Tips & Tricks

Why Cross-Drilled Cranks Don’t Work

Cross-drilled cranks kill rod bearings. Don’t believe it? You should. A long time ago, we had nothing but rod bearing grief with a high rpm, aluminum-rod big block (475 cubic inches, 9,000 rpm, wet-sump oiling system with an accumulator and a cross-drilled crank). The rod bearings looked like the engine was detonating, but the pistons were always perfect (great flame travel pattern, nice coloring and so on). We attributed it to bad bearing quality and the need to move to a more sophisticated dry sump oiling system.

Unbeknownst to us, Reher-Morrison had exactly the same trouble: Rod bearings that either looked ugly after one dyno pull or welded themselves to the crank. Through research, Reher-Morrison discovered that the cross-drilling pattern found in crankshafts (of the era) was the culprit. Basically, in a cross-drilled crank, additional oil holes are drilled all the way through the main journals. At high rpm, centrifugal force literally pulls the oil out of the main bearing. The oil has a pinwheel effect coming out of the cross drill and actually starves the rod bearing. The solution is to use a non-cross drilled crank. There are some rather high profile engine builders that believe cross drilling a nodular iron crankshaft will weaken the crank and make the shaft more prone to failure! If you’re not sure if the crank is cross-drilled, insert a welding rod in the drilled main. If the welding rod goes straight through the crank, don’t use it.

Some people recommend cross drilling #3 and #5 main journals to ensure adequate oil flow to their respective rod bearings. These bearings feed oil to rod bearings number 4, 5 and 8. In all of my experience with Oldsmobile engines I have never seen these particular rod bearings fail. Personally, I think cross drilling has done more harm than good with these engines! Rod bearing failures typically occur with bearings number 7, 6, 1 and 2; rod bearing 7 and 6 are fed oil by main bearing number 4 (cross drilled), number 1 rod bearing is supplied by #1 main bearing (cross drilled) and number 2 rod bearing is supplied by number 2 main bearing (cross drilled). Notice a pattern? David Reher of Reher/Morrison recently reported having experienced rod bearing failures in big block Chevrolet engines with cross drilled crankshafts; replacing those crankshafts with cranks not cross drilled seemed to cure the problem. Here’s the theory: a ½ groove bearing only supplies oil to the rod journal for 180 degrees of rotation, the other 180 degrees residual oil lubricates the rod journal. Cross drilling provides another source for that oil because as one oil hole passes the end of the oil supply groove, the other hole opens up to the groove. But, here’s the problem; in a cross drilled crankshaft, oil has to fight centrifugal force trying to flow to the center of the journal before flowing to the other side where it can feed the rod bearing. If pressure is inadequate to counteract the centrifugal force, oil will actually be forced out of the cross drilled passage and can potentially obstruct oil flow! Oldsmobile big blocks typically have what is referred to as a "fully grooved" main bearing. The oil supply groove runs the full circumference of the bearing. There is really no need to cross drill the crankshaft as the rod bearings have continuous oil flow. Remember, the oil feeding the rod journals from the main bearings do not go through the center of the main journal, these passages are angled toward the rod journal so oil passing through these passages face only a fraction of the centrifugal force that the oil passing through a cross drilled passage would face. Bottom line: in my opinion, don’t cross drill your crankshaft!