Crankshaft size wear limits

Zeke...PA

Zeke...PA

Well-known member
What would be an acceptable wear limit before turning an 8BA crank with no out of round or taper? Thanks Zeke
 
There can be varying answers depending on how the engine will be used.If you are talking rod journals the acceptable undersize can be influenced by bearing availability. What I am saying, is that a 2.135 could be ok with .002 bearings and 20-50 oil. Main bearings are more difficult to find in .002 so a nice, round, journal at 2.497 can be used. I think the more important question is "what are the vertical clearances when assembled"? Many engines have operated for many miles with .004 bearing clearance where as the spec calls for .002-.0025. Back in the "good old days" I have shimmed bearings, sanded caps and done about everything imaginable to keep my "rebuilt" engine cost in line with my wallet.

Loose bearings can be a cause of abnormal oil consumption because there is an abnormal quantity of oil being thrown onto the cylinder walls. This should be considered when deciding how much clearance to use.
 
Journal bearing clearances on the rod & mains are set up to keep the operating oil pressure within specs. The more clearance you have beyond recommended limits, the more of a loss of oil pressure you will have. How much an extra .001" will lose in PSI is anybody's guess but there will be a loss especially when the oil is hot.

The clearance specs.for a Mercury crank rod journal is .0005" to .003" and mains are .001" to .0026".

Kerby
 
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maybe your right about the oil pressure loss at .001 or .002 if you reinstall standard bearings, but no loss if you use the .001 or.002 replacement bearings because the clearance is taken up by the over size bearing
 
The clearances I listed above are from the 49-51 Mercury O/H Manual. My reference would be anything worn more than those clearances would start to be detrimental to the oil PSI. Most of my cranks have already been turned once so I don't think they make a .012" OS or a .022" OS just .010" & .020 and so on. I think on first run overhauls they used to make a .002" OS set of shells but I never was that lucky.

Kerby
 
In order to use those .002 bearings, the journal needs to uniform wear and that almost never happens.
 
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