Engine Wiring, Instruments & Body Tub Fitment

Well, it's the day after Boxing Day 2025 and, as you may have read in the previous post, the engine runs sweet as a nut! The clutch, gearbox, drivetrain, heater, and cooling system all seem to be working perfectly.

Before I disconnect the engine management and temporary fuel supply to drain the coolant, it makes sense to check that emissions comply with the latest IVA requirements. Our mate, Dave from Aeon Sportscars, has a Snap-on emissions tester and has offered to check it for us.

Cooling Fan Stat Installation

Whilst I wait for Dave to pop round, I decided to install one of our #FCE38 adjustable cooling fan stats near the rad. Running temperature and fan switch-on temperature are unknowns at this stage, so I reckon an adjustable temperature stat is a better option than the fixed temperature one already in the radiator side tank. It's a simple job.

Firing up the engine coincided with the coldest winter spell we'd had for a few years, so I drained a few litres of water from the cooling system and replaced it with five litres of our #COOLR antifreeze.

Instrument and Warning Light Options

My MOJO is finally on a bit of a roll so, in the meantime, I decided to take a look at instrument and warning light options. Here's the GRP dashboard panel:

It's a one-piece moulding which is almost the full width of the car and fits pretty much right around the base curve of the windscreen. You can see the hacked-out clearance for the steering column below the instrument pod, and the holes for two screen demist vents up front. On previous builds, we've sometimes bonded the dash panel into the GRP mid-section and sometimes we've made it removable—which I'm leaning towards for this build.

I remember choosing VDO instruments for our first build thirty-six years ago. Despite this P4 having modern comforts like Air-Con (and possibly even CarPlay!!), I'm leaning towards a traditional style of gauges like our Smiths 'Classic' range.

Positioning the Gauges

Symmetrical positioning of the instruments is important to me—an out-of-position gauge, even by a few millimetres, will be the first thing I see whenever I sit behind the wheel.

1. I drew a cardboard profile of the speedo and tacho with dividers.

2. Separated them by a 10mm gap and Blu-Tack'ed it in place.

3. Drilled a 1mm hole through each centre.

There is space for a 52mm gauge on the angled face on either side of the speedo and Tacho. The obvious candidates are Oil Pressure and Water Temp. I remember, on our very first car, turning some aluminium mounting sleeves and bonding them at an angle through the GRP so that the instruments were angled towards the driver.

After carefully cutting holes for the angled gauge mounting tubes, I aligned them and fixed them with a blob of superglue. I then used a simple fillet of body filler front and back to fix and blend them into the dash.

Here's one of the two aluminium gauge tubes:

• Outer Diameter (OD): 56mm (same as the gauge bezels)

• Inner Diameter (ID): 52mm (a nice sliding fit)

• Length: 25mm (same as the body of the gauge)

Picking up the 1mm central holes that I drilled for the 80mm gauges, I first scribed the exact cut-out size with sharp dividers, then cut 70mm holes for the 80mm instruments with a hole saw. I could then accurately size the holes with a flap wheel. My trusty shop vacuum cleaner, positioned right next to the wheel, caught every single particle of glass fibre dust.

Warning Lights

A pleasing result, I think. There are still Fuel and Volts gauges to fit, but I have another old plan for those. There is still space for a set of warning lights. I used a Wad Punch to make some 16mm cardboard discs and juggled their position with Blu-Tack.

These are the front-runners for the warning lights: 16mm diameter, black LED lights that only show the legend when illuminated (this one is our part number #WLFBATBK). The plan is for the dash to be finished in black Flock, so these discreet beauties will blend in nicely yet offer a bright warning when needed.

MAY 2026

Getting the emissions right is proving to be a little more problematic than I expected. Despite several visits from Dave with his Sun exhaust gas analyser and his engine analytic gear, plus a new adjustable fuel pressure regulator, the engine still seems to be running lean and we can't seem to work out why.

I have to admit to being less than helpful with fuel injection electronics these days. While Matt and Dave ponder the solution, I'm keen to take advantage of my continuing enthusiasm, so I decided to tackle the job of fitting the body tub centre section. I left all the temporary engine wiring connected—just moved it all out of the way.

Cockpit Insulation

I had to tackle a couple of jobs that would be awkward after the tub-fit. The upper and lower forward side panels seal the cockpit off from the sill sections that run between the front and rear wheels on each side. Most of the left side is taken up with the fuel tank, but both sides house the main engine cooling pipes, air-con pipes, heater pipes, brake pipes, and wiring. This is no race car, so isolating the cockpit from heat, noise, and vibration is important to me.

