Crank grinding ? 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 something

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.

[winstontj]

Nitriding is when they super cool the crank to harden it. You have to go to someone who really knows what they are doing because there is a balance of hardening and polishing back and forth to make sure all rod & journal bearings are proper sized. I know that e30m3's are hardened but I'm not sure about the m20 cranks. Someone else will have to share. If you are going to turn down the crank you might as well have it re-hardened... on that note you might as well get it lightened, turn down the rod bearings to fit something more common in NZ (like a Ford rod size) and also move the big end of the rod out on the crank to get more stroke. It can all be done and if you are going to re-harden the crank... why not go all out??