We've heard a thousand times that oil pressure and volume are keys to a healthy oiling system. We can use an oil-pressure gauge to monitor the effects of changes in our oiling system, but aside from blindly bolting on a high-volume and/or high-pressure oil pump, how can we actually control the oil flowing through our engine along with its effectiveness? Until Pfizer comes up with a pill you can drop down your valve cover (Addapreshavalvoline, perhaps), the best we can do is control our bearing clearances.
Although bearing clearance is one of the most important things to measure, it is too often taken for granted as being correct simply because "it had good oil pressure before freshening it up," or "it's a new crank," or "it was just turned." As a machinist, I can't tell you how many new crankshafts were either on the high or low side, or even completely out of specification.
To start your bearing clearance exercise, use a micrometer and take a direct reading of the crankshaft journals.
Most of the domestic top-name crankshafts coming from companies like Lunati do an amazing job of finishing cranks, which makes the task of blueprinting bearing clearances much easier. Buying an off-brand $99 Internet special "race" crank from an online auction house might seem like a deal, but when it comes to fitting the crank to your block, you might not be so happy.
Loose or tight? Generally, loose clearances will lower oil pressure, and tight clearances will raise pressure. Most hot rodders equate high pressure with a healthy oil system, but there is more to good oil pressure than just pegging the old Stewart Warner gauge.
Bearing clearance is what determines the size and effectiveness of the hydrodynamic wedge that actually supports the crankshaft. Tighter clearances make for a long, evenly distributed wedge that can support the crank over a wide radius. Make the clearance too tight though, and the bearings could scuff and lead to (gulp!) spun bearings. On the opposite end of the spectrum, loose clearances give us that low-friction feel we all like when spinning over the crank by hand (keep in mind that would be unpressurized). Too loose though, and the hydrodynamic wedge is easily pushed out of the way by the crank moving around. This unsupported crank can run directly on the bearing surface with no oil holding it in suspension, thus easily destroying the bearings. Again, not good.
Then transfer this measurement to the dial-bore gauge.
So what is a good oil clearance number to shoot for, and how do you get there? If you are on a first-name basis with your auto parts store counterman (you do spend a lot of time at the parts store, don't you?), then you could ask to see a copy of its Clevite bearings catalog. In it, you will find a plethora of information including-pause, wait for it-recommended bearing clearances. If you don't have access to a Clevite catalog, or if you can't find a listing for your engine, then a good rule of thumb is to shoot for .001 inch per inch of shaft diameter. In the case of a generic small-block Chevy with a nominal crankshaft main journal diameter of 2.45 inch and a rod journal of 2.10 inch, that would have us aiming for clearances of roughly .0024 on the mains, and .0021 on the rods. Generally, I prefer to use those numbers as my high side, and drop down about .0005 to get my low side, in this case .0019 and .0016, respectively, on a stock engine. In a high-performance engine, I'll use the rule-of-thumb number as my low side, and add .0005 to get my high side.