i see it advised against but has anyone actually experienced a failure? if so, what happened exactly?

(i don't need a new crank, i'm grinding for another reason)

JBT had a failure shortly after a regrind, but I'm not totally sure the failure can be totally atributed to the regrind, I'l let him comment, or it might be worthwhile speaking to him direct

cheers mate, i'll leave it for him to see here in case anyone else wonders

from memory, re-grinding take off the nitrate harding from the crank.

and i *THINK* it does not allow a propper oil film on the bearings which casuing the bottom end to fail.

i no of an engine which had a full re-build + New oil pump the lot.. and it failed due to bottom end knock.

The problem Is that you go through the nitride coating when you grind. It's only going to be a couple of microns thick.

The coating is like a case hardening process. I.e. the metal of the crank isn't particularly durable.

I can't comment on durability afterwards but it was coated for a reason.

An interesting conversation with a guy I knew (worked for bae with weapons, firing pins etc.) Suggested that there were nitride coatings that were nigh on indestructible. It may be you can find a metal improvement company that will take on a small scale one off? You would have it ground to size and then coated.

Have a look on Guy Croft's website for the reasons not to regrind.

http://www.guy-croft.com/viewtopic.php?f=3&t=2031

Had one regrind on my 2.0TS, polishing on 2.5 and 20VT - no issues.....BTW at least 60% of our used cars here have some kind of regrind, i saw few times an crankshaft that was regrinded 3 times and used for long with no problems with appropriate bearings.......well most of it was daily cars, not driven hard......But hope you understand the reason for that - here in the price of one new 20VT crankshaft i can buy 2 used MK1 Punto's in full working order.

i cant see the problem personally, a grind and rehardening

guy crofts article makes no mention or hardening after the grind. in theory it may produce different stress risers which may lead to cracking but this is different to the wear problems described in the forum advisory.

providing anecdotal examples of small samples of failures is not reliable, these could fail due to a multiplicity of reasons ranging from poor machining, poor build, poor components, or a multitude of other reasons....

Guy covers it some more here http://guy-croft.com/viewtopic.php?f=13&t=2746

It has come to my attention that somebody thinks my engine failure in my project thread is related to this thread, it is not. My failure was due to incorrect bore size for piston clearance. I have NEVER re-grinded my crank nor had a crankshaft issue.

I would strongly advise against re-grinding as suggested above.

So, nobody has actually experienced a bottom end failure that has been 100% attributed to a re-grind? ...but then by the sounds, not many have actually tried it either so...

the problem with the GC posts are a) I get the feeling he's mostly talking about the old skool twin cams which were probably subject to differing levels of nitriding depth and b)he's not said the failures he's seen are 100% attributed to a re-grind either.

i would assume the nitiriding layer on a modern 20v crank would be a lot thicker, as much a 40 microns but that's just speculation - need to measure it really!

thanks for the input

I went to GC's when my engine was being built with a new crankshaft. GC pointed out that this didn't have the nitridring on it and he demonstrated the difference between the 2.

He took a centre punch and a hammer and gently taped the non nitrated crank, this put a dent in it straight away, he then took an old crank with the nitrated layer, used the centre punch and used full force on the centre punch, the result? No dent.

For me, this was quite conclusive on which crank I should be using.

can you get a crank nitrated harded after the skim?

supposedly yes and as it's done at relatively low temps, shouldn't distort the crank or create stresses. however, GC has some warnings on it so some further investigation warranted me thinks.

Are 20vt cranks that rare where you can't source a good used on?

Original 20V and 20VT crank is NOT nitrated!
We did several investigations.
Therefore regrinding is absolutely no problem at all.

Juergen

You should ask Guy directly about the 20v cranks, this sort of thing is one of his pearls of wisdom.

I would like to suggest that the earlier cranks have a deeper nitride layer than the modern versions which would be a different procecss entirely. The original liquid nitriding I believe was banned due to some sort of harmfull chemical used.

Is there no treatment at all on 20v ?

now that's very interesting!

would you mind sharing a few more details of the investigations?

Please do.. im interested...

i wounder if Guy Croft will come on here to explain a bit more..

Apologies, appear to have totally missed that. So that's two saying no nitride layer on the 20v cranks.

Don't know about the rest of you but after 70k, my crank was still within std OE diameters - so if they're not nitrided, what's the deal?

Very simple test - make a hardness test (I think we did Vickers) with original crank, take a file and grind a bit of surface, repeat test.
If there are no changes in the result, there cannot be any nitride on it. We did not find any change.
Moreover I am quite sure that a grinding shop would realize a hardened crank, when trying to regrind it...
It is very very hard to regrind a nitride surface...
Juergen

Maybe if you look at my signature, you will notice that I don't have a 20vt and was talking about 16v cranks

what kind of arsey response is that? you're the one talking about 16v cranks in a 20v crank thread!

and you checked the journal diameters first to make sure they had not been ground already? did you test more than one crank?

Begbie is Mr Cranky

Pun intended?

Maybe take the time to re-read your own thread.

People were discussing about regrinds and the topic went onto nitrate on the crank. I was giving a real life example of the difference between a nitrated crank and a non nitrated crank.

well, might as well! it was not ment to be.. but Yer..