guys, no idea which car he had, or even if he ever completed the swap – all I remember is that I delivered the bonnet somewhere in Luton) and the rest of the body, including the windscreen was sold to a chap in the Midlands who wanted to convert a Gullwing into a fastback (again I have no other details). Clearly 850 CAM had ceased to exist and the now orphaned 3006 chassis plate and logbook found their way to friend who wanted to put his previously unregistered Gullwing on the road. (This car was plausibly claimed to be the ex-works car driven by Hine and Prior at Le Mans in 1962.) Said friend had crashed the car lightly Oulton Park and its own chassis plate got thrown away with the broken woodwork to which it had been attached.
I was forced to fit the build around my full time job, which was my only source of funds for the project, and in retrospect it is a miracle I completed the work in such a short time – about 21 months after I started the Tavenor ran in its first race at Brands Hatch on Boxing Day 1967; 20 years later it took me almost 5 years to build my Mantula!
During the build I also somehow found the time to create a transporter for the new car. from a Bedford furniture lorry that had lost it's roof to a railway bridge.
I first tested the car at Silverstone in August 1967 prior to completing the bodywork (still no doors or bonnet), hoping to make the grid for the Paris 1000km at Montlhery in September. It was at a club evening in torrential rain, and the only other car to venture out was a Lotus twin-cam engined Ginetta G12. The handling was amazingly good straight 'out of the box' and I had no difficulty keeping up with the G12, but I could not get past for lack of speed on the straights, a matter that I hoped would be improved when there was proper bodywork to improve the aerodynamics. As it turned out it didn't much.
At a later dry test I was lucky enough to persuade Mike Garton, later the RAC chief scrutineer but then a prominent driver of an ex-works Austin Healey sprite, to try the car with a view to driving with me at Montlhery. Mike was a nice guy who was not likely to hurt my feelings, and excused himself by saying that his BL contract would not allow him to race behind a Ford engine, but he also said some encouraging things about the handling. He also pointed me in the direction of Roger Stallwood as a promising newcomer who had just had a storming season in a clubman Terrier, blowing many Lotus 7s and Mallock U2s into the weeds. Roger was subsequently to partner me at the Nürburgring 1000km in May 1968.
Technical Stuff
I'll make my excuses now – apart from all the technical stuff the FIA rules specified minimum sizes for the windscreen, door openings, cockpit, ground clearance and, of all things, luggage space – which influenced the final appearance of the car quite unfavourably.
With the exception of the wheels, windscreen and some of the engine work I did all the work myself with the aid of a drill press, an ancient manually controlled lathe, an oxy-acetylene welding kit, a trusty old Black & Decker, and a host of hand tools. This included all the fabricated parts – subframes, uprights, wishbones, handbrake mechanism, bearing housings, mounting brackets, adapter rings etc., as well as all the electrics, the body moulds and body and even a few aluminium castings.
The Rolling Chassis
I cut the monocoque off just ahead of the seat backrest/bulkhead, sliced about 2inches (50mm) from the top of the scuttle, discarded the woodwork around the radiator, and also the doors as the openings would be changing shape. I had by now scrapped getting on for ½ of the original car if you exclude the mechanicals. In order to bring down the overall height a less upright seating position was called for which meant a more steeply angled backrest/bulkhead was necessary which in turn meant altering the transmission tunnel. Since I was planning to move up to a 1600cc engine the tunnel would in any case need to be stiffened to take the extra torque so I double skinned it. To bring the weight of the fuel as near to the centre of the car, and as low, as possible I allocated the wedge shaped spaces behind the backrest for fuel tankage and placed a second vertical bulkhead ahead of the transmission, between the wheel arches. This bulkhead together with the backrest was carried up to create the roll-over hoop, which was further strengthened by another bulkhead behind the transmission, onto which was grafted the boot area, which housed a supplementary fuel tank and an oil tank for the differential, as well as the FIA regulation luggage space. The transmission space had box sections above and below, the upper box being home to the compulsory spare wheel, the lower section acted as a suspension mount as well as strengthening the tail.
