2.4 Crankshaft

[tharbert]

I'm in the process of rebuilding my 72 2.4T engine. I just got word from the machine shop that my crank needs a regrind: .25mm. I read somewhere here that it was better to try and source a replacement rather than use non-STD bearings...and they're expensive too.

So, what options do I have? What cranks will fit and does anyone have one sitting/hanging around? I see that 70.4mm throw cranks were used from 72 through 83 but I'm guessing not all would be appropriate.

[hightuned]

No way, why else would they make bearings down to 1mm grind? My 2,7 crank is in the shop for a 0,25/0,50 grind, calculate the bearing area and you will the the diff in strength isnt much after a grind and the increased heat from have a smaller surface must also be marginal.

[Henry Schmidt]

Regrinding your crank is one option. Just make sure you have the bearings located first. Some are NLA.
Grinding the crank is only one part of the process.
You should pull the crank plugs, grind the crank to size then Nitride the crank. Nitride is a heat process that can warp the crank, so you will generally have to have it straightened. After all that you have to replace the pulls. Since they aren't readily available your crank shop will have to make them or tap the crank for threaded plugs.
All told, a std/std replacement is generally easier.


Note: if you heat treat the crank without replacing the plugs your chance of failure is unreasonably high. (IE: plug fall out after running for a short time.)

[chris_seven]

Originally Posted by Henry Schmidt

Regrinding your crank is one option. Just make sure you have the bearings located first. Some are NLA.
grind the crank to size then Nitride the crank. Nitride is a heat process that can warp the crank, so you will generally have to have it straightened

Henry,

I think some care is needed when discussing the surface treatment of 911 Cranks.

A 2.4 engine had cranks that were Tenifer Treated and this is a process that is described as ferritic nitrocarburisng and is not gas nitiriding.

Tenifer Treatments are commonly referred to as Salt Bath Nitriding and can be applied very successfully to plain carbon steels but they fall a long way short of Gas Nitriding in terms of fatigue life improvement and depth of case. This Nitrocarburising process is also commercially known as either Melonite or Tufftriding

Nitrocarburisng is not a bad process and does give some fatigue enhancement and improved scuff resistance and is very cost effective.

It is now also possible to use Plasma Assisted Nitrocarburising but I have never used this technique.

The steel used for 911 cranks was known as a CK45 which supersedes to a
C45E (http://www.saarstahl.com/fileadmin/saarstahl_extranet/images/04_produkte/walzstahlsorten/english/1191_1201_C45E_C45R.pdf) This steel is a plain medium carbon steel.

If you Gas Nitiride a medium carbon steel with this type of composition you are likely to run into serious problems as the Iron Nitrides that will form are relatively weak and quite brittle. Torsional deformation may well cause the nitrided layer to crack and subsequently spall.

Gas Nitriding :: KEY to METALS Articles

I would never 'Nitride' a standard 911 crank I would always be very clear and specify Nitrocarburising.

Distortion during these treatments is also interesting.

If you manufacture a crank from a suitable steel then Nitriding and Tenifer treatments are unlikely to produce any significant distortion providing that the hardening process is carried out at a carefully controlled temperature.

Both Nitriding and Tenifer treatments are relatively low temperature processes and if carried out correctly should not produce any thermal distortion during the final stage of the heat treatment process.

Nitiriding is normally carried out between 490 and 530 deg C (can be upt to 560 but this is becoming unusual)

It is common practice to stabilise parts at 570degC and carry out final grinding prior the nitirding stage of the process and then any subsequent distortion should be minimised.

Nitrocarburising uses slightly higher temperatures - around 570degC but as the standard crank has already been subjected to this temperature it is unlikely that a further treatment would result in any distortion.

The layer produced by Nitrocarburising is extremely thin (10 to 20 microns) so growth is also limited.

(Gas Nitriding can produce hardened layers of up to 1mm and would allow a 0.25mm re-grind without retreating and still maintain the fatigue life benefits. layers of this depth would cause significant growth of the journals and this growth is often described as a 'white layer' which must be removed which is usually done by grinding)

The thin layer produced by the Tenifer treatment is always removed when re-grinding a crank and it is common practice to retreat but this is not essential and it would be possible to run the crank without carrying out this operation.

The main cause of distortion during Tenifer Type treatments is caused by either lack of physical support whilst the part is hot (better to hang cranks if the available plant is large enough) or by uneven heating - thermal shocks caused by not preheating the crank thoroughly prior to immersion in the Salt Bath.

