So i'm putting in my new 22re I just build and i didn't tape off the hole for the knock sensor when i painted it. I'm just wondering if i need to clean out the threads for it. I'm somewhat familiar with the knock sensor circuitry, but does it ground through the body of the sensor into the block?

 

Yes on the 1987 engine you should just one wire coming to the sensor and it should be black.

you might get lucky when you thread it in then maybe not.

My luck it would work when it felt like it.

 

One wire on my 86 so yes it grounds thru the threads.

 

It's a piezoelectric sensor. So it's generating the voltage signal being sent to the ECU. Being a DC circuit, current/power can only flow from the source(+) to ground(-). As in from the KS(+) to the ECU(-), NOT vice versa. Only ONE wire required(and some sheilding, in this case).

 

The threads are 'natural' pipe threads so they taper smaller the deeper they go. Eventually, threading anything in there will cut through the paint and make contact.

Other than that, yes, it does ground through the block since like any microphone requires a return path, and since the knock sensor has a single wire connection, it must ground through the engine block, just like MH said:
The signal wire from the KS to the ECU is also shielded and grounds near the ECU which should help reduce any noise on the circuit.

 

Piezoelectric devices are AC devices. You can read more about the knock sensor HERE Page 32, or HERE and not auto/toyota specific piezos HERE.

NPT Taper is "National Pipe Thread Taper", http://en.wikipedia.org/wiki/National_pipe_thread
Academics aside..

As mentioned above you probably won't need to clear out the threads, but it wouldn't hurt you can pick up a tapered tap online, the local hardware store, even walmart maybe, or file out a a clearing relief in the old one. Even just clearing out the outside of the upper thread would do there isn't alot of current involved.

 

Thanks guys! The fact that the sensor generates the current is what was throwing me off a bit. I cleaned up the threads a bit with a green scotchbrite pad, threaded it in, and the motor is running great. Thanks!