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Discussion Starter #1 (Edited)
New crankshaft bearings are almost always a must when rebuilding an engine. When you remove the old bearings, inspect them for unusual wear or damage such as scoring, wiping, dirt or debris embedded in the surface of the bearings, pitting or flaking. Anything other than normal wear may indicate an underlying problem that needs to be corrected before the new bearings are installed.

Dirt contamination often causes premature bearing failure. The underlying cause may have been a missing air filter, air leaks into the crankcase (missing oil filler cap, PCV valve, etc.), or not changing the oil and filter often enough.

If the engine has a "spun" bearing, it is likely the bearings were starved for oil - possibly as a result of a failed or badly worn oil pump, an obstruction in the oil pump pickup screen, or too low an oil level in the crankcase (leaky gaskets or seals).

Excessive heat can be another cause of bearing failure. Bearings are primarily cooled by oil flow between the bearing and journal. Anything that disrupts or reduces the flow of oil not only raises bearing temperatures but also increases the risk of scoring or wiping the bearing. Conditions that can reduce oil flow and cause the bearings to run hot include a worn oil pump, restricted oil pickup screen, internal oil leaks, a low oil level in the crankcase, aerated oil (oil level too high), fuel-diluted oil from excessive blowby or coolant-contaminated oil from internal coolant leaks.

Misalignment is another condition that may indicate the need for additional work. If the center main bearings are worn more than the ones toward either end of the crankshaft, the crankshaft may be bent or the main bores may be out of alignment. The straightness of the crank can be checked by placing it on V-blocks, positioning a dial indicator on the center journal and watching the indicator as the crank is turned one complete revolution. If runout exceeds limits, the crank must be straightened or replaced.

Main bore alignment can be checked by inserting a bar about .001 inch smaller in diameter than the main bores through the block with the main caps installed and torqued. If the bar does not turn easily, the block needs to be align bored. Alignment can also be checked with a straight edge and feeler gauge. A deviation of more than .0015 inch in any bore calls for align boring. Line boring must also be done if a main cap is replaced.

The concentricity of the main bores is also important, and should usually be within .0015 inch If not, reboring will be necessary to install bearings with oversized outside diameters.
Connecting rods with elongated big end bores can cause similar problems. If the rod bearings show a diagonal or uneven wear pattern, it usually means the rod is twisted. Rods with elongated crank journal bores or twist must be reconditioned or replaced.

Uneven bearing wear may also be seen if the crankshaft journals are not true. To check the roundness of the crank journals, measure each journal's diameter at either bottom or top dead center and again at 90 degrees either way. Rod journals typically experience the most wear at top dead center.

Comparing diameters at the two different positions should reveal any out-of-roundness if it exists. Though the traditional rule of thumb says up to .001 inch of journal variation is acceptable, many engines can't tolerate more than .0002 to .0005 inch of out-of-roundness (always refer to the specs).

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If you are rebuilding a Vee engine and the main bearing bore needs to be aligned bored/honed consider re-installing the (bare) heads and the (lower) intake manifold first. This puts the same stresses on the valley of the block which might pull the main bore slightly out of shape/alignment. This is certainly a must for "high performance" applications.

Also, if you are switching intake manifolds, use your new one for this task.
 
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