Skip to main content

Reply to "351C with 35,000 miles - Need to rebuild?"


Originally posted by René #4406:

... I read in the Clevite catalog a whole text that explains that a 180° groove is the best solution ...

... What do you think ...

Cleveland series engines have always had problems with low hot oil pressure and connecting rod bearing damage. The problems were interrelated because rod bearing damage is what drove the oil pressure down. These issues were not isolated to engines operated at high rpm; even 2 bbl carbureted engines operated strictly at low rpm in mundane transportation applications developed these issues. The “strategy” to adequately lubricate the rod bearings varies to some degree depending upon whether or not the production reciprocating assembly is in use. Some folks suspect that the oil passages drilled within the OEM crankshaft may be in part responsible for the difficulty in lubricating the rod bearing journals. Others point to the weight of the OEM connecting rods and the associated pistons. Most folks with experience dealing with these issues will agree there is something making it difficult to lubricate the rod bearings adequately. Whatever that problem may be, it’s a long established precedent to use fully grooved main bearings in applications involving the OEM crankshaft because they more than double the amount of oil flowing to the rod bearings, and the extra supply of oil is needed to adequately lubricate them. This is not a new idea that originated with me.

Fully grooving the main bearings accomplishes the same thing as cross-drilling crankshafts once did; it supplies lubrication to the rod bearings through 360° of crankshaft rotation thus more than doubling the amount of oil supplied to the rod bearings. The factories did not cross-drill cast iron crankshafts for fear it would weaken them, thus the main bearings were fully grooved instead. Fully grooved main bearings were standard equipment in the Boss 351, and were once standard in all heavy duty bearing sets for the 351C. There have never been any problems or issues arising from the use of fully grooved main bearings; the 351 Cleveland has never had any problems in regards to lubrication of the main bearings.

However with the advent of modern low viscosity oil (0W and 5W) the bearing manufacturers no longer manufacture fully grooved main bearings because the groove in the lower half of the main bearings channels low viscosity oil away from the gap between the bearing and crankshaft much like the rain grooves of a tire channel water from between the tire and the road. The solution here is an easy one, don't use 0W or 5W motor oil. This is why I always very specifically specify using 10W30.

Clevite once included fully grooved main bearings with ALL of their heavy duty bearing sets for the 351C. Unfortunately a “corporate” change in policy regarding fully grooved main bearings does not take into account the needs of specific older engines. They focus on main bearing performance which has never been an issue for the 351C, while the subject of reduced oil flow is disregarded. If the main bearings aren't fully grooved, but additional oil to the rod bearings is needed, how are you going to accomplish delivering additional oil to the bearings? There is no alternative solution provided!

Without a doubt modern motor oils are better performing than the motor oils of decades past, but owners continue to find excessive wear occurring to 351C rod bearings (the silver colored top layer of heavy-duty bearings wears away exposing the copper colored layer below). For this reason I consider it counter-productive to do anything that would reduce the amount of oil supplied to the rod bearings in 351 Cleveland applications utilizing the factory iron crankshaft, factory connecting rods, or pistons weighing close to the weight of the OEM pistons.