No Arches, No Drama: Sizing 12-Inch Minilites for the Hillman Imp

The Hillman Imp is a small car with a big packaging problem. The engine lives where the boot should be, the rear suspension is coil-sprung on semi-trailing arms, and the arch lips were drawn by people who assumed nobody would ever fit anything wider than a 5.20 crossply. The received wisdom, repeated at every show you go to, is that 12-inch Minilites mean a session with an angle grinder.

They don't. A well-chosen set of 12x5s — and in most cases 12x5.5s — will sit under standard Imp arches, provided you measure before you spend. This is the process we'd use on any Imp, van or Chamois that rolls into the workshop.

Start with a template, not a shopping basket

The single biggest cause of Imp fitment misery is buying wheels off a spec sheet. Imp bodyshells are sixty years old, many have had a light tap or two, and the rear subframe bushes sag. Two cars of the same year will not measure identically.

So build a template. The cheapest version is a scrap 12-inch Imp steel rim with the centre cut out and a plywood disc bolted to the stud pattern — 4 x 3.75in (95.25mm) PCD. Add a vertical batten down the outer face and another across the inner face at the widths you're considering, and you can sweep the whole arc of the wheel through suspension travel without spending a penny.

Do the measuring with the car on level ground, and jack under the trailing arm to bring the rear suspension to normal ride height. Then jack further to simulate full bump. If your template touches anything at full bump, the real wheel will touch harder.

What you're actually gaining

An original Imp rim is 12 x 3.5in. Going to a 12 x 5in adds roughly 38mm of total width, and a 12 x 5.5in adds about 51mm. Where that extra width goes depends entirely on offset — which is why offset matters more on an Imp than on almost any other British classic.

The rear semi-trailing arm sits close to the inner barrel, and the arch lip is only a few millimetres from the tyre shoulder at the top of its travel. There is very little margin to give away on either side.

Offsets that work in the real world

As a starting point on a standard car, a 12 x 5in Minilite in the region of ET30 to ET35 will usually clear both the trailing arm and the lip. A 12 x 5.5in generally wants to sit slightly further outboard, around ET25 to ET30, to keep the inner barrel away from the arm. These are starting points, not gospel — the template decides.

Two things worth knowing. First, the wheel nut seats on Minilite-style alloys are usually a 60-degree taper, so make sure you're running the correct nuts and not the domed items off a steel rim. Second, check your stud length: an alloy wheel is often thicker at the hub face than pressed steel, and you want full thread engagement with the nut, not three turns of it.

Tyre choice and rolling radius

The original 5.20-12 crossply sat around 560mm in diameter. That number matters, because the Imp's speedometer and its gearing were built around it.

• 145/80R12 — roughly 537mm. About the closest sensible radial to the original diameter, and a natural match for a 12 x 5in rim.
• 155/70R12 — roughly 522mm. Slightly shorter, so the speedo will read a touch optimistic and the car will feel a fraction busier on the motorway. Fine on a 12 x 5.5in.
• 165/60R12 — noticeably short and wide. It can be made to fit, but you are now fighting the arch lip for no real gain on a car with 875cc and a swing to its step.

Stick to a reputable brand with a proper speed and load rating. Cheap 12-inch tyres are cheap for a reason, and the Imp's rear weight bias punishes a floppy sidewall.

The three clearance points to check on a test fit

1. The trailing arm inner face. You want 8-10mm of cold clearance minimum. The rubber bushes deflect under cornering load, so what looks fine on the drive can kiss the arm on a fast left-hander. If it's tight, a couple of millimetres of spacer or a slightly different offset is far cheaper than a new arm.

2. The arch lip at twelve o'clock. Load the suspension through its full travel with a jack under the trailing arm and look up into the arch with a torch. Check the tyre shoulder, not the tread — the shoulder is what catches.

3. The front, on full lock. The Imp's front arches are tighter than most people expect, and the tyre can foul the inner wing or the anti-roll bar at the extremes of steering. Turn lock to lock with the car on the ground and get someone to watch.

MOT and the paperwork

The MOT doesn't 'approve' modifications, and there's no separate test for aftermarket alloys. What the tester will look at is the condition of the tyres, that the wheels are secure, that nothing is fouling, and that no damaged or distorted components are present. A wheel rubbing an arch isn't a testable item in itself — but a tyre worn through by contact, or a suspension component damaged by it, very much is.

Wider wheels that protrude beyond the bodywork are a Construction and Use matter rather than an MOT one, and the police take a dimmer view than the test station. On a standard-arch Imp with sensible offsets you shouldn't be anywhere near that argument.

Finally: tell your insurer. A change of wheel size is a modification, and a five-minute phone call now is worth a world of grief later.

What we'd fit

For a standard-arch road Imp: 12 x 5in Minilites all round, 145/80R12 tyres, correct taper nuts, and a fresh set of studs if the originals look tired. It keeps the gearing honest, keeps the speedo close, and — crucially — keeps the angle grinder in the toolbox.