DDK - Die Deutschen Klassiker

Is there any easy way to tell if a mark on a crank journal can be polished out or if it needs grinding?

It's not accurate nor technical but on a VW crank I've worked on the theory that if you can feel it rather than see it then it'll need re-grinding.
I run (what's left of) my fingernail along the mark to see if it catchs.

Hi Luke

I did the first couple of years of my apprenticeship at a company that made new crankshafts, so I should know, but I struggle to remember!
We only used polishing to 'super finish' cranks that were going in top spec engines, things like Judd V10s and Metro 6R4s, didn't bother with the Maestro Diesel ones! The crank had to have a pretty good 'ground' finish to start with, the polishing would just improve what was there, removing any micro sratches from grinding and any dullness to provide a super shiney low friction surface. Think the maximum amount of material we ever removed using this process was 0.002mm, so naff all!!

I dont know if crank re-finishers use a corser grit or anything to remove bigger marks, but I suspect that you would only polish a crank (as opposed to re-grind) if it measured within original tolerance and had only the lightest of marks or looked dull/oil stained after years of service.

I'll stop waffling on now! hope that helps your crank selection for your new engine!

Dom

thanks guys, I'll let my fingernails grow a bit and then test it out!

Should you file and polish your fingernails first though?

When considering buying a used crank it's also worth checking the 'roundness'/ovality of the journals (using a micrometer) & the overall straightness of the crank (not easy for the 'layman' to do !).

thanks guys I've manganged, in the course of a day, to secure a Std/std magnaflux, and polished crank c/w with reconditioned rods. So it looks like I won't be needing to polish anything!

Oh the wonders of the internet!

Result!
And you kept your manicure intact. Cubist will be please