Testing Bearings on a Pressure-Lubricated Engine; Factory Methods of Fitting Engine Bearings; Repairshop Procedure in Adjusting, Refitting, and Renewing Engine Bearings; Maintenance of Connecting-Rod Lower Bearings; Maintenance of Main Bearings; Example of a Pressure Lubricated Main Bearing Layout; Relation of Engine Lubrication to the Life of the Bearings.
This subject deals with suggestions for locating loose connecting-rod and main bearings and the procedure for reconditioning them.
The repair operations on the engine crankshaft and connecting-rod lower bearings are as follows:
1. Testing for looseness
2. Tightening or adjusting
3. Refitting
4. Renewing
The job that most mechanics do when an engine is brought in with loose bearings is to tighten or "snug" them up, which is accomplished by removing the bearing caps, removing one or more shims or reducing the depth of the bearing caps and then re-assembling. This can be done only when the bearing metal and bearing journals are in good shape and where the amount of looseness is very slight.
When any hearing is allowed to run loose for any length of time, it will usually be found necessary to refit it in addition to the snugging-up operation previously described.
Renewing a bearing is necessary when one or more have been pounded or burned out due to misalignment, lack of lubrication, or other causes.
How to Determine What Bearings Are Loose

Before any actual work is done on the rod or main bearings, the mechanic will want to know just what bearings are loose and the approximate degree of looseness.
One of the easiest methods for determining the condition of the bearings in an engine oiled by pressure lubrication consists in disconnecting the regular oil pump and passing oil through the system by means of pressure from an outside source. This test will give a very accurate indication because it makes known the amount of contact between each bearing and journal, based on the fact that a bearing with 90 per cent or better contact will allow very little oil to leak through past its journal.
It will detect a bearing that is o. k. as regards clearance, but which is incorrectly fitted as regards contact or spotting of the babbitt.
Fig. 1. A method of testing for loose bearings in which oil is passed through the system while engine is static. Besides detecting a loose bearing it will also detect an obstructed passage. As it is possible for a bearing to be tight and at the same time pass too much oil, this method of testing gives the most accurate check on the quality of a reconditioned main or rod bearing.
Special observations should be made where piseon pins and camshaft are pressure lubricated.
i By Paul Dumas, Technical Editor of Automobile Trade Journal. Reprinted with permission from Motor Age and Automobile Trade Journal.

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Carburetor Manuals: Engine Bearings