The layout of the i.r.s followed contemporary formula one practice and consisted of uprights (fabricated sheet steel) hung on upper wishbones and lower reversed A frames, with trailing torque/radius arms, and coil-over spring/damper units, and an anti-roll bar. The mechanical components consisted of a BL 'A' series final drive and differential – chosen over Ford because there was a wide range of inexpensively available ratios – housed in a magnesium casing of Lotus 11 ancestry, with Lotus 11 drive flanges, sliding spline drive shafts specially sourced from Hardy-Spicer, terminating in Hillman Imp stub axles and hubs.
The brakes were mounted inboard on the sub-frame to which the differential unit was bolted, and used Girling AR (Aluminium Racing) callipers on Girling solid discs. All the joints in the system with the exception of the inner pivots of the upper wishbones, which were graphite impregnated nylon bushes, were aircraft spec. Rose joints of various sizes. No rubber anywhere, so no squirming about. The geometry was such as to provide some rear wheel steering in response to roll and bounce in order to increase slip angles when cornering and also to provide straight line stability over bumps – it worked superbly if the car was pushed hard, but made it very twitchy if you weren't sufficiently committed, as a couple of enthusiastic but non-racing friends of mine found out on a test day – fortunately without hitting anything.
At the front I modified the Marcos suspension brackets to increase the track by a similar amount to that of the rear, and retained the Triumph Herald/Spitfire uprights (Brabham used them on their cars, so I figured they ought to be OK on mine), and the lower wishbones, modified to give a choice of pickup points for the anti-roll bar. I fabricated new upper wishbones with a wider base than the Triumph parts to improve stiffness under braking loads, Rose jointed to allow for camber adjustments. The steering arms were reshaped to maintain proper Ackerman effect with the altered track and wheelbase, and I recycled the steering rack from the Marcos. The Triumph brakes were replaced with Girling racing items, as on the rear of the car.
I used 13” steel wheels widened to 6” at the front and 7” at the rear. The widening was beyond me and was done by Revolution Wheels, then in their very early days. I forgive Klaus for using Minilite pattern wheels in the restoration, as I would have used Minilite Magnesium wheels if I could have funded them!
As mentioned above, the main fuel storage was ahead of the rear axle in two wedge shaped tanks, one either side of the transmission tunnel. The tanks were made by bonding g.r.p directly to the inside of the woodwork, and contained some fairly intricate baffles and interconnecting pipes to prevent fuel surge causing too much weight transfer, and further trick pipework to minimise any possibility of spillage if the car should be inverted. The filler was mounted high on the roll-over bar.
A pair of high pressure SU fuel pumps were housed in the transmission tunnel, again carefully plumbed to ensure that there would be no surge induced fuel starvation. These two tanks contained about 60 litres, and there was a secondary tank of similar construction behind in the boot, containing the remaining 25 litres of the permitted fuel. Fuel from this tank was transferred into the main tank by a single high pressure SU pump.
The radiator from the Marcos was recycled, but was mounted on a new subframe that brought the bottom down to the level of the underside of the car. It was raked back so that in all the top was about 6 inches (150mm) lower than on the original, the hose outlets being cut off and refitted to match the angle. An outsourced bespoke remote stainless steel header tank was mounted ahead of the scuttle. A pair of oil radiators was mounted in front of the radiator, with dedicated ducting. Initially exhaust radiator air was ducted out through the top of the bonnet, but I abandoned the duct for the Mk 2b as it was a s*d to install, and experiment showed it made no difference to the cooling. The hole in the top of the bonnet remained and the air probably adopted it as the path of least resistance anyway.
The subframe also provided a hinge mounting for the bonnet.
