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Polish crankshaft journals


turbogrill
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Hi,

 

What is your take on polishing a crankshaft journals? 

 

Internet seems to have different opinions.

 

My cranks looks good much better than one another one that came out of a running engine. I did scuff it a little with 600 grit sandpaper (that is what the rebuild book I am following recommended).

I can see mirror reflections but if I look really close I can see some "scratches". It's not a perfect mirror finish.

 

I could take it to a machine shop but I am getting tired of spending money....

 

Edited by turbogrill
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I have used crocus cloth and lubricant to polish crank journals before. https://www.mcmaster.com/crocus-cloth

 

i believe the experts run that in a thin belt sander and have the crank chucked up so it can rotate while they polish.  By hand it is tedious and you probably need to convince yourself it is super important in order to have the patience to make a difference.

 

 

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29 minutes ago, turbogrill said:

Hi,

 

What is your take on polishing a crankshaft journals? 

 

Internet seems to have different opinions.

 

My cranks looks good much better than one another one that came out of a running engine. I did scuff it a little with 600 grit sandpaper (that is what the rebuild book I am following recommended).

I can see mirror reflections but if I look really close I can see some "scratches". It's not a perfect mirror finish.

 

I could take it to a machine shop but I am getting tired of spending money....

 

 

If your cost options are final polishing of the crank vs affording good tri metal (ie clevite H" or very high quality bi metal bearings , I would highly recommend spending the money on the bearings. My gut says your crank might already be better prepped than several I have seen work....

 

Also depends what bearing type you run, the bi metals tend to "polish" the crank some on their own. Higher performance engines using tri metals might show more response to the smooth surface. 

Edited by Black Magic
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With all the time and money spent on one of these race weekends I would never skimp on something like this. I would rather pay a machine shop to make it perfect than possibly have an issue due to it and end up on the trailer on Saturday. 

 

First e30 engine we refreshed one team member didn’t want to spring for new stock rockers. I should have stood my ground and just replaced them all. So at a test day a rocker snapped, the part still left on the rocker shaft got wedged against  the retainer holding the valve open. This all transpired at 6k. Never again will I not replace something gay I know should be replaced. As a result we have had pretty good reliability as a team. 

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I think there also a discussion if polished is always the best. And how much? Seems like there is a relationship between the amount of polished a crank should be and clearance.

 

The rebuild book I am following is very picky about straigtness and roundness but dont go much into polish of a crank.

 

Just having a mediocre machine shop polish a crank might not always result in a better result.

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Straightness & roundness are paramount. You can polish it to a whatever-micron finish you want, but if it's running out .0025" then it doesn't mean sheet, it's going to wipe out the bearings. For polish, as long as there are no raised/bruised areas, that's step #1, then worry about actual surface finish. 

 

Edit: you should be able to rotate the crank w absolute minimal finger pressure without bind when installed with a light oil on the bearings. Or there is the old "wing it around" trick - you should be able to wing the crank around by hand and it should free spin a couple of revolutions (no rods attached obviously). I'm legitimately not an engine builder, but I have been in the machining trades for my whole adult life and this should be common sense for anyone you allow to help or touch your engine components.

Edited by pintodave
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14 hours ago, pintodave said:

If it binds, it's no good. Period. 

 

With the caps on I can rotate it with my hand and a firm grip. But not with a finger around the snout.  It should be able to free spin after you give it a spon. something is wrong.

 

I am using very thick assembly lube. Could that be it?

 

Have bought a set of mics and will double check everything again.

Edited by turbogrill
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Do you have the crank seals in place? I've never gotten crank to spin with assembly lube, at best it's a half turn on a loose crank. To tell the difference between lube and bind, compare the resistance when turning very slowly (should be light) vs attempting to turn  it fast (obviously more resistance).

 

If you find a spot that has more resistance than other places and free in another, you have a bent crank and the bores aren't in line.

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