<VV> turn my crank

[AeroNed at aol.com]

In a message dated 11/2/2006 7:55:19 P.M. Central Standard Time,  
tdrinkr at yahoo.com writes:

what is  done when a crankshaft is
turned? And what are the consequences of doing  this?
I.e. will over-sized bearings be  needed?




The crank is placed in a lathe so that the main and rod journals are  located 
in the center of the spinning lathe. A cutting tool, basically a sharp  of 
steel, is used to remove a small about of the journal. The amount of the  
material removed depends on how bad the journal is originally. The goal is to  cut 
all of the journals to the same diameter. It is not uncommon to have the  mains 
at one new diameter and the rod journals at another.
 
The consequences are that any surface hardening of the original crank is  
lost. Furthermore, the crank has lost some of its life. Just like cylinders,  
cranks can only be turned a finite number of times. It will also loose a small  
amount of its strength, but for anything but a really hot racecar, this loss of 
 strength is insignificant.
 
The shop will tell you how much they cut the crank, 0.010, 0.020 or even  
0.030 under size. You should get this for both the mains and rods, if different.  
Then you get the appropriate bearing sizes.
 
This is not a big deal, happens all the time. Having the crank turned is  
usually cheaper than buying a replacement. Of course if you're building the  
engine after the shop turns the crank, make sure to check their work. Measure  the 
journals with a micrometer if you have one or at least use plastic gauge to  
measure the bearing clearance.
 
Hope that helps,
Ned