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Discussion Starter #1
I pulled this from a post from a friend on EvoM this was done on an Evo 8 crank, it was a very well known shop in Germany that did this test. I provided a crank for this test.

We did grind a crank down all the way to determine the deep of the hardening.
The hardening is not nitrite, its inductive hardening ( thats the reason why you can polish the crank.
It was an used Evo8 crank with an rod bearing failure on cyl.1, which was trash anyway due to the failure
We started to grind the number 2 Rod surface.
ALL dimensions in metric (mm) here. Hardness is not an absolute value in our case, its relative measurement!!

Initial : 47.993mm Fillet depth: 0.49mm Hardness: 88
1st grind : 47.893mm Fillet depth: 0.39mm Hardness: 88
2st grind : 47.743mm Fillet depth: 0.24mm Hardness: 88
3st grind : 47.493mm Fillet depth: none Hardness: 86
4st grind : 47.243mm Fillet depth: none Hardness: 84
5st grind : 46.993mm Fillet depth: none Hardness: 68 hardness drops fast

So , in conclusion, you can grind the Crank Rod pin ( only rod testet ) down.
so, 0.25 mm is possible when the fillets should be there and 0.50 mm if fillet doesnt matter or can be refreshed.
Any more does not make it because fillets gone and also hardening drops ( extreme loss after the -0.75 grind )
Also, the grinding should be done in an really smooth way, so it should not be done like an V8 crank. There should be removed only a small amount of material, with cool down times in between. The hardening can change extreme if to much heat is transfered into the crank. ( tried that at Rod position 3, more material in one grind, hardening changes 5 more point than rod 2 )

But, there is an other problem you will run into.
All Bearing manufacturers doesn't make the right bearings for the turbo application.
The max grind to get Turbo bearings is -0.25mm. smaller to get bearings from them.The bigger OS will be only for the N/A 4g63 / 4G64.
This was approved from King bearings, the materials and coats used will be different than and they do not advise to use in high HP applications.
Same Information from Clevite, ACL and Glyco.
Also, Mitsubishi has different bearings ( but same dimensions ) for the 4g63T and 4g63 n/a Engines.

It may be possible that someone has get Bearings as an custom order, but not "off the shelf".

For my self: For an almost stock Evo ( stock turbo ) i would use an grinded crank (-0.25) when an new one is not in the price range..
In any high HP application ( 500++) i would never used an grind crank.
Get an new or good used one. Better safe then sorry.

here in Germany, its easier and cheaper to use an new crank all time, because the grinding is expensive. Grinding all 4 rod sections is about 500$.
In the states, i assume, it will be much cheaper because you have more Engine shops then here in Europe.


my 2 cents

** These are not results I gathered, I am just passing along info I have found on another forum. All credit for doing this test belongs to the Original poster. **

My personal thoughts on why so many ground crank builds fail is from improper cleaning of the crank after the grind, trapped metal and stone in the dead section of the oil galley being washed into the rod bearings on start up.
 

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Great information!
 

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You need to walk a line with the hardness though, depth of hardness can vary wildly between parts. Generally it is at least .01" but beyond that it is going to be sketchy.
 

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Discussion Starter #6
Sean, it depends on the method of treatment, and the application of that method.

I know I have machined and built 6 7 bolt 4g63 that are running in the "local" area with cut cranks, over the past several years, with only 1 report back of a spun bearing, and that was not from the crank being ground undersized, that was from a bad install and tune, crossed up injector harness and dead times waaaaaaay of and the oil being thinned out from gas in the oil pan.

I always found it funny How it was preached that a 4g crank could not be ground and not fail, it was the only crank I have ever heard that about. Domestic or import...
 

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I always found it funny How it was preached that a 4g crank could not be ground and not fail, it was the only crank I have ever heard that about. Domestic or import...
So weird.

I don't want to step on toes or anything, but I'll say that the last couple hundred cut 4g cranks I've used haven't had an issue, even when cut beyond .010.
 
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