Blue spots on crankshaft
- Thread starter Booner7M
- Start date
IJ - maybe we should say .25 mm?
Every 7m crank will have blue on it from the hardening process when it was made, as IJ pointed out. If you grind past .010 on the rods or mains, you may be past the hardness of the crank, as IJ pointed out. If you go as far as IJ has, you will have to nitride or heat treat the crank to bring back the surface hardness, as IJ pointed out.
Grinding is typically done to remove larger scratches and to make the crank straight. Grinding is used to remove.003 and more. Polishing is done after grinding, or without grinding to just resurface the journals. Polishing will remove most light scratches. Polishing can remove from .0001 or less up to .002. Polishing will not staighten the crank. If you need more than .002 removed from the crank, send it out to be ground. If you have had rod knock, the crank will be bent. You may or may not have a crack leading from the oil hole also. You can not see it by eye, it will need to be fluxed to show up. If it is cracked, the crack will need to be ground out with a die grinder, welded, reground, hardened, and then polished.
Every 7m crank will have blue on it from the hardening process when it was made, as IJ pointed out. If you grind past .010 on the rods or mains, you may be past the hardness of the crank, as IJ pointed out. If you go as far as IJ has, you will have to nitride or heat treat the crank to bring back the surface hardness, as IJ pointed out.
Grinding is typically done to remove larger scratches and to make the crank straight. Grinding is used to remove.003 and more. Polishing is done after grinding, or without grinding to just resurface the journals. Polishing will remove most light scratches. Polishing can remove from .0001 or less up to .002. Polishing will not staighten the crank. If you need more than .002 removed from the crank, send it out to be ground. If you have had rod knock, the crank will be bent. You may or may not have a crack leading from the oil hole also. You can not see it by eye, it will need to be fluxed to show up. If it is cracked, the crack will need to be ground out with a die grinder, welded, reground, hardened, and then polished.
IJ. said:
Cant wait till i grind my rod journals .200 and see what happens with the price.......:momaru:
Nope, used copper pennies to check for roughness back in the day - now I just don't bother - polishing the crank is dirt cheap.
it's a simple thing to do to see if it needs to be polished...
though I doubt anyone is pulling a good running engine apart...
though I doubt anyone is pulling a good running engine apart...
Slide: Measure it first as ANY work on that is going to be minimum of a grind to 10/10 and if it's been done already 20/20 then nitrided.
It would be cheaper to source a Virgin crank.
It would be cheaper to source a Virgin crank.