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Crankshaft bearings

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David64MGB

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Measured my main and big bearing diameters on the crankshaft today. Only had vernier so the measurements are only within a thou. My big end bearings are currently .020 undersize, but are another .002 to .004 under that, apparantly from wear. The big end bearings are .030 undersize and another .002 to .004 under that. Should I turn these down to the next size?
 
Crankshaft journals are measured in tenths of thousands, so I would measure everything with a mic and let someoene who knows how to use one to measure such tight tolerences do it. Your dial caliper measurements are nearly enough acurate in this type of application. In many measureing applications where the tolerence range is +/-.001" - .002", a dail caliper might be close enough, but in the case where the tolerence level is +/- .0005", the caliper is just not acurate enough for the job.
 
I am definitely more than tenths out of tolerance. The accuracy of the vernier is plus minus .0005 (plus the operator errors), but that isn't enough to bring them in range.

I was trying to get one of the guys from the local car club to mike the parts, but that wasn't happening so I figured I'd try it myself to see if they were even close. I know I have to turn the journals, I am just looking for someone more knowledgable to tell me this. This is my first engine re-build.

The next trick it to find a reputable local shop that understands MGB engines to turn them.

Thanks,
 
Take the crank to the local machine shop and let them mic em to see what there is to see, in any case they should be polished, worse case turned to next smaller size. Not a big deal.
 
What is the original size of the main journals on an 1800 5 main crank? Also the rod journals and the original cylinder bore diameter. I have a 72 engine that I'm going to rebuild when I get the time. /ubbthreads/images/graemlins/cheers.gif PJ
 
Your local machine shop will have all the sizes etc etc on his computer.
 
At 40-under you're getting about to the end of that crank's life....IMHO
 
Paul get yourself bearing book, I bought a old Vandervell bearing book off Ebay, I also got a Federal Mogul one from the machine shop. bearing books have all the jouranl sizes, housing bores and shell thickness in them. I'm at home now or I would give you the specs, I can remember 1275 specs off the top of my head, but the others escape my mind.

Tony, I know you made a typo, but just not confuse anyone reading this, crankshafts are ground under, not over.

Biggest thing to worry about when getting someone to grind your crank is that they put a nice filet radius on the journal, go to a shop that does alot of race work, they'll do it right.
 
2.1265"~2.127" main journals, 1.8759"~1.8764" rod journals. (Have a Haynes manual to hand) /ubbthreads/images/graemlins/jester.gif

IMO .040" under ~is~ the "last cut". And the fillet-cut edge radius is quite important to reduce stress risers.
 
Thanks, Hap - I did...I'll change it.
 
Thanks guys, The info helps a lot. /ubbthreads/images/graemlins/thumbsup.gif /ubbthreads/images/graemlins/thumbsup.gif PJ
 
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