Grinding Crank ? Nitriding ?

[Mops]

Hello guys.

Haven't posted here for a while here.

Background information:
I'm building m20b27 stroker motor for turbo engine target output about 400hp (1 bar of boost). I got stock high compression m20b25 motor and i got m20b27 bottom end for it's crank and rods. I got most turbo parts prepared and ready to go, but i need to actually build the motor first so i have something to put turbo on

The m20b27 crank i got is kinda worn and engine builder told me it will have to be ground/polished so oversize bearings (+0.25mm) can be installed. Aswell he told me that after grinding it has to be nitrited (or nitrided ? not sure of the spelling) to make strong, otherwise it will wear quick. Other local engine builder told me that nitriding it optional depending on the type of crank. I'm told that if the crank is hardened in a special way from the factory then it might not need nitriding.

Can any of you advise whether nitriding is neccesery ? it is an expensive process, i was quoted NZ$140 for grind and addiditonal around NZ$350 for nitriding.

I live in new zealand and m20b27 cranks are very rare here. I was looking for it for a year and first one i found was damaged beyond reasopable repair and this one is ok, but will need grind and oversize bearings, so before you suggest it... no... finding another crank is not really an option. finding one in good condition on the wrecker yard is next to impossible and cranks alone are hard to buy, i'd have to shell out for a complete used bottom end or really a complete m20b27 engine...

Much appreciated.

Thanks in advance.

[edited to fix typos]

[MJG]

Nitriding is a hardening process which infuses nitrogen into the surface. If it is nitrided from the factory then get it done. Unfortunately I can't say if they were or not, but I suspect not. Nitriding should be done BEFORE grinding too if anything. Personally I wouldn't bother nitriding it, as all the counterweights etc would then be nitrided, which, if they were already nitrided, would make them brittle.

I deal with nitrided gas turbine engine parts at work, so know a bit, but not that much.

[Andyboy]

You can't nitride a 2.7 crank because it's cast iron and Nitriding requires EN40B nitriding steel. You can however tuftdride it - a similar process which puts a hardened layer on the surface. It's well worth doing but some say you still need a steel crank to do this, something like EN16T.

[Templ8e30]

You can't nitride a 2.7 crank because it's cast iron and Nitriding requires EN40B nitriding steel. You can however tuftdride it - a similar process which puts a hardened layer on the surface. It's well worth doing but some say you still need a steel crank to do this, something like EN16T.

Spot on Andy, unless it's an alpina (324TD crank).

I used to work for Bodycote in Manchester who specialise in Nitride treatments. The slow process involves heat and ammonia gas.

Cheers,

Iain T