Problems with tight connecting rod bearings

Written by chris stevenson
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Problems with tight connecting rod bearings
Tight connecting rod bearings can cause a multitude of engine problems. (pistone image by Michele Campini from Fotolia.com)

Rod bearings receive enormous loads of stress and rank high on the list of catastrophic engine failure. Tight or stiff bearing problems can be the result of many factors that lead to their malfunction. Rod bearings can spin, freeze, stretch, deform and suffer out-of-round conditions. Rod bearings have precision, machined surfaces that must handled in a contamination-free environment during inspection and repair. Human error, lack of lubrication and structural failure account for most bearing failures.

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Heat

When inner rod bearings have reduced clearance between the bearing surface and the crankshaft journal, a proper oil film has no room to reach between the two surfaces. This increased friction produces heat, causing the bearing to expand in the race, further cutting off the oil delivery. When temperatures reach as high as 204 degrees C, it leads to annealing or galling of the bearing to the journal race. This will often tighten or freeze the bearing against the crankshaft journal. Bluish-black marks appear on bearings and journals that have suffered extreme overheating.

Torque

An overly tight bearing can be the result of an out-of-sequence torque procedure, or a torque pressure that exceeds the manufacturer's specifications. The bearing surface will crush against the journal, causing excessive friction and heat. Over-torqued or over-tightened bearings cause the rod end cap to bow in the centre, resulting in an out-of-round profile. The centre gap in the bearing will chafe and vibrate with each revolution, degrading the bearing material and removing minute metal particles. This causes vibration and rod knock that gets progressively worse.

Lubrication

Tight or crushed bearings can deform the oiler hole alignment between the crankshaft journal and the bearing oiler hole, causing a reduced flow of oil to the bearing surface inside the connecting rod journal. Without proper lubrication to keep the bearing from contacting the journal, temperatures rise and bearing material becomes scuffed. Repeated scuffing leads to excessive clearance and rod knock. Excessive rod knock can throw the rod as a result of the major imbalance in the connecting rod rotation.

Bolt Stretch

Tight bearings cause excessive connecting rod bolt stretch. Bolt stretch causes the threads to weaken and tear the threads inside the rod cap. Bolts under such stress fatigue prematurely and lose their tensile strength; they cannot be reused and expected to indicate proper torque numbers. Bolt stretch gauges have to be used to measure the length of bolts when they have been fully loaded and torqued to specifications.

Bearing Spin

Bearings tightened beyond specifications will heat up from friction and cast off bits of metal that degrade the bearing surfaces. The gap between the journal surface and bearing surface deteriorates, opening up with excessive clearance. This causes the rod to pound in its seat, setting up a vibration in the rotational axis of the connecting rod. "Throwing a rod" or a spun bearing results.

Removal

Tight bearings result in removal difficulty of the connecting rod bolts. The bolts can freeze in position, requiring the use of air power tools to remove them. Bolts that refuse to move in their threads must be heated or extracted with special tools, such as sharp fluted bolt and nut extractors. Bolt breakage requires Easy-Out tools, including extensive drilling and tapping.

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