Clearance to Run

How Much Bearing Clearance Is Enough?

Mike Petralia Jan 17, 2000 0 Comment(s)

Step By Step

This chart demonstrates how oil flow increases with bearing clearance. An increase of only 0.001 inch from 0.0025 to 0.0035 will increase the oil flow rate around the bearing by 150 percent, or 1.5 gallons per minute. This excess oil is thrown off the crank and creates windage, which robs power and creates oil control problems for the piston rings.

This shows bearing temperature rise versus bearing clearance. As you can see, temperature rise is minimal as long as the clearance stays above 0.0020 inch. Reduce the clearance to 0.0015 and the chart shows well over a 50-percent increase in temperature.

Here you can see how load capacity is affected by bearing clearance. The highest load capacity shown is at 0.002 inch; however, a wider clearance of 0.0025 inch is desirable since it provides more oil for cooling while sacrificing very little in terms of load capacity. A higher load capacity is always preferable.

The best way to measure bearing clearance is by using a dial-bore gauge with the bearing torqued in place as shown. Subracting the crank's journal diameter from the bearing i.d. will give you the actual bearing clearance. Generally, an extra 0.0005 inch is added to the No. 5 thrust bearing to ensure plenty of oil gets to the thrust surfaces.

Design is as critical as clearance when determining how long bearings will live under harsh conditions. The newest performance bearings from Speed-Pro feature a radiused groove three-quarters of the circumference of the bearing. A radiused groove is more beneficial than a straight-cut groove in that it helps to better control oil flow around the bearing without sacrificing load capacity.

One or two thousandths of an inch extra doesn't sound like much, but it can mean the difference between life and death for your engine bearings. We're talking about the effect bearing clearance has on engine life and power. Too much bearing clearance creates problems because all the oil that's pumping in to fill the gap between the bearings lowers engine oil pressure. This excess oil is then thrown off the crank, creating windage, which robs power and generally decreases engine performance. Years of research have been devoted to improving bearing technology, but there are still some inexperienced engine builders who run very loose bearing clearances thinking it will make power.

0.0025 IS THE KEY

Callies Performance has studied the effects bearing clearance has on oil temperature, flow, and load capacity with an emphasis on how reductions in clearance affect the life and performance of the bearings. Callies research has shown that a bearing clearance between 0.0020 and 0.0025 inch is ideal in almost all street and race applications. Although the difference between these two dimensions is small, there is much to be gained or lost if bearing clearance deviates far from it. Bearing clearance between 0.0020 and 0.0025 inch is the best compromise between high bearing load capacities and low bearing temperatures. The three graphs on the previous pages represent what Callies research determined. CHP

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