Much like owning a cat or the Cincinnati Bengals, lathe ownership brings with it both advantages and disadvantages. One of the downsides is a tendency on the part of the owner to devise incredibly complex solutions to otherwise simple problems, largely as an excuse to fabricate something complicated and beautiful.
Take the matter of fitting an analog TPS onto a 5-speed equipped 1.6 Miata, for instance. Traditionally, there have been two distinct approaches: the easy way and the hard way.
The easy way, of course, is to acquire the complete throttle body assembly from a car originally equipped with an automatic transmission and bolt it on. The hard way, as espouced by Ben, is to purchase a TPS from a certain vintage BMW, file out the center shaft, drill a few holes, and bolt the whole thing together.
It turns out that there is a third approach: the ludicrously difficult, complex, and time-consuming (though cheap) way.
I initially chose the easy way. Through the help of a fellow list member I tracked down and acquired the throttle body assembly from an automatic car. Upon installing it (a completely painless procedure) I found to my dismay that the TPS itself was more or less kaput- an ohmmeter confirmed that the resistive element inside was shot. I reflected upon this for a while, and contemplated purchasing a replacement TPS for this throttle body. Unfortunately, the cost of a new unit exceeded $150, and I realized that the very youngest of the US-spec 1.6 cars is now 15 years old, making the acquisition of another used TPS a crap shoot at best- 15 years is a pretty long lifespan for an analog sensor in a hostile environment.
Some searching on eBay turned up no BMW sensors, but I did find one vendor selling new aftermarket TPSs for a variety of cars, mainly Hondas. One of the cheapest ($19, shipped) was listed as fitting, among other things, í94-í00 Integras, which from experience I know to use an analog TPS. So I bought it.
Now of course this thing doesnít even come remotely close to fitting the Mazda TB. Since the auto TB (with the non-working sensor) was on the engine at the time, I decided to work with the old manual TB. First order of business, cut down the shaft. Yes, thatís the whole TB (minus ISC) mounted in the lathe, having its shaft trimmed down.
Next, a mushroom-shaped cap was made to fit over the end of the shaft and hold a screw which would interface with the receptacle on the bottom of the TPS. Also, a collar to hold the TPS a set distance away from the TB and center it properly around the shaft. Both of these started as a 2.5Ē diameter solid bar of aluminum, since thatís what I had lying around:
And the whole thing assembled:
What have we learned from this experience? Well, on the plus side it works perfectly, and I know where I can get cheap replacement sensors if need be. On the minus side, it took nearly a full day to fabricate this, and if I ever have to do it again, Iíll be looking a bit harder for a BMW TPS.