Hot Rod Engine Tech How to Avoid Rod Bearing Fillet Ride

How to Avoid Rod Bearing Fillet Ride

Arrow indicates location of the fillet and the chamfers on the rod and the bearing insert.

Note chamfer on the rod and adjacent bearing insert.

Fillet ride is a potential cause of rod bearing failure in racing and high performance engines. This condition most often occurs with racing crankshafts that have a larger fillet radius on the transition from the journal surface to the side thrust face of the rod journal. Crankshaft manufacturers commonly enlarge the journal radius to provide a smooth stress free transition at this point to minimize the potential for cracking or breakage as might normally occur with a sharp transition or ninety degree angle (see arrow).

Unfortunately the larger and stronger fillet radius provides greater opportunity for the rod or the bearing insert to contact and ride directly on the fillet with catastrophic results. Racing rods and bearings are chamfered to accommodate larger crank fillets, but engine builders are obligated to check each one to verify compatibility.

Most rods are adequately chamfered, but some bearing shells have insufficient chamfer for proper clearance so builders often machine the bearing for proper clearance. Fillet ride can also cause issues with connecting rod side clearance as it prevents the rod from using the full amount of side clearance available between the journal shoulders. In a heartbeat the crank can be ruined and the rods damaged with insufficient side clearance for proper lubrication between them. Engine seizure may occur. As shown in the accompanying photo both the rod and the bearing must adequately clear the fillet radius on the crank journal. Close visual inspection of the area indicated here will reveal any fillet ride issues if present. If the condition is noted, minor machining of the rod and/or the bearing insert will usually alleviate the problem.