Inspecting crankshafts

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Inspecting crankshafts

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.

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).


Also measure each main journal for taper (measure the journal area at two locations, toward the front of the journal and toward the rear of the journal). Maximum allowable journal taper is generally about 0.0004-inch.

Also, be sure to measure each main journal at several radial locations to check for journal out-of-round. Maximum allowable out-of-round is usually around 0.000118-inch or so (check the make/model engine specs).

Next, measure each rod journal diameter at several radial locations on each rod journal. The tolerance range (min/max) will generally be around 0.0008-inch or so (for example, rod journal diameter might be listed at 2.0991 – 2.0999-inch).

Measure each rod journal for taper (at each end of the journal surface). Maximum allowable rod journal taper is generally around 0.0002-inch. Also measure rod journal width (base of fillet to base of fillet — in other words, the front of the journal and the rear of the journal relative to crank length), and compare this to the listed spec. If journal width is too tight, you’ll have insufficient connecting rod sideplay.

If any beyond-tolerance areas are found in terms of journal diameter, taper, width or out of round, this can be corrected by re-grinding on a dedicated crankshaft grinding machine. In order to correct journals, you’ll end up moving to an undersize (smaller diameter than original), in which case you can easily purchase a set of undersized-I.D. main and/or rod bearings (bearing pairs with a smaller I.D. and thicker walls).

Just remember that bearing size needs to be uniform — if one main journal must be re-ground to then accept an undersize main bearing, then all of the main journals should be ground to that same size. The same holds true for rod bearings. If even only one rod journal needs to be undersized, then all rod journals need to be ground to the same diameter. Always check with your bearing supplier to first find out what undersize bearings are available (-0.0005-inch, –0.005-inch, –0.010-inch, –0.020-inch, etc.). This will determine the diameter of the re-grind.


If a used crank checks out OK and you intend to re-use it (with no need to re-grind), each journal can be polished on a crankshaft belt polisher, using 400 grit, stepped up to 600 grit. Small surface scratches can also usually be eliminated by polishing. NOTE: Different equipment makers may specify different grit-grade abrasives for polishing. The journals should not be “mirror” polished, since microscopic scratches are needed to provide oil cling.

 Buying a replacement crankshaft

Your customers have several choices when purchasing a crankshaft, including a new OE crank, a reconditioned OE crank or an aftermarket crank. OE crankshafts are available in the original stroke dimension, while aftermarket performance cranks are offered in a range of strokes from the OE spec through increments of longer strokes. Quality aftermarket crankshaft makers include Scat, eagle, Lunati, Crower, Ohio Crankshaft and others.


 General tips

1. If the journal surfaces are damaged (scratched, scored, gouged, burnt), further inspection is required. If the scratches are light enough, the journals may be saved simply by re-polishing with 400 grit, followed by 600 grit abrasive paper. This should be done on a dedicated crankshaft polishing stand, where the crank rotates at a slow speed while an arm-mounted abrasive belt is lowered onto the journal. If the surface damage cannot be eliminated by polishing, the journals mayneed to be re-ground with an abrasive stone wheel on a crankshaft grinder.


2. If a crankshaft’s mains, rods or both are re-ground to an undersize, the crankshaft MUST be labeled to easily identify any undersizing by stamping or etching the undersize on the forward face of the front counterweight. For example, if the main journals are fine but the rod journals are re-ground to, say, 0.010-inch undersize, the stamping or etching should say “ROD 010,” OR “R –10”, etc., to clearly identify the rod journals as having been undersized by 0.010-inch. A negative symbol (-) preceding the number makes it clear that the re-grind factor of 0.010-inch has been removed.

3. Also make sure to inspect all fillets (the shoulder area where the journal surface blends into the counterweight or throw area). A journal should never be ground to create a sharp corner, since this can lead to an eventual stress riser, which can result in crank failure.


4. Inspect all threaded holes (the center hole in the front snout and the flywheel holes in the rear flange). Make sure that the threads are clean and are not damaged. A chaser tap (as opposed to a cutting tap) can clean these threads without cutting and removing too much thread material.

5. Inspect all main and rod journal oil feed holes to make sure that they’re drilled through, and that they’re not plugged up with debris.


6. As far as crank oil holes are concerned, simply deburr the holes to break off any sharp edges. It was commonplace for years for builders to radius-sweep the holes, but you get too much bleed-off doing that, so it’s better to simply deburr the holes, removing as little material as possible.

