How to determine if crank needs to be cut?

[Pablo]

I'm curious to know how a machinist makes the determination. I am about to take my stuff to the machine shop and the machinist told me that they will hopefully only have to polish the crank, if not, the cutting and nitriding will cost around an extra 250 dollars.

In my particular case the crank came out of an engine with decent looking bearings (nothing spun, all the bearings were still grey). I see and feel some light scratches on some of the journals. I measured the journals all to stock diameters.
So what is the deciding factor? Can a scratch be acceptable so long as the material is all below (no raised edge) the rest of the journal surface?

 

[forcedfedv6]

Rule of thumb is, if there are major scratches on it as in gouched up from a spun bearing, or the crank has hot spots from a spun bearing from lack of oil, it needs to be cut, i wouldn't go no less than .20 under, if scratches are minor then a good polish will do just fine dont forget to have them chamber the oil feed holes in the crank while your at it for better oil flow.

 

[trev0006]

machine shop will know

 

[dr_frankenstein]

run the edge of a penny over the crank journals.

journals must be clean and dry.

if there is even the slightest hint of copper left on the crank - turn it.

if not - have the machine shop give it a quick polish.

:biggrin: A.j.

 

[ijames]

It is a combination of measuring the current journal sizes, knowing how much material polishing will remove, and having the experience to look at the scratches and surface finish and "knowing" if polishing will be enough given however much material is available to polish away based on the measurements. A few light scratches with no sharp edges are ok.

 

[turbot990]

polish it up you will be fine

 

[charlief1]

There are several factors. You have to mic several locations per main and rod to determine if the journals are egg shapped first. Then set it up in a v block fixture and run a concertricy check to see if the crank is bent or if the journals have any undue wear. Then go from there. On one of our motors not doing this will cause problems sooner or later.