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Old 06-27-2013, 02:09 PM
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Crankshaft Grinding Limits

Hi,
Mate has a 355 sbc that he is freshening up. Upon visual inspection, the crankshaft big end journals, particularly one, show some scoring and will need to be ground. The crank is already 20/20 under. It is a GM steel crank and he doesn't really want to fork out for a new piece if its not necessary.
It's a 12.75 :1 500hp street/strip car.
My question is, what is regarded as the 'safe' undersize limit for the big end crank journals to be cut?
Thanks for your opinions.



Duke

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Old 06-27-2013, 02:17 PM
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Duke-I've seen them 30/30,but mostly for standard re-builds.Nothing performance.You could start to get into the hardening of the crank and alot of that depends on who is cutting the crank.Why don't you look at it this way??. Is the savings worth the risk of losing it all??.

Oh yeah.Just so you know the kind of guy I am,if you have been following a thread I started about a 468 BBC,in there are parts guys say I don't need to run the ET I want.But I always want a safety margin to work with and that has worked extremely well for me for many,many,yrs.

Last edited by 1Gary; 06-27-2013 at 02:26 PM.
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Old 06-27-2013, 02:21 PM
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030 without worries. many have gone more
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Old 06-27-2013, 03:10 PM
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Thanks guys, so what's the risky part, going through the hardening on the crank? Or having to much soft metal (bearing) ? Or both???



Duke
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Old 06-27-2013, 03:16 PM
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The NASCAR guys run a Honda bearing I think, 1.888", so I would think you could cut the crank way past 0.030"/0.030" if you could get bearings to fit.
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Old 06-27-2013, 06:59 PM
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Hardness is usually gone at .010" IF it is actually a hardened crank. These are usually a dark or black color.

Bearings are available to .060" under, then to 2.00" (small journal) which are available to .040" under. Then as Tech says Honda. these are Also available in .25-.50-.75 MM undersize..(.010-.020-.030")

So you can get very small journal size. Just be sure a rod is available in your combo..and the oil hole ends up in a good place.

It always amazes me.. A guy will worry about a .030-.040" or more undersize crank. But think nothing bad about ,, say,, a 400 crank cut for 350 mains or Big Block crank cut to 2.100" small block rods.
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Old 06-27-2013, 09:01 PM
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But the NASCAR cranks are not a OEM steel crank.
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Old 06-28-2013, 01:53 AM
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Quote:
Originally Posted by BOBCRMAN@aol.com View Post
Hardness is usually gone at .010" IF it is actually a hardened crank. These are usually a dark or black color.

Bearings are available to .060" under, then to 2.00" (small journal) which are available to .040" under. Then as Tech says Honda. these are Also available in .25-.50-.75 MM undersize..(.010-.020-.030")

So you can get very small journal size. Just be sure a rod is available in your combo..and the oil hole ends up in a good place.

It always amazes me.. A guy will worry about a .030-.040" or more undersize crank. But think nothing bad about ,, say,, a 400 crank cut for 350 mains or Big Block crank cut to 2.100" small block rods.

Thank you all for your comments!

Yeah, I had a 400 crank cut to 350 mains size but used 'standard' not undersize bearings , obviously. Didn't give a thought to the hardening??
But it's been ok so far..

Ok, let's see if I got all this right..

The grinding of the journal to a smaller size is not an issue , if NASCAR can run 1.88" or so, then...

The hardening is gone by .010" if it existed at all, so again, a non - issue...

That leaves the thickness of bearing material (the soft metal).
I was told once, years ago , that it will 'pound' if it is too thick, under a lot of load , or is that just b/s too?



Duke
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Old 06-28-2013, 03:18 AM
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Quote:
Originally Posted by 1Gary View Post
But the NASCAR cranks are not a OEM steel crank.
do you think the lesser leagues are not running the same tech as the big boys on the older type stuff.. even the local dirt guys do on their junk..
buy good bearings and forgetaboutit.
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Old 06-28-2013, 03:22 AM
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Originally Posted by thinwhiteduke View Post
That leaves the thickness of bearing material (the soft metal).
I was told once, years ago , that it will 'pound' if it is too thick, under a lot of load , or is that just b/s too?



Duke
good bearing (tri metal) are not soft metal all the way through it's thickness.. the hard metal backing is the bulk of the bearing the soft layer isn't that thick no matter the undersize.. again with good bearing,
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Old 06-28-2013, 03:35 AM
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By the time you waste and money turning a crank and getting bearings................what does a new OEM steel crank cost anyways????. Kind of a moot point ant it??.
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Old 06-28-2013, 04:45 AM
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Quote:
Originally Posted by 1Gary View Post
By the time you waste and money turning a crank and getting bearings................what does a new OEM steel crank cost anyways????. Kind of a moot point ant it??.
maybe here in the usa, but he's down under a 350 forged crank might be harder to come by.
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Old 06-28-2013, 04:59 AM
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"That leaves the thickness of bearing material (the soft metal).
I was told once, years ago , that it will 'pound' if it is too thick, under a lot of load , or is that just b/s too? "

A complete load of b/s..

The bearing material stays the same thickness. The steel backing is made thicker as undersize increases. Another load of b/s is the old saying that the thicker bearing doesn't transfer heat out to the rod..
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Old 06-28-2013, 09:02 AM
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It isn't to be argumentative,but if I saw a crank in Racing Junk that was 30/30,there is no way I would spend the money for it.To me that is just another band-aid repair not intended for high performance use.
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Old 06-28-2013, 10:49 AM
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Back in the 70's when Bill "Grumpy" Jenkins was building his little 327 Pro Stock motors he was using cranks cut .030. He said they had less friction and would rev faster.
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