A note about undersize grinding

If a crankshaft (rod and/or main journals) is to be re-ground to an undersize, this is done on a dedicated crankshaft grinder, using specific-width abrasive stone wheels. When main journals are ground, the crankshaft is mounted and rotated “straight” with zero runout. When rod journals are ground, since they are offset from the crank centerline, the crank is adjusted on the machine to run at an offset, with the rod journals positioned at zero. Cooling fluid is applied during grinding to cool and clean the journal surfaces.

As far as crank service life is concerned, if a crank’s rod or main journals need to be re-ground, say – 0.020-inch, you’ll lose the initial surface hardness. While some builders (or customers) may assume that the crank is no longer usable simply because the surface hardness has been lost, in reality this isn’t a problem. Simply send the crank out for nitriding after the corrective grinding has been accomplished.

General clearance recommendations

Start with 0.0010-inch of clearance per inch of journal diameter. For example: 2.100-inch journal diameter X 0.0010 = 0.0021-inch clearance. For high performance applications, add 0.0005-inch. If, for example, initial clearance is determined to be 0.0021-inch, add 0.0005-inch for a final clearance of 0.0026-inch. From this point, tighten clearance as your experience dictates in specific applications.

NOTE: Use of a dial bore gauge is always the recommended method of measuring oil clearance. Instead of measuring journal diameter and then measuring installed bearing diameter, zero the bore gauge at the actual journal diameter. When you measure bearing diameter, you’ll obtain a direct clearance reading without the need to perform math procedures, avoiding potential math mistakes.

If clearance modification is needed, do not increase or decrease clearance by modifying housing size outside of tolerance limits. An undersize housing will over-crush the bearing; and an oversize housing will reduce crush and bearing retention.

Today’s leading bearing manufacturers utilize finite element analysis computer modeling to examine the elastic deflections of all bearing-related areas. EHL, or Elasto-Hydrodynamic Lubrication, allows engineers to more accurately determine the effects of dynamic forces in relation to forces and oil clearances. This understanding of loads, metal deflection and effects on clearance has allowed a more precise view of what the bearings are subjected to, and furthers engineers’ ability to develop bearings that will function properly in high-stress dynamic racing applications.


If you really want to get nit-picky with regard to bearings, pay attention to not only suggested clearance, but also take into account the bearing surface are from an anticipated load standpoint, as well as bearing speed, based on journal circumference.

In higher end engines, where you plan to run smaller journals sizes, you really need to pay attention to the load carrying capabilities.

In order to provide adequate oil delivery, some high-end race engine builders sometimes drill extra oil holes in the bearings and partial-radius grooves in the housing or saddlearea of the mains to create multiple oil supply points. This is especially important in engines that use smaller bearings and will experience higher loads (don’t try this at home).

As far as bearing clearances are concerned, for street engines that see higher loads, some builders tend to run somewhere around 0.003-inch for mains and around 0.0025-inch for rods. For engines that will see lots of heat for extended periods, such as endurance engines or marine engines, tighter bearing clearances are the norm, to compensate for the fact that clearances will loosen under hot conditions.

In a high-speed, high-load engine application, experienced builders tend to run a fairly high crush (where bearing shells mate together), while maintaining this within an acceptable range. Considering bearing load and journal and housing deflection, you want to make sure that the bearing is securely held in place. Where you have oil films that are in the tenths of thousands clearance, the bearing gets very hot. If you don’t have adequate crush, you won’t get enough heat transfer. Avoid taking housings to their maximum size, to avoid inadequate heat transfer.

When a builder opts for smaller journal diameter crankshafts (to reduce mass) they sometimes modify the crankshaft journal oil holes in order to drive more oil to the rods. As you shrink the rod journal diameter, the load goes up. In order to get extra oil to the rod bearings, they create a slight teardrop groove to the crank main oil holes. The leading edge (attack side) of the oil hole is slightly grooved. As the crankshaft rotates, this slight teardrop-shaped cavity fills with oil and is then force-pumped into the oil hole, increasing boost pressure. This can cure problems with rod bearings that were otherwise seeing too much load. This can be done with a grinder, but is best performed on a CNC machine. However, you need to pay strict attention to the dimensions of the teardrop groove, in terms of width, length and depth. Generally speaking, this teardrop groove is usually around 0.300-inch to 0.400-inch in length. If the groove is too aggressive, you could start starving the mains for oil. The specific profile of this groove controls the amount of oil pressurizing into the rod.

Again, this is nothing for the weekend builder to mess with, and is certainly not necessary for street applications.   ●

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