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First off, although we make and sell wheel spacers, this isn't a sales post!
I always get messages from people wanting to know what size spacers they need for their Exocet. The truth is, if it's a road car, so long as the wheels and tyres clear the arch stays, they'll be fine.
Some people aim for the fabled Zero Offset which is fine, but making up a one-off pair of 32mm spacers is usually more expensive than a set of 25 or 30mm which are more widely available. You aren't going to notice those few mm.
Good advice is to fit the arch stays to the hub, then chuck a wheel on. It will no doubt hit the arch stay but don't worry, get a pile of M12 washers and start packing the studs out until the wheels clear the arch stays. Give it a measure and pick the closest size available.
There's really 3 different types of wheel spacers available (well 4 but I'm not going to recommend those cheap non-hubcentric slot on with extended studs, they're bloody horrible!)
Slot on spacers
These are normally thin shim style. You can get them in generic fitment which hang off the studs or you can get the spacers machined to the correct PCD and Centrebore. The latter are better as they fit the hub perfectly, rather than the sloppy fitment which never quite centre properly. On the MX5 hubs you can safely go to a maximum of **6mm **in this size
Slot on hubcentric spacers
These are normally thicker than the shim style as they have a hub centering ring machined into them to allow the wheels to locate on the spacer instead of the studs. There's plenty of debates on the internet about whether the shoulder supports the weight and all that, but I find it centres the wheel better than relying on tapered nuts. On the MX5 these generally start at **13mm ** and go as wide as you like. You will need to fit longer studs to the hub with this setup though.
Bolt on hubcentric spacers
These are thicker again. They're designed in the same way as the slot on hubcentric spacers with the correct PCD and centrebore but have new studs supplied with the spacer. The spacer is secured to the hub with nuts, then the new studs are located in the face to allow you to bolt the wheel onto these. The smallest thickness these start in is 15mm (the smallest we do is 17mm) and again go as wide as you like. Most people prefer this style as it's usually more secure, and don't require a lot of messing around to get them to fit.
Bolt on hubcentric spacers also allow the option for PCD and centrebore change - so long as the stud count stays the same ie 4x100>4x114.3 or 5x100>5x114.3. The outer centrebore can also be machined to the correct size to fit the wheels instead of running spigot rings.