Tech Talk #51 – Crank Calls, Part II

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“Under most circumstances, a cross-drilled crank is going to cause big problems.”

There’s big money in sequels. Just ask George Lucas; he’s managed to extend the Star Wars series to nine movies. Unfortunately I don’t think this second installment of my two-part column on crankshafts is going to challenge Revenge of the Sith at the box office. After all, a crankshaft is just not as exciting as a light saber.

If you saved your May 27, 2005 edition of National DRAGSTER, now is the time to retrieve it from your workbench, nightstand, or race car. For those who have somehow have misplaced this issue, I’ll briefly review Part I.

It’s my opinion that the availability of affordable aftermarket crankshafts has fueled the move toward big-inch drag racing engines. We’ve seen a rapid escalation in engine displacement in sportsman competition, spurred by the growing popularity of Top Sportsman, Quick 32, fast bracket racing, and similar eliminators that put a premium on horsepower and top speed.

There are some significant differences among crankshafts, however. Last month I pointed out the importance of proper internal balancing and counterweight positioning in a long-stroke crank. This time my topic is oiling.

I’ll begin with a rather bold statement: Don’t use a cross-drilled crankshaft. There are a few exceptions to this rule, but under most circumstances, a cross-drilled crank is going to cause big problems.

Unfortunately cross-drilling is one of those terms that’s become part of the jargon of hot rodding. People who know very little about racing engines have heard of a “cross-drilled crank,” and mistakenly believe they’ve got to have one. In fact, cross-drilling simply refers to the position and routing of the holes that carry pressurized oil from the main bearing journals to the connecting rod bearings.

In a cross-drilled crankshaft, oil feed holes are drilled completely through the main journals so the passages are open on both ends. Holes from the rod journals are then drilled at an angle to intersect the holes in the main jouranls at the centerline of the crank. This system was thought to ensure a continuous supply of oil to the rod bearings because one end of the passage drilled through the main bearing is always exposed to the pressurized oil in the upper main bearing insert.

So what’s wrong with this picture? The pressurized oil that enters the feed hole through the main bearing journal must overcome the centrifugal force created by the rapidly spinning crankshaft before it can reach the passage to the rod journal. If the pressure created by the oil pump is not strong enough to counteract the centrifugal force that is pulling the oil away from the rod journal feed hole, then the rod bearing is starved for lubrication. Since the pinwheel effect of the centrifugal force increases with rpm, when the rod bearing does run dry and seize, the resulting carnage is usually catastrophic.

I learned my lesson about cross-drilled crankshafts the hard way. Back in the early ’80s we started to turn our engines faster. We’d been running stock Chevy cranks in our 287-cubic-inch small-blocks and B/ED motors without any problems. Eventually the supply of usable cranks became exhausted, so we ordered aftermarket cranks – “California cranks” as my Texan friends called them. These cranks were much prettier than the factory forgings, and they all had trick cross-drilled main bearings. It didn’t take long for those cranks to turn blue when the rod bearings burned, sometimes on the first or second dyno pull. Then we’d bolt in an old 283 crank and the engine would live forever. So what was the difference? The difference was the cross-drilling.

Today most racing crankshafts have a “high-speed” oiling system, which is essentially just how Chevy drilled those stock cranks. The oil feed holes for the rod bearings intersect the main journals at or near the surface of the journals. The pressurized oil does not have to overcome centrifugal force to reach the oil feed holes for the rod bearings, so the supply of lubrication is constant even at high rpm. There have been some refinements made to the angles and positions of the oiling holes to “time” the oil supply, but the basic design hasn’t changed significantly.

It’s easy to spot a cross-drilled crankshaft. Insert a piece of welding rod or coat hanger wire into the oil hole drilled in the main bearing. If the wire comes out the other side, the crank is cross-drilled. My advice is not to use it.

It is possible to crank up the oil pressure high enough to overcome the negative effects of cross-drilling. However, excessive oil pressure creates its own set of problems, increasing parasitic losses due to windage, excessive oil on the cylinder walls, and the power that’s consumed by turning a high-pressure oil pump.

It’s possible to manufacture a 5-inch stroke big-block crankshaft without cross-drilling the main journals. However, as the stroke becomes longer than 5 inches, the overlap between the main journals and the rod journals is reduced to the point that there is insufficient material for the oil feed holes. The crank manufacturer must then change the angle of the holes and drill them to intersect a cross-drilled passage in the main journal. When using this type of long-stroke cross-drilled crankshaft, it’s absolutely essential to increase the oil pressure and install a big dry-sump tank, because this engine is going to circulate a lot of oil.

It’s critical for anyone assembling an engine to inspect the crankshaft carefully. The first thing we do with every crank that comes into our shop is get out a pen light and a welding rod and check the oil holes. It’s not uncommon to find an oil feed hole that’s blocked or not drilled quite far enough. When assembling an engine, make sure that every oil passage is open and drilled where it is supposed to be.