Re: More crank bearings

Posted by lee on January 12, 2005 at 13:39:54 from (24.24.95.246):

In Reply to: More crank bearings posted by CNKS on January 11, 2005 at 18:19:12:

This is my take and I'll have to say I did not read all that below word by word. An engine is an engine to me, tractor or automotive, so I don't look at old tractors as though quality of work should be anything less than say for a daily driver automobile. The plastigage is just a means for checking or verifying an assembled journal or rod bearing clearance in lieu of a more elaborate process requiring the proper measuring tools. Plasigage must be used without oil as it will dissolve in oil. Typically an outside micrometer calibrated on size and good to a resolution of .0005 (.0002 for closer work) should be used to determine the journal diameter measurements. First off the crank must be clean and a thorough visual inspection made to evaluate the ground journals for surface finish, nicks, scratches, etc.. It goes without saying it will be near impossible to measure a beat up crank precisely. Assuming it looks and feels real good, several readings would be taken on each diameter to locate the high and low readings, subtracting to get out-of round. On a very good journal the difference may not be discernable ie., the journal is round. At least two readings would be recorded on paper at the high and low spots usually 90 degrees apart. Sometimes more readings are taken and recorded at equal spacings around depending how anal the machinist may be. After all the measurements are taken and recorded an assesment of the condition of the crank and how much to grind it can be done against the specs. Cranks tend to wear out of round or to have surface finish problems so I would look at surface finish and out-of round very close against specs. If surface finish is a problem the crank should be ground. If out of round (i won't quote a spec) beyond spec it needs to be ground. As stated above it is difficult to get good measurements if surface finish is poor. It helps to know what a good journal looks and feels like.
This sounds like a lot, but with experience a guy can evaluate a crank rather quickly.
Moving on. Ideally a dial bore gage is used in a similar fashion on the assembled mains with the bearing shells in place or on rod big ends with bearing shells in place to determinie the inside diameter of the assembled bearings. If all is round as it should be the assembled clearance is determined by subtracting the appropriate inside and outside diameters. The dial bore gage step is often time skipped at a shop. They may have a crank ground at .010 under and buy the appropriate bearings. Plastigage is then used to determine the fit-up conditions. Since the crank was ground they can easily verify it for round and proper size. So plasitgage can be a fairly easy check for clearance at assembly in a case like this. I'll stop here. I hope it makes some sense. The problem with your question is it cannot be answered in short form. Even this long write-up does not do justice.

Follow Ups:

• Re: More crank bearings CNKS 14:47:03 01/12/05 (6)
  • Re: More crank bearings brtx 16:09:31 01/12/05 (0)
  • Re: More crank bearings lee 15:50:31 01/12/05 (4)
    • Re: More crank bearings CNKS 17:30:32 01/12/05 (1)
      • Re: More crank bearings lee 17:59:40 01/12/05 (0)
    • Re: More crank bearings lee 17:04:50 01/12/05 (1)
      • Re: More crank bearings CNKS 17:52:58 01/12/05 (0)

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