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scrape the bearing to a fit before the 

 bearing cap is tightened up. It may be 

 necessary to clean up the crankshaft 

 journals as these may be scored due to 

 not having received clean oil or having 

 had bearings seize upon them. It is not 

 difficult to true up the crank pins or 

 main journals if the score marks are not 

 deep. A fine file and emery cloth may 

 be used, or a lapping tool. The latter is 

 preferable because the file and emery 

 cloth will only tend to smooth the sur- 



Fig. 15. Top half of crankcase, 

 showing method of crankshaft re- 

 tention by three main bearing caps 



face while the lap will have the effect 

 of restoring the crank to proper con- 

 tour. 



If a crank pin is worn out of true to 

 any extent the only method of restoring 

 it is to have it ground down to proper 

 circular form by a competent mechanic 

 having the necessary machine tools to 

 carry on the work accurately. 



After the crankshaft is trued the next 

 operation is to fit it to the main bearings 

 or rather to scrape these members to 

 fit the shaft journal. In order to bring 

 the brasses closer together, it may be 

 necessary to remove a little metal from 

 the edges of the caps to compensate for 

 the lost motion. A piece of medium 

 emery cloth is rested on the surface plate 

 and the box or brass is pushed back and 

 forth over that member by hand, the 

 amount of pressure and rapidity of move- 

 ment being determined by the amount of 

 metal it is necessary to remove. This is 

 better than filing because the edges will 

 be flat and there will be no tendency for 

 the bearing caps to rock when placed 

 against the bearing seat. It is important 

 to take enough oiT the edges of the boxes 

 to insure that tliey will grip the crank 

 tightly. The outer diameter must be 



Popular Science Monthly 



checked with a pair of calipers during 

 this operation to make sure that the sur- 

 faces remain parallel. Otherwise the 

 bearing brasses will only grip at one end 

 and with such insuf^cient support they 

 will quickly work loose, both in the bear- 

 ing seat and bearing cap. 



Scraping Brasses to Fit 



To insure that the bearing brasses will 

 be a good fit on the trued up crank pins 

 or crankshaft journals they must be 

 scraped to fit the various crankshaft 

 journals. The process of scraping, while 

 a tedious one, is not difficult, requiring 

 only patience and some degree of care 

 to do a good job. The surface of the 

 crank pin is smeared with Prussian blue 

 pigment which is spread evenly over the 

 entire surface. The bearings are then 

 clamped together in the usual manner 

 with the proper bolts and the crankshaft 

 revolved several times to indicate the 

 high spots on the bearing cap. The high 

 spots are indicated by blue, as where the 

 shaft does not bear on the bearing there 

 is no color. The high spots are removed 

 by means of a scraping tool, which is 

 easily made from a worn out file. These 

 are forged to shape and ground hollow 

 and are kept properly sharpened by fre- 

 quent rubbing on an ordinary oil stone. 

 To scrape properly, the edge of the 

 scraper must be very keen. 



When correcting errors on flat or 

 curved surfaces by hand scraping, it is 

 ' desirable, of course, to obtain an even- 

 ly spotted bearing with as little scraping 

 as possible. When the part to be scraped 

 is first applied to the surface-late or 

 to a journal in the case of a bearing 

 three or four "high" spots may be indi- 

 cated by the marking material. The 

 time required to reduce these high spots 

 and obtain a bearing that is distributed 

 over the entire surface depends largely 

 upon the way the scraping is started. If 

 the first bearing marks indicate a decided 

 rise in the surface, much time can be 

 saved by scraping larger areas than are 

 covered by the bearing marks ; this is 

 especially true of large shaft and engine 

 bearings, etc. An experienced workman 

 will not only remove the heavy marks, 

 but also reduce a larger area ; then, when 

 the bearing is tested again, the marks 



