Knock sensor thread question

[Tynee]

Good evening. My first foray into the metal fab section, so be gentle…

I’ve got a buddy who’s working on his Malibu boat with some flavor of older Chevy Small Block in it. He asked to borrow a tap to clean up the threads in the block before he reinstalls his knock sensor. I couldn’t get a good read on the thread size and pitch on the knock sensor he handed me, so I started googling. Believe it or not, that ended in confusion and frustration…


Is this guy saying that the threads on the knock sensor are 1/8”-18 NPT, and the threads in the block are 1/2”-20 UNF? How could that possibly be true?

 

[Wrench97]

I've found them to be 1/4" pipe thread on GM's

 

[paulsomlo]

Something not right there - 1/8 NPT standard thread is 27tpi and 0.405 major diameter; it's not going into a 1/2-20 hole. If you've got the knock sensor, get a caliper or micrometer and a thread gage and go from there.

 

[Tynee]

Something not right there - 1/8 NPT standard thread is 27tpi and 0.405 major diameter; it's not going into a 1/2-20 hole. If you've got the knock sensor, get a caliper or micrometer and a thread gage and go from there.

Yeah, .495 diameter. 20 tpi makes sense, but a 1/2-20 die was too small. That’s when I got confused and started googling.