Engine bearing clearance-FAQ

Engine bearing clearance is very important and can cause early failures if not correct.

Engine bearings should operate with the least amount of engine bearing clearance as possible. As a result tighter oil clearances produce less peak loading on the bearings and the engine runs smoother with less vibration.


Measuring con-rod
Measuring con-rod
Measure crankshaft
Measure crankshaft

Remember, you can mix bearing sizes to achieve desired oil clearances.

Therefore in order for bearings to maintain a stable oil film, which is vital to their life, there must be a perfect match with,

  • Engine bearing clearance
  • Oil viscosity
  • Engine operating  temp
  • Crank surface finish and geometrical alignments

Just remember that bearing size needs to be uniform. Also a general rule of thumb for determining oil clearance is .001 of clearance for every inch of shaft dia. Ex .001 x 2.00 shaft dia.  = .002 oil clearance.  For clearances smaller and larger than the rule of thumb ,refer to the factors listed below:

Factors for tighter oil clearances:

  • Smaller journals
  • Lower viscosity oils
  • Surface finishes min 4 RA
  • Near perfect crank and engine block geometry
  • Higher engine operating temperature  due to decreased oil flow
  • Perfectly balanced cranks

Factors for looser oil clearances:

  • Larger journals
  • Higher viscosity oils
  • Surface finishes rougher than 5 RA
  • Not so perfect crank and engine block geometry
  • Lower engine operating temp due to increased oil flow
  • Weaker housing bores causing distortion at high RPM’s

The oil escape from a pressure fed bearing increases by roughly the square of the engine bearing clearance. Thus a clearance of 0.002” (0.050mm) can pass almost twice as much oil as with 0.0015” (0.038mm). If the oil pump capacity cannot meet this demand the pressure will fall. This will result in bearing failure. This illustrates the importance of accuracy in fitted bearings.

Multi-viscosity synthetic motor oils flow more easily than conventional straight weight oils at both low and high temperatures. So they can handle cold starts as well as elevated operating temperatures (which is really important with turbochargers). Hence to reduce friction and improve fuel economy, most late model stock engines are factory-filled with 5W20 or even 0W20 oil. In addition combined with tighter engine assembly tolerances, these oil and bearing combinations work relatively well for everyday driving and even mild performance use.

Especially if the engine has a flat tappet cam that requires plenty of ZDDP in the additive package.

Engine bearing clearance
Engine bearing clearance

Excessive crush

  • APPEARANCE -Extreme wear areas visible along the bearing surface adjacent to one or both of the parting line.
  • DAMAGING ACTION -Before the bearing cap is assembled, a small portion of the bearing extends just a little beyond the edge of the bearing housing.

The bearing is forced against the housing when tightened in place. Consequently that portion of the bearing which extends beyond the housing is called “crush”.

When there is too much crush, however, the additional compressive force created by the surplus crush that still remains after the bearing is fully seated causes the bearing to bulge inward at the parting faces. This bearing distortion is called “side pinch.”

POSSIBLE CAUSES

  • Bearing caps were filed down.
  • Excessive torquing.
  • Not enough shims.

CORRECTIVE ACTION

  • Rework the bearing housing of the engine block if it has been filed down.
  • Also replace the connecting rod if its bearing cap has been filed down.
  • Check journal surfaces and regrind if necessary.
  • Most important install the new bearing and follow proper installation procedures by never filing down bearing caps and using the recommended torque wrench setting.
  • Correct the shim thickness (if applicable).
  • Also check for out-of-roundness of the inside diameter of the assembled bearing by means of an out-of-roundness gauge, inside micrometer, calipers or prussian blue to assure that any out-of-roundness is within safe limits.
Bearing loose in rod
Bearing loose in rod

Insufficient crush

  • APPEARANCE -Highly polished areas are visible on the bearing back and/or on the edge of the parting line. Areas of pock marks or build-up due to metal transfer between bearing and housing. This is commonly referred to as “fretting”.
  • DAMAGING ACTION -Because a bearing with insufficient crush is assembled in an engine, it is loose and therefore free to work back and forth within its housing. Because of the loss of radial pressure, there is inadequate contact with the bearing housing, thus impeding heat transfer away from the bearing. As a result, the bearing overheats causing deterioration of the bearing surface.

POSSIBLE CAUSES

1. Filed down parting faces in a mistaken attempt to achieve a better fit, thus removing the crush.

2. Due to dirt holding open bearing caps.

3. Insufficient torquing during installation (be certain bolt doesn’t bottom in a blind hole).

4. The housing bore was oversize, or out of round.

5. Over use of shims.

CORRECTIVE ACTION

1. Clean mating surfaces of bearing caps and inspect for nicks and burrs prior to assembly.

2. Check journal surfaces for excessive wear and regrind if necessary.

3. Check the size and condition of the housing bore and recondition if necessary.

4. Correct shim thickness (if applicable).

5. In addition Install new bearings using correct installation procedures (never file bearing parting faces).

Using “Plastigage”

Plastigage
Plastigage

So most auto parts stores have it available. Also it comes in various different crush dimensions to coincide with the engine bearing clearance figures for your engine.

Start with loosening the bolts of bearing cap number one. Remove the bolts along with the cap (and bearing). Consequently wipe all traces of oil from the crankshaft and the bearing surfaces.

Next, tear off a short piece of Plastigage (it’s sold in a long, thin paper envelope). Place a section of Plastigage on the center of the crankshaft journal, oriented front to back or diagonally.

Install the bearing cap and bolts. Torque to specifications, then loosen the bolts once more and remove the cap. You’ll find the Plastigage has crushed on the crankshaft journal.

Use the envelope the Plastigage was packaged in. You should find a scale on one end. Compare the scale to the crushed Plastigage on the bearing journal. This is the clearance figure. Now if the clearances are within specifications, you can move forward. Clean the journal (it wipes off with a towel soaked in brake cleaner) and repeat the process for all bearing journals.

You can also use the same format for checking connecting rod bearing clearances.

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