crankshaft wear: what to do

[Peterfrans]

I received the report from the machine shop today. The results of their measurements are as follows:
all rod and main bearings: 56,97mm except no3 cyl rod: 56,98 and main bearing 4: 56,96

My little spec book says that the wear limit is 56,96

Basically my options are:
1. only have the surfaces polished, they are still within limits and i am not going racing or use it as a daily driver

2a. have the crank ground down to .25 undersize

2b. have the crank ground down to .25 undersize and have it nitrated.

Obviously 2b is the most expensive option. I am partial to this as I don´t like cutting corners which I might regret later. On the other hand saving some money in these times is not also not bad. Open to advice!

[Wayne 962]

I don't have my spec book in front of me, but if the crank is in spec, just have it polished, and use it again. Grinding cranks is not a good idea, as they cannot get them as hard as they were when they were new...

-Wayne

[911pcars]

Peterfrans,
As I recall, nitriding involves elevated temperatures during processing. This is not ideal for leaving crankshafts straight. Tufftriding™ or Tennifer™ treatment is a lower temperature surface hardening process which is preferred. Tennifer is what Porsche uses in the fatherland. B. Anderson says Tufftriding is no longer available. Don't know, but you can do a search on the internet to confirm. A standard, "0-0" crank is preferable, but if you can maintain surface hardening with a "10-10" or "10-0" crank (0.010" undersize), and are able to source the bearings, go for it.

Sherwood Lee
http://members.rennlist.org/911pcars

[snowman]

More important thing to check is to see if the crank is ROUND. There is a seperate spec for this. If it is still within the roundness spec, then have it polished. If it is not within the roundness spec have it ground and re hardned at a place that does lots of porsche cranks, even better nothing but Porsche cranks. After gringing you might consider having the radius s shot peened for extra measure. The shot peening will prevent cracking at the radius, one of the most likely failure modes.