<VV> Re: crank polishing

[djtcz@comcast.net]

Message: 12
Reply-To: <dsjkling@sbcglobal.net>
From: "Dan & Synde" <dsjkling@sbcglobal.net>
To: "Virtual Vairs Submission" <virtualvairs@corvair.org>
Date: Fri, 21 Jan 2005 19:20:01 -0800
Subject: <VV> Procedure for polishing a nitrited crankshaft's journals?

>Does anyone know how to polish a nitrited crankshaft?  I have one that was
>ground and nitrited but never  finished.  It is a dull dark gray all over.
>I don't know if this can be done by hand or if it needs to be done on
>special equipment.

"Primum non nocere" - first, do no harm.
http://www.engineparts.com/Motorhead/techstuff/bearingfailures.htm#
Click on the picture labeled 'accelerted wear" 
1 - Machine polishing with a fresh belt can pretty easily put a few 'tenths" of taper or an undesireable concave profile on a crank journal.  The tufftride '"white" hardness layer can be mighty thin, so some claim it is possible to bust through that by polishing too.  The desireable tough fatigue resisting layer goes deeper.
2 - Maybe even more important is the importance of polishing in the 'right direction". On Tufftrided, Nitrided, and nodular iron cranks there are tiny surface scales and flaps that need to be smoothed down like petting a porkypine in the "right" direction.  This can only be done by polishing the crank as if it were spinning just like it will in the engine.
http://www.federal-mogul.com/cda/content/front/0,2194,2442_2875_6752,00.html
http://www.engineparts.com/Motorhead/techstuff/crankgrind.html
I use 400 or 600 grit, dulling it first by "shoe-shining" a tool holder or junk allen wrench a few times. Polishing is Easier but dangerouser when done spinning the crank in a lathe, but possible to do manually with the crank in wooden v-blocks.
The older "Chevy power" books had a procedure that used a shoe lace wrapped around 400 grit paper on the journal, but that would not give the "directional" result