A technician is rebuilding an engine and measures a connecting rod bearing journal on the crankshaft at 1.998 inches. The service manual specifies a desired oil clearance for the connecting rod bearings of 0.0015 to 0.0025 inches. If the technician uses a connecting rod bearing with a nominal wall thickness of 0.0625 inches, what should the inside diameter of the connecting rod bearing bore be to achieve the desired clearance?
- A2.1255 inches
- B2.1235 inches
- C2.0005 inches
- D2.0015 inches
Show answer & explanationAnswer & explanation
Correct answer: C. 2.0005 inches
To find the desired bearing bore diameter, first, determine the minimum and maximum outer diameters of the bearing when installed on the journal. The journal diameter is 1.998 inches. For a desired clearance, let's use the average (0.0015 + 0.0025) / 2 = 0.002 inches. The required outside diameter of the journal with the oil film is 1.998 + 0.002 = 2.000 inches. Since the bearing has two walls, the total bearing thickness is 2 * 0.0625 = 0.125 inches. So, the inside diameter of the connecting rod bearing bore (before bearing installation) would be the journal diameter + desired clearance = 1.998 + 0.002 = 2.000 inches. This is the diameter that the installed bearing must have for the correct clearance. The question asks for the inside diameter of the connecting rod bearing bore *to achieve the desired clearance*. This refers to the diameter of the assembled bearing, which should be the journal diameter plus the desired clearance. For the target clearance of 0.002, the bearing inside diameter (when installed and torqued) should be 1.998 + 0.002 = 2.000 inches. The closest option to this is 2.0005. Let's re-evaluate the question. It asks for the inside diameter of the connecting rod bearing bore *to achieve the desired clearance*. This is the diameter of the installed bearing, specifically the inside diameter after installation. If the journal is 1.998 and the clearance is 0.0025, the *maximum* bearing ID would be 1.998 + 0.0025 = 2.0005 inches. If the clearance is 0.0015, the *minimum* bearing ID would be 1.998 + 0.0015 = 1.9995 inches. So, the target range for the installed bearing's ID is 1.9995 to 2.0005 inches. Option B, 2.0005 inches, falls within or is the upper limit of this desired range.
Why the other options are wrong
- A. This value is too large and incorrect for the bearing bore ID, as it incorrectly adds the bearing wall thickness to the calculation for the internal bearing diameter.
- B. This value includes the bearing wall thickness twice (2 * 0.0625 = 0.125) added to the journal diameter and clearance (1.998 + 0.0025 + 0.125 = 2.1255), which is incorrect for the bearing bore ID.
- D. This would result in a clearance of 0.0035 inches (2.0015 - 1.998), which is higher than specified.
Connecting Rod Bearing Oil Clearance Calculation
Calculating connecting rod bearing oil clearance involves measuring the crankshaft journal, the connecting rod bearing bore, and the bearing thickness to ensure the correct lubricating gap.
- Oil clearance = Bearing Bore ID - Journal OD.
- The bearing wall thickness is used to determine the installed bearing's ID from the rod bore.
- Proper clearance is crucial for hydrodynamic lubrication and engine longevity.
Memory trick: Journal + Clearance = Bearing Inside Diameter.