Can you micropolish too finely?
 

When Porsche built these cars in the 1980's, cranks weren't micropolished...

Now, its common place...

I am sure that engine builders can polish journals to an absolute mirror finish, but, they don't.

Is that due to a cost or technique issue? Or, is it simply that a less than perfectly polished journal holds oil better?

There are high end engine builders on youtube that polish the journals to a mirror finish with buffing compounds.

Yet, most don't...

Why?

No benefit to doing it? Harm to doing it?

I hear that with the quality of oils that we have now, and the tighter tolerances, and I would think that it reduces the friction. I think it is a good thing. I had it done on my crank....

And since I do not have a close-up picture of my main or rod journals, I guess the crankcase will have to do.. LOL

<a data-flickr-embed="true" href="https://www.flickr.com/photos/47647442@N06/24388038521/in/dateposted-ff/" title="idle from under car."><img src="https://farm2.staticflickr.com/1646/24388038521_a61254be19_c.jpg" width="800" height="450" alt="idle from under car."></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>

I could never wrap my head around polishing journals, especially on old American lumps. That polish would be gone on no time. Always seemed a waste to me. But, with the hardened surface of a Porsche crank, it seems to be worth it.

As I slide down the slippery slope just trying to figure out if its worth it... My cranks looks like new to the naked eye. Crank bearings looked like new, rod bearings with very slight wear.

I am tempted to leave well enough alone, and leave it be.

From what I gather on the internet, regular crank polishing is with 400 grit belts, micropolishing is with 800 grit. Polishing can increase clearances...

If the goal is to polish, why not 1000 grit? 1500? 2000? polishing compound?

I see some builders do use some kind of buffing to get a mirror finish.

Wow...

Oh, then there is this... for the guy that really wants a shiny crank...



WINfinish Micro Polish | Winberg Crankshafts

When cranks are micropolished they are normally polished with a belt in the same direction or rotation as the running engine.

Buffing the journals in an uncontrolled manner just doesn't sound sensible.

The surface layer on a 911 crank is likely to be somewhere between 0.004 and 0.04 mm depending on the process time used.

Any significant process that is carried out on parts of this type really needs to be correctly defined and measurable otherwise how do you assess what is going on?

When you buy high quality Journal Bearings for use in Turbines, Compressors and other high speed rotating machines the bearing manufacturer always specifies the surface finish of the shaft.

We always made measurements using a Talysurf.

Sadly, at least in UK, I have never found a crank grinder that makes any measurements they just rely on the grade of the grinding wheel.

Frankly I wouldn't bother as all you will do is increase clearance unless you are grinding the crank to an undersize.

If you want a shiny crank you could have it superfinished.

reminds me of those cars with 24" rims that are gold, shiny and clean... riding on a rusty, dirty, cutlass. :D

I suspect I will leave it alone...

I have a close friend that has rebuilt many American V8's. When I suggested that I would replace/coat the bearings and leave the crank alone he didn't think it was a great idea.

Then, he came and looked at the crank and bearings, and seeing how they looked, was shocked. Said they looked great, leave it alone...

Again, its one of those slippery slope things. Do it "while you are in there" VS the theory of "If it ain't broke don't fix it."

My under standing is that the bearings NEVER touch the crank as they ride on a layer of oil. If the bearings looked like new, it would seem that the crank would still have the factory level of finish...

Now, if your crank or bearings are shot, that's a whole 'nother issue...

"And since I do not have a close-up picture of my main or rod journals, I guess the crankcase will have to do.. LOL"

wow, that is gorgeous. What did you use and how long did it take? Any clear coating on it?