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Whether it’s alloys or split-rims, buying aftermarket wheels can be minefield. Neil ‘the wheel’ Boath leads us safely through with this guide to how to buy used alloy wheels.

Wheels are probably the most important aspect of a classic Ford — they set the tone, tell you exactly what the car’s all about and let’s be blunt, make or break it. There’s only one other equally important aspect and that’s stance — get the two right and you can almost forget about bodywork, rust and visual tricks, when it sits right with spot on wheels, you’re onto a winner.

But the world of wheel choice is equally mind blowing and it’s dead easy to go with what you’ve got, convincing yourself that it’ll work. Being blunt and honest is the best thing you can do for your car and yourself — but what you equally need wheel-wise is a good knowledge of what’s out there. One person who’s almost obsessed with round lumps of alloy — especially of the split rim type — is Neil Boath, and his collection of rims is massive.

One thing we’ve never done is discuss the options and look at the myriad of types and styles that are available on the second-hand market. And while this could take us years to wade through – which is what Neil’s been doing all his life. Consequently, he’s in a great position to explain the rare ones, especially the split-rims, what ET and PCDs all about, and how to measure them easily so you don’t get it wrong.

What have you got?

Somewhere on your wheels there will be an indication as to their size. Often it’s cast in, or it can be stamped according to the material the wheel is made from. This size reference can be set/raised into the front face, the rim edge; in fact almost anywhere — you just have to find it. Sometimes it’s really obvious other times it isn’t; or, it’s just not there at all.

What we need to do is either interpret what the codes mean or if there aren’t any, measure it all for ourselves — and it doesn’t hurt to double-check the code either; so you know it’s right.

how to buy used alloy wheels

As an example to aid measurement, we’ll use one of Neil’s ultra-rare ones as an example. These are one-piece cast wheels and are new-old ‘80’s stock EXIP Megas he bought direct from Germany. 

how to buy used alloy wheels

Lifting the centre cap reveals the wheel fitment holes and therefore the PCD — we’ll get onto that in a moment — plus, the cast-in info that we’re after.

how to buy used alloy wheels

To double-check, measure the overall diameter of the wheel followed by the rim width across the flats as shown here. This wheel takes the form of 7Jx16 — this means it has a 7 inch wide rim, with a 16 inch diameter and a J-profile to the lip.

Measuring offset (ET)

What we’ve got here are two letters followed by a figure — in this case ET 35. The first bit is German, standing for Einpresstiefe — and translates roughly as offset. The number next to it relates to the amount in millimetres (mm) the bolt face is offset from the centre line of the wheel – this will either be a positive number or a negative, indicated by a negative sign (-). Positive (+) isn’t normally written.

A quick way of remembering what’s what: negative means dish and positive means flat. So in this case ET35 means the bolt face is set 35 mm to the outside of the rim, creating a flatter-looking wheel — the dish is all on the inside face rather than the outside.

how to buy used alloy wheels

The first measurement we need is the overall width of the wheel. This is different to the rim width we’ve just measured, and the common mistake is taking that 7J bit and assuming the width we need is the metric equivalent of  7 inches — it isn’t! 

So with a flat surface and a tape measure, we can determine our overall width is
202 mm — if we divide that by two, it gives us the centre line measurement, which is 101 mm.

how to buy used alloy wheels

The next bit we need is to measure the back-space of the wheel from the outside of the inside face lip to the inside face of the bolt face — here Neil’s using a spirit level as a straight edge…

how to buy used alloy wheels

…And measuring from it to the bolt face (left), determining a figure of 136 mm.

For the next part it’s important you get in the right sequence as you subtract the wheel centre from the back space so:

136-101 = 35 mm — a positive number meaning the bolt face is offset 35 mm to the outside face of the wheel.

If you subtract in the wrong order — 101-136 — you’ll get -35 mm, meaning it’s dished the other way — just looking at our wheel tells you that’s wrong. But if it weren’t so obvious, you could be caught out when you fit the wheel to the car — it’ll probably catch where you don’t want it to.

Measuring Pitch Circle Diameter (PCD)

how to buy used alloy wheels

The next bit we need to sort is the Pitch Circle Diameter — more commonly known as PCD — and this can be confusing because there are four-stud and five-stud fixings — the latter is more difficult to measure as there’s no definite start/stop point. But, as we’ll see there are some handy gadgets you can get to make life easier. The traditional way to measure this is with a pair of Vernier callipers…

…Or with a tape measure. Neil’s not started at the end of the tape but at the 10 cm mark as it’s easier to get a more accurate reading. Then you measure from the hole’s outside edge, across the bolt face to the outside edge of the opposite hole – yes, this is the same as measuring centre to centre, it’s just an easier and more accurate way of doing it because you don’t have to approximate where the centres are. The standard four-stud Ford fitment is 108 mm, which ours reads and so is spot on.

