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Probably overthinking this one too but see attached photos, one of the knock sensor setup and the other of the block with the boss circled where it goes. I'm replacing the stock knock sensor (and putting in Plex) with the Bosch donut style variety. To accomplish this I'm using an adapter that turns the M12 hole into a M8 stud. Should I bottom the adapter against the block? Looking at it it seems it's designed to be bottomed as the bottom portion of it is unthreaded and looks like it's meant to contact the block, but concerns are what effect would this have on the sounds picked up by knock detection? I worry if the thing chatters against the block that could make knock detection useless however I can tighten it a little against the block to eliminate that but then am is it eventually going to wear through the block in that spot? The stock sensor appears that went into the same M12 hole seems to have not made contact with the bock (in the bottom of the hole) rather it contacted the block on the face of the boss it of the M12 hole.
At a minimum the bosch donut style sensor would be seated firmly against the face of the boss (the same as the factory sensor) but perhaps it would be to it's benefit if the stud it is mounted to (which goes into that boss) also contacts the block surface?
Nobody knows the answers to some of your questions. But keep asking, you never know perhaps it really is something to be worried about and someone will have their experience to share.
If the start of a bolt/stud is unthreaded, it's usually to avoid cross threading in soft block materials like aluminum.
Thanks David, appreciate that.
I expect you are partially correct regarding the stud - because of normal thread tapping methods during manufacture it is not possible to fully thread the holes. With conventional studs (and bolts) it is normal practice for them to be fully threaded to the end - this means as the stud nears the bottom of the hole the threaded diameter decreases as the threads aren't cut - so as the stud is tightened it will damage the threads on it and, in extreme circumstances (usually butchers using too-long bolts), it is possible to crack the area around the hole. With the stud thread being relieved at the end, this 'wedging' affect shouldn't happen and it would be expected for it to be fully inserted, and no more than FINGER TIGHTENED (depending on the location I would possible back it off a turn from bottomed because...) - if it is over-tightened it is possible to 'punch' through thin castings. Use a thread lock if you wish.
Knock sensors are intended to be held firmly against the block, with the mating surfaces to be clean and flat, the fastener is only required to do that and over tightening them can cause crush damage. There should be a torque value, use it. Not too sure why you feel the stud might need to be bottomed out, as it will be firmly held against the block through the tensile force on the threads, anyway.
This turns out to be another one of those questions that answered itself when I actually installed it. So it turns out that if you bottom the stud finger tight then after sliding on the donut knock sensor and putting the M8 nut over that I could not get all the threads in the but engaged with the stud so the stud had to be backed out some. Not that I think the stud would move around that much if at all once the nut is torqued to spec but I backed out the stud from the block enough to not only provide for full thread engagement on the M8 nut which in turn pulls back the base of the stud far enough from the block that they won’t chatter together.
Regarding stud adapter you’re using - it’s irrelevant if it bottoms out our not as it will have no effect on the knock signal. Treat it like a head stud and using something like locktite 248 or 273 wind it in until it just comes to a stop, there will be a torque setting to work to but it’s probably 10, 15, 18Nm something really low in that region.
The important part is the sensor which should be tight against the block and tightened to the correct torque setting.
If you are at all concerned about overtightening the stud and the Bosch sensor is a permanent install consider something like a Keensert to allow you to use a standard cap screw.
Thanks Scotty& Gord
Thought I’d add this in in case someone comes across this in the future. Re-read the instructions and the answer was in there. See attached: “When installed only the center metal part of the sensor should
make contact with the engine (red colour)”