[chris_seven]

There is some poor information circulating about 911 cranks and if the correct surface treatment is carried out it should all be quite straightforward.

911 Cranks are generally manufactured from a medium carbon steel (CK45) and then Tenifer treated.

Tenifer treatment is also known as Tuftiding and more accurately this process is best described as ferritic nitrocarburising.

It was traditionally carried out in a Cyanide based Salt Bath and is a relatively low temperature process which is carried out at around 595 degC and should not cause any significant distortion. Modern variants of this treatment have eliminated the Cyanide but they are still effective.

The depth of the surface layer is very limited and is normally present as a 'white layer' which should not be removed. This layer is generally only a few microns thick.

This process will destroy the 'bungs' in the crank and after this process has been carried out the crank oilways need to thoroughly cleaned and the crank re-bunged.

In the Sixties and Seventies this process was sometimes referred to as 'Soft Nitriding' or 'Salt bath Nitriding' and this has led to much confusion.

Many High End Race Cranks are made using special 'Nitriding' Grade Steels which are then Gas Nitrided. They are generally made form steels such as EN40B which have significantly different alloy additions to CK45 which help to create a very high level of surface hardness and greater case depth of between 0.5 and 0.6mm.

The heat treatment process is much more sophisticated than Tenifer Treatments and more costly.

Tenifer treatments help in significantly improving scuff resistance and do provide some small improvement in fatigue life but not in comparison to a correctly applied Gas Nitriding process which is significantly better.

The confusion between 'Nitriding' and 'Soft Nitriding' has meant that form time to time efforts have been made to Gas Nitride a CK45 Standard Porsche Crank which, due to the lack of appropriate alloying elements is never successful.

When relatively simple steels are Gas Nitrided they generally produce surface layers that are predominantly Iron Nitride which is not substantially hard but is brittle and has a tendency to crack and subsequently spall.

It is therefore unwise to Gas Nitride standard 911 Cranks.

There is a second issue that needs to be considered. Gas Nitriding also produces a 'White Layer' and this layer is predominantly Iron Nitride and it is customary to remove this layer by either grinding or chemically.

This has led to come cranks that have been Tenifer treated having the White Layer produced by this process being removed which takes away all of the surface enhancement.

There are several competent machine shops ion the UK that can carry out this process successfully and if you send me a PM I will send you their contact details.

They will also Magnaflux and Micropolish the crank if needed.