Where can I get a crank re-nitrided after grinding?

[Dave Erickson]

I have a 912 crank that needs to have the rods ground to 3rd under due to wear. I understand that the issue with doing this is that 912 crank journals were nitrided, so the surface hardness is not enough for third under. Which grinder would you recommend to do the third under and re-nitride the crank? Or is there more to it than that?

-Dave

[C J Murray]

Buy a new crank.

[Dave Erickson]

Buy a new crank.

Explain, please. I used to rebuild VW's and occasionally used third undersize without problems.

[Martin Benade]

VW cranks are not prone to cracking in half, so there is less to worry about.

[C J Murray]

Hi Dave, add up all the money you will have invested in your rebuild including the rebuilding of the carbs($2K) and distributor that you don't yet know are a problem. Now calculate the difference between all the machining and heat treating of your spindly 40 year old crank vs the cost of a new Scat crank. Now imagine you made the wrong choice and not long after the rebuild your 40 year old crank goes BANG, taking out your cases, even though Billy Bob's Super Precision Machine Emporium assured you that his DNA slathered all over your 40 year old crank would render it stronger than the crank in a 16000rpm Formula 1 engine. Buy a new crank or you are going to toss and turn in your sleep every night.
Cliff

[Dave Erickson]

Thats not so hard: cost to grind and nitride the crank $4-500. Cost of a new Scat crank: $2k? Cost of Solex rebuild: $200 (includes cost of rebushing and boring, new parts from Alfa1750, replated fasteners). Cost of distributor rebuild $50 (includes recurving). My labor is free, I'm retired.

But that misses the point of the question. It is an engineering question, really. Why is a 3rd under crank that has been nitrided any more likely to be a problem than a 2nd under that has not been nitrided? From all appearances, the rod journals on this crank are butter soft, because the original nitriding probably didn't go deep enough for even a second under grind. The mains are still standard. Nitriding does not introduce any stress into the crank from what I have read, so there is no reason to think that it would increase the tendency to crack. I think the reason that 3rd under has a bad rep is that the Porsche factory did not recommend it because they switched to nitriding cranks instead of induction hardening, and nitriding does not go nearly as deep.

-Dave

[Jacques Lefriant]

Hi Dave
nitriding cost is high unless you send the crank to a high volume crank specialist that can mag / grind the crank properly and has low overhead(i have access to these firms). you might be able to find a better used crank that has been done properly to 10 or 20 under. 30 under rod bearings are dealer only and there are no 40 under. There is no economical/theorectical way to predict the service life of repaired cranks. you can get a new forged crank from AA or any of our vendors The Scat crank is overkill for most applications and the German new crank is sky high in cost. call me if you want 619 247 7495
j
BTW
there are different Nitring methods and some will soften the previous previous hardening.

[Wes Bender]

From an engineering standpoint, can you regrind and nitride your old crank? Yes.
From a practical standpoint, is the risk/reward factor worth it? No.

Is your car going to be a museum piece? If so, nitride that sucker and then park it.
Going to drive it and have fun with it? Go for the replacement crank.

I broke a crank in my first 356C and it's hard on the wallet, the ego and your disposition.

Cheers,
Wes