I retained the gearbox from the Marcos, this being a Hewland modified Ford 105E casing containing a 5-speed geartrain, utilising straight cut gears with face-dog engagement. Noisy, and without synchromesh a little tricky to use, but light in weight. I hoped it would be robust enough, and in any case I could always upgrade later if necessary. Although it was a bit marginal and I chose to make a number of precautionary bearing and thrust-washer replacements during the 1968 season, the only thing that ever broke was the gearlever, which came apart in Roger Stallwood's hands at the 1000 km race. I also retained the “Porsche” type clutch. This had a diaphragm sprung pressure plate (unusual in those days) acting on a single sintered metal faced unsprung centre-plate. The rather dainty splines in the centre-plate used to be a bit of a worry even on the Marcos, but since the facings on the plate were only good for a handful of starts anyway, it just meant fitting a new splined boss to each replacement plate, instead of re-using the old one. A bit of an expensive nuisance as the only way to get hold of one was to take apart a standard sprung 105E centreplate, and I did not have time to find a reliable source of scrap ones.
The windscreen was the cause of much head scratching. I consulted a manufacturer of laminated screens about the cost of a one-off, and then had another think. One of the Rootes Group cars, the Humber Sceptre I seem to remember, had a screen with the sort of multiple curvatures that I was going to need, and looked as if with a bit of luck it could be cut down to give me what I wanted. The manufacturer agreed to try the cutting down at my risk. Remarkably the final shape of a laminated screen is down to the skill of the operative, as it is made by sitting the glass on an appropriately shaped cradle ( the expensive part of making a one-off special), and heating it until it sags the desired amount in the right places. They kindly made me one with even more exaggerated curvature than the standard item, and I took it away and propped it up on the car to see what needed to be done to make it fit on the scuttle. Having marked the screen up I took it back and they succeeded in chopping off the surplus without drama. I then set about making a windscreen frame to fit the glass. The final step was to take the rolling chassis to a local body shop and have the glass bonded to the frame – which in those pre-Sikaflex days was a fairly specialised technique used on only a very few volume-build cars.
The Powerplant
The first engine I built was based on my old 997 Cosworth engine. I used a 1340cc 109E(?) 3 bearing crankshaft and conrods with a set of 4mm larger than standard pistons, the original block being over-bored and linered to suit, which gave the rather odd capacity of 1475 cc. The crank and rods were hand polished by me, the rod/piston sets were equalised in weight and balanced end for end, and the crank and flywheel were professionally dynamically balanced. The head was a 109E casting which I had professionally reworked with equalised combustion chambers, enlarged and gas-flowed ports and oversize valves. I used a Cosworth A6 camshaft which, learning from my Marcos experiences, I modified to improve the oil supply to the Cosworth rocker shaft.
I would have dearly loved to be able to afford a Tecalamit-Jackson fuel injection kit, but instead I settled for re-using my old Weber DCOE 40s, rejetted and with larger choke tubes. The whole setup was guesswork backed up by asking around and hoping that the people I asked would know more than I did!
I consulted an (Italian) mechanical engineer for guidance on a possible safe rev. limit based on the 8000 rpm limit of the trusty old 997 Cosworth and the increased stroke, and he came up with 9000, so I decided on a more conservative 8500 for my initial testing forays, and was amazed when it held together, and felt capable of more.
One track marshal at the car's first dry test offered his expert technical opinion that “it don't half sound nice mate.”
The first engine did not last long – see Racing History for more of the oily details. The second engine was a bit more substantial and had a 5 bearing crank and was basically the bottom end of a small valve Lotus twin-cam engine, again carefully polished and balanced, mated to the original 109E pushrod head. I did not go for a twin-cam head because I could see no way of fitting the cross flow arrangement into my engine bay without substantial redesigning and rebuilding of the driver's footwell and of the pedal and master cylinder mounting arrangements, a task I was not prepared to contemplate in mid season. I am not sure what crank I used, because I see from my old race programs that until much later in the year I was still declaring the engine as 1475cc rather than the 1558cc of a twin-cam, and there would have been no advantage in 'forgetting' to make the change. I do remember with certainty that to begin with I was using the small-valve pistons, which had small valve pockets in the crown, whereas later on I was forced to use big-valve pistons which had substantial valve pockets (see Racing History for the grief this caused me) so I must have changed the spec of something else at the same time – possibly a change to using a twin-cam crankshaft. Perhaps some Ford historian out there will be able work out what the combination of parts must have been.
© Raymond Nash 2010-2023