Polishing crankshaft journals

[siteunseen]

Is this something I could do or just pay somebody at the machine shop? I wouldn't do a hack job because I'm in no hurry. Anybody's advice please.

[siteunseen]

I forgot about my "How to Rebuild" by Tom Monroe. He tells how. Thanks anyway.

[cygnusx1]

Can you share your experience with this? When I had my crank out, I considered doing it, but I ended up leaving it alone.

[240260280z]

I ran a cotton rope and scotch brite on the crank like this:

[ozconnection]

I believe the trick here is to get 'even' rub all the way along the journal. A piece of rope will apply too much pressure in one area and not enough everwhere else, potentially doing more 'harm' than 'good'.

You could try a flatter material such as a carefully cut piece of rag or a fairly wide shoelace or similar.

My real solution would be to take the crank to a machinist and pay the man a few pennies!

Piece of mind.

[siteunseen]

You get a feel of when to move the rope around. It's a dance like all the other stuff that makes them fun.

[240260280z]

Put a two wraps of the scotchbrite and several wraps of the cotton rope around it.

[Phred]

Crank polishing has been done many ways, probably 90% of which are wrong. All due respect to blue, scotch brite is only a cleaner and does not polish a hard crank journal in the technical sense. The idea in polishing is to smooth down any micro (or bigger ) scopic peaks on the journal, to obtain a flat, smooth surface finish. Quality crank grinders will polish after grinding to remove any grinding marks, even if the journal looks smooth. Polishing is usually done with a machine about the size of a chain saw. It spins a continuous belt about an inch wide. This can also be done manually, with inch wide polishing paper, swinging your hands back and forth. Polishing, or sanding paper in bulk, called a shop roll is used. If the finish is poor, start with 320, work through 400 and then to 600. The crank should be spun in the direction the crank turns in service. This will microscopically "flatten" the grain in the proper direction. Think of running your hand through your hair. It lays the hair down in the direction your hand travels. Polishing the journals lays the grain down in the same way. A good micrometer that reads in tenths is used as the polishing proceeds. It's all too common to polish a journal till its not flat, but smaller in the center of the journal than towards the sides of the journal. So a good crank polish tecnician will mic as he goes, adjusting the areas being polished to maintain a flat surface. This is the most common way small shops polish a crank. Many custom crank repair shops, and racing crank builders, use a very expensive machine which clamps the journal in a flat polishing attachment, that is designed to put a very fine "micro polish" on the journal and maintain the all important flat journal. The meaning of flat, being that when checking the journal with a micrometer across its full width, from the radius of one side to the other, it should be the same size to within .0002.

This is just one paragraph to give you a general idea of how cranks are polished. It is an area often put aside as not "that important". That all depends on your point of view, and what you are expecting out of your engine as far as performance, and life expectancy. If you're really interested, there is some good info on the web. Just do a search on crank polishing. With 30 + years as a racing engine builder, I have my preferences. With few exceptions, the engines I build will have polished crank in it.

Phred

[siteunseen]

I cut a piece of 600 the length of the journals then taped it together, few drops of oil too,  then wrapped small rope around the paper like people do a stick when making a fire (like in Blue's picture).  Back and forth very lightly then went back with 800.  They were all in good shape to start with I just feel better now.

Edited January 30, 2012 by siteunseen