knock sensors came out in 81 with the chrysler ignition. volvo surmised the valve ticking might be seen by the sensor and pull the timing down. never tried a chyrsler car run with out them, so i can't say go without.

They are, in fact, "hushers". They force the tappet bucket up against the cam lobes for quieter valve train operation. They are cheap and easily available from Volvo and all mail order companies that deal with Volvo parts. As you note--they don't last forever and the engine will run just fine without them. If you are going to use them be sure to make your valve clearance measurements with them NOT in place. If you've done your measurements before removing the cam with the petrified hushers in place you'll be OK. I use compressed air to blow out any solid bits surrounding the valve springs. Use a rag over the works so as not to get a face full of oil/debrit -- Dave