The picture below shows the upper aluminium panel covered with our DYNAMAT XTREME. This will help insulate the cockpit but will also dampen noise and vibration.

 

 

And here's the left side, lower panel. You can see the coolant pipe and the two heater hoses. Here, I've glued our HEATMAT as a reflective insulation. All the hot and cold pipes inside the sill sections will also be covered with our TEMPSLEEVE sleeving.

Preparing and Fitting the Body Tub

On one of the first warm-ish days of the year, I granted the centre section day-release from its lonely isolation in a corner of the workshop and perched it upside-down between a couple of tables.

Here you can see the inside of the roof, which has been reinforced with 3mm coremat where the sander is sitting. Left and right are the door frame reveals. Below the sander in the picture is the steel-reinforced windscreen hoop, and above the sander is where the roll-bar hoop will sit. Above that, below the strip of coremat, you can just make out the opening for the mid window.

I filled any irregular areas and rubbed it down smooth with the orbital sander and a 40-grit disc.

The Flocking Process

I've probably written about the 'flocking' process previously in this blog. Basically, millions of 2mm long, fine nylon fibres are given an electrostatic charge that makes them stand on end in a brushed or sprayed coating of two-pack epoxy adhesive. When cured, this leaves a velvety smooth surface like you used to find inside glove boxes.

And here it is—finally sitting in its rightful place!

Sadly, there are no pictures of the actual fitting process. It's, at least, a two-man job, so I enrolled Leo, an enthusiastic young barman (and car-nut) from the pub to help. The tub alone is a hefty and awkward lump. It has to be lifted and fitted with the side frames and fuel tank secured inside the sill area with gaffer tape, and the whole assembly raised high above the (already raised) chassis, then carefully lowered over the roll bar and sprung over the coolant pipes before locating the front squarely over the pedal bulkhead.

I built a raised platform on the floor either side of the chassis with storage boxes and scaffold planks. Leo took the heavier side with the fuel tank and we carefully climbed up on the planks, lifting the tub by the side frames in the door reveals. It was way over the comfortable lifting limit of my 72-year-old bones, but with half an hour of gentle springing and manipulation of the fixed components, we managed to sit the tub cleanly on all of its chassis mounting points with no damage to the chassis powder coating. Thanks Leo!

Securing the Body

Many moons ago, when I was finishing the chassis construction, the tub was trial-fitted to the chassis and some drilled steel plates were welded along each side of the bottom chassis rails to mount the sill edges. Some mounting holes were drilled through so I could reproduce the exact location when the time for a final fit arrived. That's now. I opened up all the holes to 5mm and 'Cleko'd' every one.

I removed the Clekos and injected a bead of our black polyurethane sealant/adhesive all along the join line, then replaced some equally spaced Clekos and riveted the remaining holes with our 5mm diameter, black body rivets.

Fabricating Wheel Arch Scallops

There is some work to do 'weathering-in' all the body-to-chassis joints—especially around the wheel arch areas. Here is work in progress: working out how to mould a 'scallop' into the front inner wing as clearance for the front brake pipe mount.

1. I cut a short piece of 35mm aluminium tube and, with my Powerfile, ground away a section of the glassfibre that would match the curve of the ali tube.

2. My plan was to build up a mound of filler just inside the opening and tape the tube over it so that when the filler set, I'd remove the tube and be left with a perfectly formed cut-out. Well—that didn't work. The filler just sagged and spread inside the panel.

3. The solution was to spray some aerosol expanding foam inside the cavity, tape the tube in place, and when it set, just carve away enough foam to let me build a stable layer of filler against it. Again, I taped the tube against the filler and this time—success.

The finished scallop with the brake pipe refitted. Same for the other side.

3:00 AM Problem Solving

Although I've built many of these cars in the dim and distant past, I'm still discovering new or improved ways of putting one together. These days, sleeping on a problem often results in a satisfactory solution—usually at around 3:00 am.

There is a gap of about 8 to 10mm between the body tub and the sideframe on each side of the car. It's slightly hidden by the bottom door reveal lip (the strip where the door seal pushes on). Previously, we've injected a bead of Polyurethane sealant, which has kind-of done the job but still allows a little movement as the poly compresses. An improvement would be to use body filler to fill the joint, but there's no way to get it in there neatly and cleanly with a spreader—or even a finger.

