A minor design problem appears to be causing an increasing number of bearing failures within the accessory drive.
As they progress; these failures usually manifest at low RPM, first by a rattling noise, then a low grinding sound, and in worst cases, a rather loud screech (at any RPM) that is sometimes mistaken for a failed main crankshaft bearing. Early signs of failure can be masked by the normal clicking and rattling sounds of the large idler and timing gears within the aft housing.
At issue, is the fact that the rearward slant of most Atomic 4 installations results in the oil supply to the forward bearing within the accessory drive being cut off or greatly reduced. During overhauls, we frequently find little or no oil within the forward bearing, and the oil seal directly behind the alternator pulley is mostly missing from running dry.
Thanks to the robust design of the accessory drive, the actual failure rate is still quite low (2 or 3 a year have been coming to our attention). However, our concern is that there might be a lot more bearings in the fleet that are presently at risk. Obviously; high time engines, or those with rearward slants of approximately 5 degrees or more, would be at most risk.
Fortunately, there is a relatively easy and inexpensive fix which will be discussed later in this Technical Note.
GENERAL DESCRIPTION OF ACCESSORY DRIVE (Engine level):
The accessory drive is mounted on the forward face of the aft housing, and on the same side of the engine as the starter. The role of the accessory drive is to redirect engine power for turning the alternator, the water pump, and the distributor (which mounts on top of the drive). The first photo shows the accessory drive removed from the engine.
The gear on the opposite end of the housing from the pulley is the drive gear, and it meshes with a larger gear (an idler gear) inside the aft housing. As the engine runs, the gears fling oil up and over the housing of the accessory drive, some of which collects in a small catch basin directly between the drive gear and the mounting flange.
The shaft of the accessory drive rotates within two identical roller bearings. There is a rear bearing near the drive gear, and a forward bearing immediately behind the pulley. Photo 2 shows an enlargement of one set of these bearings, each of which consists of an inner race and an outer bearing head.
In a cutaway of an accessory drive in a level position (next photo); we can see that after entering the catch basin, engine lubricating oil drips over a cone shaped spacer and on to the worm gear that meshes with a small gear on the lower end of the distributor shaft. These internal gears fling the oil throughout the inside of the accessory drive, some of which enters the front oil passageway and flows forward to help lubricate the front bearing.
Eventually, all the oil collects in the lower part of the accessory drive, to a level that just reaches the lower edge of the worm gear that drives the distributor. This level is identified in the Photo as “Original Oil level.” At that level, oil flows back into the main engine through a small drain hole in the rear face of the Accessory Drive housing.
THE EFFECT OF SLANTING ENGINES REARWARD: The shallow slope of the front oil passageway becomes practically horizontal at rearward slants approaching 7 degrees. Photo 4 shows how the oil pool within the drive changes so that, as this degree of slope is reached, oil barely touches the worm gear, and there is no oil reaching the base of the front bearing.
THE FIX: This photo shows the original 1/4″ drain hole in the forward face of the Accessory Drive housing, threaded and plugged with a 1/16″ allen headed steel pipe plug. A second drain hole is drilled through the housing so as to establish an oil level approximately 1/2″ above the original level.
The oil level established by the new upper drain hole is shown as the upper line in Photos 3 and 4. Notice that, even in the slanted configuration, the new oil level reaches well above the lower part of the front bearing so as to make up for the fact that oil coming forward via the front oil passageway is practically nil.