We have been successfully Tufftriding a wide range or completion car cranks for more than 40 years - (911 cranks for 7 years) and experienced very few problems but have used the same contract heat treatment company throughout this time.

[hightuned]

Impressive material knowlege Chris, im a design engineer by trade and was wondering why Porsche didnt use a deep hardening process for the cranks as there is always talk about rehardening it after a grind? I'll prob have some questions for you when i get my crank back if you dont mind?

[Henry Schmidt]

Chris
I guess I could have been more precise in my description of the Nitriding process but because no one is nitriding cranks in their garage, I use the generic term to describe what needs to be done. I mention "nitriding" mainly to remind people that plugs will/can fall out if not replaced after heat treating.

The process we use is Ion Plasma Nitriding and I believe that process is as good as we can do in the environment in which we operate.
Cheers

[swade]

I had a plug fall out of my 72T crankshaft but caught the rod knock prior to any pieces departing the metal engine case and shut her down. The rod bearing did not have much thickness left and the journal was a lovely shade of blue. Bought a new crank, had my engine case sides shaved to bring the bore in line, then the spigots shortened to make up for the shaving the case got. All for a little plug, time consuming, expensive, car not on road. Install plugs after heat treating and save going through this.

[chris_seven]

My concern was that , as has happened here in the UK, that someone may have taken a used crank for Gas Nitriding and then had a major engine failure.

We are currently making 906 Cranks and we have chosen to use a Nitriding Steel and we are Gas Nitriding them to produce the fatigue life improvement as I know an understand this process well.

I was tempted by Plasma techniques but as I don't have any first hand experience of the process I thought it best to stick to traditional materials and techniques.

I guess if we had time (and money) to do some testing we would probably Brinell the fillet radii and then Ion Beam Nitiride which is a room temperature process.

[tharbert]

I'm not extremely familiar with machine shops and their capabilities. Do most have some ability to harden or otherwise treat bearing surfaces? I googled the different processes described and found a few hits but nothing in the St. Louis area.

BTW, I've sourced a STD crank from another 72. I'll hang the old crank in the garage for another ten years at which point it may be worth repairing to someone.

[cmcfaul]

Oversized bearings needed for a re ground crank are ridiculously expensive. Plan to spend $1000 for over sized bearings....those simple curved metal parts. Standard bearings are close to $50.

You will likely find it cheaper and better to look for a standard / standard crank (standard main bearing size and standard rod bearing size).

I just did exactly this. Found a crank from a 2.7 that works perfectly for $400. Going to make a lamp out of my polished, one over, crank ;-)

Chris

73 911 E

[hightuned]

Originally Posted by cmcfaul

Oversized bearings needed for a re ground crank are ridiculously expensive. Plan to spend $1000 for over sized bearings....those simple curved metal parts. Standard bearings are close to $50.

You will likely find it cheaper and better to look for a standard / standard crank (standard main bearing size and standard rod bearing size).

I just did exactly this. Found a crank from a 2.7 that works perfectly for $400. Going to make a lamp out of my polished, one over, crank ;-)

Chris

73 911 E

Right you are, im used to working on chevy v8's so i never gave the bearings a second thought, this is ridiculous and i cant even find the 0,50 bearings i need! Think im gonna call the shop and tell em to chuck the crank in the bin, rebuiling my 2,7 just isnt economical im stating to wonder if im not better of buying a 3,2 or give renegade a call and drop in a LS v8?

[chris_seven]

Originally Posted by hightuned

Think im gonna call the shop and tell em to chuck the crank in the bin, rebuiling my 2,7 just isnt economical

It must be worth keeping as all of the 'good used' cranks will eventually dry up even if they are 'good' to begin with.

You will just never know how much fatigue damage a used crank has stored up and by grinding and re-nitrocarburising you effectively have a crank back at zero hours.

I am sure that at some time another manufacturer will spot an opportunity and make some bearings.

[hightuned]

Originally Posted by chris_seven

It must be worth keeping as all of the 'good used' cranks will eventually dry up even if they are 'good' to begin with.

You will just never know how much fatigue damage a used crank has stored up and by grinding and re-nitrocarburising you effectively have a crank back at zero hours.

I am sure that at some time another manufacturer will spot an opportunity and make some bearings.

Yep i give up, the shop called today and said they have to go with atleast a 0,50 grind so im getting another crank instead. Glad found out abouth the bearings before a wasted money on a regrind and rehardening of the crank, im shure the old crank will make a nice lamp foot or something?