My early morning idea was to find an almost empty sealant cartridge, clean it out, mix a batch of body filler with a smaller-than-usual amount of hardener to delay the curing time, 'spoon' it into the clean cartridge, and inject it into the gap as quickly as possible before it cures. With a little online research, I discovered that new, empty 300ml cartridges are available on eBay and Amazon for about £2.50 each.

Job done. A good, solid bond between body and chassis where there was previously a little movement.

Windscreen and Chassis Support

There are two more small areas of reinforcement required between the chassis and body. It is still possible—with some considerable weight—to flex the GRP windscreen surround area a millimetre or two on each side of the dashboard position around the 'A' post area.

Permanently bonding in the dashboard moulding would fill the gap and complete the structural integrity. But, as on the last few cars, I chose to make the dash panel removable—secured around all contact areas with screws in rivnuts.

My solution has been to bridge the gap on either side of the dash position, between the side frame mount and the screen surround. I experimented with cardboard patterns, then 1mm aluminium, and finally fabricated these 3mm aluminium support brackets. The drilled dimples will 'key' the filler to the bracket.

You can see the fuel filler hose and the side frame bolted to the dash frame tubes. I taped around the edges of the gap between the plate and the GRP with Gaffer tape, leaving one end open, then injected body filler using my last empty cartridge and the custom injection procedure described previously.

Fuel System Setup

The fuel tank will soon have to be located and safely secured, but for now, I can slide it along inside the sill to give me easy access to the fuel level sender mounting hole. Our #FUSEND has a matching resistance range for Smiths Classic Gauges, so I measured the tank depth and, using the included chart, shortened the sender and float arm to the recommended length. I tied a short piece of string to the float and inserted the sender through the hole almost to its mounting flange. By pulling and releasing the string, I could check that the float could move, unhindered, from 'empty' to 'full'.

Swirl Pot & Filters

And at the other end of the fuel tank is the space where the fuel filters, pump, and swirl pot will live. The large (12mm) pipe on the tank is the outlet and the smaller (8mm) is the return. The reason for installing a swirl pot is simple: the 12-gallon tank is long but relatively shallow. Although it's baffled inside and won't suffer too much 'surge' during normal driving, there's a chance that if parked facing downhill on a steep slope with a low fuel level, the outlet could be above the fuel line.

I'll have to sleep on the exact location, mounting options, and plumbing.

I reckon the most practical mounting position for the swirl pot, at least, is on the rear, vertical sill panel. A full swirl pot will be quite heavy, so I decided to reinforce the GRP with an internal, 3mm aluminium panel riveted and bonded to the inside.

The fuel tank is sitting in position, so I used 400mm of our flexible fuel hose #FFF51 and a couple of wire hose clips to connect it to the base of our #AST35KIT. This is a beautifully made aluminium fuel cap with a screw-on 3 ½" BSP neck with a 51mm hose outlet, a breathing, lockable internal insert, and a smooth roller catch. Another 'dummy' one will sit in an identical recess on the other side of the nose. The smaller pipe on the right is the tank breather; I'll modify the cap neck with a hosetail to connect to this.

Pipe Insulation with TEMPROTECT

The 38mm aluminium main coolant pipes run from the engine to the radiator inside the sills on both sides of the car.

• Passenger's Side: 15mm heater pipes run from the engine bay to the under-dash heat/AC unit.

• Driver's Side: Air-con refrigerant pipes do the same.

Without insulation, the cockpit would quickly become an oven that the AC would struggle to keep cool, and the fuel in the tank would also heat up.

This is a job for... TEMPROTECT. This is a silicone-coated, braided glassfibre sleeve that wraps around your pipes and secures to itself with sewn-on Velcro edges. It's available in black or blue for pipe diameters of 19mm, 25mm, 32mm, and 38mm. Even with restricted access like on our gap between the fuel tank and chassis, it slides easily along the pipe, wraps around it, and secures tightly.

This picture shows the two heater pipes wrapped together with 32mm Temprotect, and underneath them, the 38mm aluminium coolant tube. There's an offcut in the picture to show how the hook and loop Velcro strips are sewn onto the fabric.

That'll do for this post, so I'll wind it up with a picture taken from behind without the tail.

NICE ARSENAL!