Advanced Diagnostics

Advanced Diagnostics: B Scan Engine Inspection Service

The B Scan is the most comprehensive engine inspection available without tearing apart the engine. The B Probe, designed for both spark-ignited and diesel engines, gives a fast, convenient method to identify problems and perform repairs before the engine suffers severe damage or downtime.

How Does B Scan Work?

For a B Scan, the engine’s spark plug or injector is removed, and a specially designed probe (B Probe) is inserted into the opening. Next, 80 psi air pressure is blown into the cylinder and the leak rate is measured. Leakage of greater than 1X the cylinder bore, measured in standard cubic feet per minute (SCFM), is considered abnormal.

A vacuum is then induced by reversing the flow through the cylinder, lifting the piston and rod. A dial indicator measures the amount of piston travel and the dial indicator’s needle will experience a momentary pause when the weight of the connecting rod is first encountered. The amount of piston travel to this point is equal to wrist pin clearance. More vacuum is applied and the total lift achieved. Subtracting the total lift value from the wrist pin clearance value results in the rod bearing clearance. The engine’s OEM supplies acceptable clearance values for comparison.


An excessive leakage rate indicates anomalies with the rings, liner, valves, or head. Excessive or minimal clearance indicates rod, wrist pin, or busing problems. For some models, master rod clearances can also be measured. Once this information is recorded, we perform a complete borescope inspection of the cylinder, including pictures of all components. The time spent in each cylinder is approximately ten minutes, so in an average of two hours, we have performed a comprehensive top-end examination of the engine.

Components Inspected

  1. Wrist Pin Bearing Clearance
  2. Rod Bearing Clearance
  3. Master Rod Bearing Clearance
  4. Liner
  5. Ports
  6. Head
  7. Valves
  8. Rings
  9. Piston

Case Histories

Case History #1:

Waukesha 7042 Engine, in midstream service

Findings: Cylinder 1R exhibited a connecting rod bearing clearance higher than the wear limit as recommended by the manufacturer, Waukesha. Cylinder 6L leakage rate was higher than 1X the bore of 12”. (Refer to Figure 2.)

Case History #2

White 8G-825, in a power plant

Findings: Cylinders 1-4 exhibited high leakage rates (Figure 5). All wrist pin and bearing clearances were within OEM specifications. Cylinder 1 rod bearing and cylinder 8 wrist pin were near maximum life and will be trended.

Case History #3

Cooper GMXD-10, in a midstream plant

Findings: Cylinder 5R had excessive leak rate, higher than 1X bore, due to a broken top piston ring. Several other cylinders had carbon in the ports but indicated good leak values. Bearing clearances were all within specification, several are near the maximum life and will be trended. 2L/2R master rod clearance would not lift. The recommendation was for the company to check the bearing.

Recommended Inspection Intervals

  • Annual Shutdown Inspection. B Scan inspection is ideal for pinpoint precision maintenance needed for Condition Based Maintenance, extending the life of components beyond the OEM recommended replacement intervals.
  • Engine Baseline. For a vibration and performance program to precisely trend component condition, baselines are essential. B Scan inspection allows a visible starting point to give the trends absolute validity.
  • Equipment Condition Status. Many companies are buying used equipment and moving it from one oil field to another. Knowing the on-the-spot condition of the engine can qualify the purchase.
  • Spot Checks. These are useful when excess bearing or cylinder liner vibration is detected or when high exhaust temperatures occur.

T.F. Hudgins, an Allied Reliability brand, is the only contract service company in the United States that utilizes the B Scan as part of its analysis service offering on natural gas and diesel engines. We have been performing B Scan services for 15 years, with the most experienced analysts available in the market. Contact Allied Reliability’s Advanced Diagnostic Group today to start seeing the benefits of the most comprehensive engine inspection service available.