A serious grinding or screeching noise in the accessory drive usually means that the one or both of the roller bearings in the drive have failed. The usual bearing to fail is the one directly behind the pulley that drives the alternator.
There is a report of accessory drive bearing failures in the tech tips section of our web site giving additional background information, but basically, the only remedial action is to remove the accessory drive and replace the bearing or bearings. If you want to confirm that the accessory drive is the real problem, you can press against the pulley with a small piece of wood to impose a slight side load on the bearing. If the bearing is bad, the noise will usually alter significantly, either getting louder or sometimes softer. In either case, you can be quite sure that the bearing is in trouble. Obviously, this check assumes that you have sufficient access to the accessory drive and, more importantly, that you can perform the check safely. – Updated: November 4, 2003
We believe these noises are caused by backlash and general looseness within the gear train between the small drive gear on the aft end of the crankshaft, the gear on the accessory drive, and the large idler gear (5” dia) which connects these two gears, These gears are all rather “crude” with diagonal teeth (which appear to be forged). The idler gear itself has a brass bushing pressed into its center and it rotates on a steel spindle that is pressed into the aft end of the block. This brass bushing is fed by the pressure oil system through a small hole in the spindle, and is loose enough (by design) to allow a generous amount of oil to flow out along the gear itself to be “flung” unto the accessory drive gear, providing an oil supply to that unit. We have seen these bushings to have .003” to .004” on very low time engines. It is important to note here that we have never seen or heard of any serious problems associated with these noises. In years past, we have disassembled engines to investigate some of the more pronounced clicks and rattles, and have never successfully defined any measurable differences between engines that have the noises and those that do not. – Updated: November 4, 2003