A strain or deflection gauge is used to check the crankshaft alignment on large diesel engines. It is a specially adapted dial indicator that fits between the crank webs. The strain gauge reads the flexing motion of the webs directly as the crankshaft is slowly rotated (correct engine rotation). The gauge dial reads in 0.00l-inch graduations.

The strain gauge consists of a dial indicator, contact point, balancing attachment, clamping nut, spring plunger, rods and extension, and bezel.

Before you take a reading, be sure the engine is completely assembled and cold. Place the strain gauge between the webs of a crankthrow, as far as possible from the axis of the crankpin. The ends of the indicator should rest in the prick-punch marks in the crank webs. If these marks are not present, consult the manufacturers technical manual for the proper location of the marks. Ensure that the strain gauge is at the same temperature as the engine. A temperature differential may cause inaccurate readings. Readings are generally taken at the four crank positions; top dead center, inboard, near or at bottom dead center, and outboard However, the manufacturers technical manual for the specific engine provides information concerning the proper positions of the crank for taking readings. In some situations, due to the position of the dial, you may need to use a mirror and a flashlight to read the gauge. Once you have placed the indicator in position for the first reading, DO NOT touch the gauge until you have taken and recorded all four readings. Variations in the readings taken at the four crank positions indicate distortion of the crank, which may be caused by any of several factors, such as a bent crankshaft, worn bearings, or improper engine alignment. The manufacturers technical manual will provide you with the maximum allowable deflection. Figure 2-8 shows the locations for taking crankshaft deflection readings.