As I’ve mentioned before, assembling the Westfield often involves a bit of fettling. There are a number of tools which are useful for fettling, such as files. But my personal favorite is the adjustable reamer. This is a tool which is used to slightly enlarge a hole. It’s extremely useful on the Westfield.
Those two nuts are holding the ends of six blades which slide in slots which run the length of the shaft. The slots are slightly sloped. This means that when you turn the nuts and make the blades slide along their slots, they move slightly in or out. This lets you adjust the diameter you’re opening out the hole to.
Here’s a picture of one in action.
The Westfield uses an English size bolt through the front mounting hole of the Miata’s differential, while the Miata used a metric bolt. This means that the Westfield kit supplies a different sleeve to go in the hole. Unfortunately it was a couple of thousandths of an inch too large. I could have turned down the sleeve a little bit, but it seemed simpler to open up the hole a tiny bit instead. That’s exactly the sort of job which calls for a reamer.