But there’s a much easier way. For the sake of about £12 most of the wheel manufacturers sell these PCD measuring tools — a pair of callipers which you slot in the holes…

…Although you only put the pins in the adjacent holes…

…And read the PCD off on the scale. Dead easy!

Five-stud PCDs are always more difficult as you need to use an imaginary centre line onto which to measure if you’re using the tape measure/Vernier callipers method — but with the gauge, again use the adjacent holes, while there’s a separate scale for five-stud, too.

Common Ford PCDs and Unilug

how to buy used alloy wheels

As we’ve mentioned the majority of Ford PCDs are 108 mm and that can be four and five stud. There are different PCDs though — the Granada uses a 112 mm five-stud, while American Fords are Imperial at 4.5 inch PCD (114 mm).

Another American trait is for the Unilug fitment (above) — basically an oval-shaped hole that covers a variety of PCDs — handy if you’ve different fitment front to back, plus one wheel will fit more than one car. 

Split Rims

We’ve covered cast wheels, which have a fixed back spacing and can only be altered within small tolerances via machining – these are all mostly road wheels, while heavy duty examples of this type are best suited to the extremes of rallying.

Race cars often run what’s known as split rims — or to give them their correct term, modular wheels. These can be two-piece or three-piece but the idea is that the offset and width can be changed for whatever reason. There are plenty of manufacturers but some of the most well-known are BBS, Compomotive, Schmidt and slightly more recently, Image and MB Racing. Neil’s collection consists mostly of Compomotive, BBS and Revolution.

Two Piece

In this type, it’s the rim type that significantly differs — these are normally cast, or they can be machined from billets, but the rim is of fixed offset and the centre bolts in from the front or the back. If you want to change offset, then you need to buy a new hoop. This type is generally orientated towards road-based motorsport. The part that’s relatively common to both types of modular wheel is the centre which is usually cast — and often that’s in magnesium alloy so they’re super-light — but prone to crack damage and corrosion. Alternatively, centres can be machined from billets of aluminium, so custom-design wheels are easy to make.

Three Piece

These are similar to two-piece, but the outer rims are two-piece, usually from two pieces of spun aluminium — this means they’re machined in a lathe and formed by pressure over a former of predetermined shape.

This is a spun aluminium hoop — the fronts and backs are usually determined by a hole for the valve, which is usually at the front – although for the clean look it can be round the back, too.

Obviously, the offset and the width can be changed simply by swapping in different sizes of front and rear hoops, so an infinite combination of wheel offsets can be catered for. You can see in the rim area, the join between the two hoops.

The back of the sandwich mount also has a sealing ring determining that this is meant for a three-piece wheel — the two-piece front mount may also have a sealing ring but it also may have an alternative system of sealing.

Wheel centres

There are several types of these but the main ones are front or sandwich mount — on the left is a Compomotive Turbo rim, sandwich mount — note the sealing ring. On the left is an earlier version of the same type of wheel but this is for front mount.

Split-rims: What to look for

Apart from the usual kerbing damage, which can of course buckle a rim too, there are a few things to check on a modular wheel.

Centre cracks

These are especially prevalent on magnesium centres, which you’re advised to have crack tested. Cracks are not necessarily caused by damage in service — although a stainless hoop can transmit damage through to the centres.

Are they Ford fitment?

Don’t necessarily assume they are Ford fitment because the seller says they are — hence the PCD check. There were other makes taking part in motorsport apart from Ford! When buying wheels, Neil either takes his PCD gauge or has taken a spare hub with studs.

Are they Magnesium?

A quick way of checking is placing a small amount of vinegar on the back of the wheel —if it fizzes, it’s magnesium! A common misconception is there’s a paint code for the centres — as in what’s BBS’ gold colour? Well there isn’t one — it’s a Chromating process, undertaken with chemicals and necessary because the centres will simply corrode and disintegrate in normal air. You can paint the centres but they need to be chromate-coated first otherwise the reaction will simply be going on beneath the paint!

Sealing split-rims

Modular wheels need sealing but how depends on their design — usually that’s with a Nitrile-cooled sealing ring — like a big O-ring. As we’ve seen previously, this can be from front or back via a machined in groove to take the ring ­ or, if that’s not present then with a separate sealing ring, which itself carries an O-ring.

Split-rim bolts

Modular rims are bolted together with special 12.9 high-tensile bolts torqued to a spec determined by the manufacturer. They should be tightened like wheel nuts – in opposites.

This is the bolt face for a two-piece Compomotive rim that has a cast-in centre mount fitment and pre-drilled holes for bolt fixing.

The full version of this how to buy used alloy wheels guide first appeared the April 2016 issue

Words and Photos Jon Hill

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