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Old 08-07-2007, 06:12 AM   #1
Starkiller
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Default Micro Polishing crankshaft

I'm rebuilding a block and would like to micropolish the crank before putting it back in.
What should I use for a solvent and grit?
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Old 08-07-2007, 05:37 PM   #2
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machine shop..
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Old 08-07-2007, 07:45 PM   #3
SaabTuner
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Stock crank is already nitrided isn't it? I'm not sure how micropolishing would affect that.
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Old 08-08-2007, 01:21 AM   #4
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Quote:
Originally Posted by SaabTuner View Post
Stock crank is already nitrided isn't it? I'm not sure how micropolishing would affect that.
I thought only the spec c's were...or the ver 9's...basically only motors we don't get
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Old 08-08-2007, 02:31 AM   #5
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Huh ... that sucks then. Almost every other performance engine has either nitrided, or induction-hardened, journals. Seems odd that the WRX wouldn't. But I guess I shouldn't be TOO surprised.
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Old 08-08-2007, 05:04 AM   #6
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The proper way to micropolish the crank requires a large lathe and a crank polishing accessory that uses a narrow continuous belt of very fine grit emery.

The proper grit and technique depends on what you want done. The smart way is take the crank to a shop that does this as a service and get it done with the proper equipment. It is really not terribly expensive (at least when I used to do it). They can also custom size the crank journals to give you specific bearing clearances as well.

Special hard coatings and antifriction coatings are also available and that would be a good time to do it.

Trying to micro polish by hand can result in flat spots or taper in the crank journals or other odd problems. On cranks that have been nitrided the coating is very very thin (10µm - 15µm) and can easily be taken off by polishing by an uninformed mechanic. The direction of motion of the abrasive relative to the normal direction of rotation of the crank also matters so a low tech polishing by hand with a back and forth motion could cause more problems than good.

Larry

Last edited by hotrod; 08-08-2007 at 05:31 AM.
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Old 08-08-2007, 03:51 PM   #7
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well, couldn't one micropolish the "counter weights" by hand and then have the crank balanced...certainly won't remove too much weight that way...I wouldn't touch and bearing journal by hand though.
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Old 08-08-2007, 05:07 PM   #8
hotrod
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Not much point in polishing the counter weights, except to remove flash and roughness to help it shed oil and improve aerodynamics of the leading edge of the counterweights. Sure you can do that by hand (actually it has to be done by hand) but you would want to get it coated with an oil shedding coating if you did that.

Micropolishing usually refers to putting a very good rms finish on the bearing surface, and in some cases slightly changing the nominal journal size. In stead of buying special odd sized bearings to increase oil clearence you can have the journals polished to .0005 under or what ever additional clearance you like and get both improved surface finish, but special clearences while using standard bearing inserts.

Larry

Last edited by hotrod; 08-09-2007 at 03:19 PM.
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Old 08-08-2007, 08:43 PM   #9
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micropolishing by hand is asking for trouble
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Old 08-09-2007, 02:56 AM   #10
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Quote:
Originally Posted by hotrod View Post
Not much point in polishing the counter waits, except to remove flash and roughness to help it shed oil and improve aerodynamics of the leading edge of the counterweights. Sure you can do that by hand (actually it has to be done by hand) but you would want to get it coated with an oil shedding coating if you did that.

Micropolishing usually refers to putting a very good rms finish on the bearing surface, and in some cases slightly changing the nominal journal size. In stead of buying special odd sized bearings to increase oil clearence you can have the journals polished to .0005 under or what ever additional clearance you like and get both improved surface finish, but special clearences while using standard bearing inserts.

Larry
yes, I know micropolsihing reffers to the journals but I was talking of just polishing for the same reasons you listed above...
and to think of it to polish the weights by machine would require at least 4-axis
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Old 08-10-2007, 12:18 AM   #11
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I've had cranks polished. They rotate the crank and use this thing with a gritty tape on it that looks like a fishing pole or a bow (like bow and arrow). It's a very simple process and removes maybe a couple tenths at most. Sorry I don't know the grit they have always been done by a machine shop.
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