Tech Stuff

Inspecting crankshafts

After the used crankshaft has been cleaned, the very first check should be for cracks. Here a crank is passed through a magnetic field on a particle inspection station.
<p>After the used crankshaft has been cleaned, the very first check should be for cracks. Here a crank is passed through a magnetic field on a particle inspection station.</p>

Regardless of which crank you choose to use during a customer’s engine rebuild or fresh build (used OE or new aftermarket), take the time to inspect the crankshaft. In the case of a new crankshaft, check for dimensions and runout. With a previously used crankshaft, you’ll also need to check for flaws (cracks). Inspecting the crankshaft before installation verifies its condition and allows you to avoid problems and/or comebacks.

First and foremost, especially when dealing with a used crankshaft, clean the crank thoroughly, preferably in a jet wash or hot tank. Once clean, always inspect the crankshaft for flaws/cracks. This is best done on a magnetic particle inspection station (commonly known as a magnaflux machine (even though “Magnaflux” is actually a brand name, other equipment makers, such as DCM, for example, make magnetic particle inspection equipment). The crank is mounted horizontally on the inspection bench and passes through a large diameter circular magnet and inspected with an ultraviolet (“black”) light. Any cracks are easily found, visible as whitish lines.

If a crank is cracked, don’t even debate the issue — sell it as scrap metal and buy a new one. By crack-checking first, you’ll avoid wasting time by performing further dimensional inspection.

Next, check crankshaft runout. With the crankshaft mounted level on a pair of level V-blocks (resting on the front and rear main journals), set up a dial indicator at the center main journal, placing the indicator probe slightly offset to avoid hitting the journal’s oil feed hole. Preload the indicator by about 0.050-inch and then zero the dial.

Once a magnetic field has been obtained, an ultraviolet light is used to inspect for cracks.
<p>Once a magnetic field has been obtained, an ultraviolet light is used to inspect for cracks.</p>

Slowly rotate the crankshaft while observing the gauge. Record your reading. For example, the maximum OE-spec for allowable runout may be listed as 0.000118-inch. If the indicator gauge doesn’t read in the hundredths of a thousandth of an inch, you’ll be hard-pressed to actually determine that tiny number. Generally speaking, if the crank shows less than 0.001-inch runout, it’s probably fine. If the crank shows more than 0.001-inch runout, it needs to be either straightened or replaced. Crank straightening is a precision task that should only be handled by a skilled specialist. Not all cranks can be successfully straightened, by the way.

Using a micrometer, measure the main journal diameter (at the center of the journal) of each of the main journals. Record your measurement and compare this to the specifications for that particular engine.

Published specs will include a tolerance range (max/min), usually of about 0.001-inch (for example, 2.558- to 2.559-inch). Bear in mind that, if using a reconditioned crankshaft, the main journals may have been re-ground to a smaller diameter in order to maintain serviceability (for example, the mains may have been ground –0.010-inch undersized).

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