Grinding of crank

[Pellesin]

I`ve got a 3,2 crank that has suffered some beating due to rod knock. I measure the damage to be just above 2.nd oversize dimentions - so the crank should be salvageable.
According to mr Wayne Dampsey - atleast what I understand of what he writes in his book is that it is useless to get the crank grinded down, unless the surface is rehardened after grinding.

Does anyone have expirience with this? I live in Norway and I`ll give our domestic machine shops a call.

Any machine shops i California USA that can perform this? I`m going over seas in a couple of weeks, and can bring the crank with me.

Best regards from Norway

[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.

[tocobill]

Marine Crank outside of LA. They do alot of work. I sent them my 993 crank about a year ago for a grind, weld, re-harden (nitrite) polish, and cross drill. Pretty fair price ... even more so when you dont have to ship it coast to coast. Im sure others will chime in about Marine Crank or other shops.

If you end up sending out to MC, they will drill and tap your "bungs" to accept set screw plugs. IIRC this is a standard procedure for them vs oem plugging. This is nice because you can pull them to clean the crank passages if you end up doing another overhaul or if its a race engine that gets opened often. Also makes it a breeze to clean the crank when you get it back before reassembly as they clean but insist you reclean as a CYA disclaimer. Mine was very clean but 3 cans of carb clean and some pipe cleaners just to make sure. Would hate to ruin a new set of gt3 bearings due to some FOD in a passage after the whole process.

[turbobrat930]

Armundo of CCR ( Custom Crankshaft Repair) repaired my crank, and offset ground it as well (stroked to a 78.8mm stroke). He did an excellent job!! He is now located in TX, USA

[Dan J]

Do you have Armundo's contact info? I used him when he was in Ca but the old phone doesn't work

[turbobrat930]

Originally Posted by Dan J

Do you have Armundo's contact info? I used him when he was in Ca but the old phone doesn't work

Sorry, I have not spoken to him since he moved to TX. When I used him, I went thru JB Racing. Call them, and see if they have updated info...I am sure Jim would be more than happy to help out

[Flat6pac]

Info in PM
Bruce

[Pellesin]

Thanx for all the info guys! I really apprechiate it.

Did everybody ger their crank surface hardened after grinding? Or did someone just grind, (clean) and then rebuild as is fairly common with other engines.

Best regards

[Flat6pac]

The bearings for a cut crank are VERY expensive
Bruce

[harvardma]

Chris_seven mentions the hard layer on a standard crank is only a few microns thick. Does this get removed if you just micropolish the crank?

[Pellesin]

Originally Posted by harvardma

Chris_seven mentions the hard layer on a standard crank is only a few microns thick. Does this get removed if you just micropolish the crank?

This is a good question. What does it look like when it's finished? Wonder if anyone has a picture to shear. If I understand Cris_seven correctly you grind, polish (or just polish the ones you didn't grind) and then treat with Tenifer/Tufftriding. After this it is time for assembly. I guess the surface still has a polished finish after the treatmemt as well.

The bearing halves for a cut crank are more expensive - this is true. On the other hand - a new crank is as well....

Here in Norway the "word is" that you don't grind. "Everybody has heard that you should't" but no one can confirm why.... in general no machine shop has heard of this treatment. But that really doesn't matter all that much... with a national population equal to a medium American village - no wonder we havent heard it all..
An engine builder I spoke with the other day could confirm that a fiew cranks from german buildt cars needs this treatement, and things could go wrong without.
I catch myself thinking that things obvoiusly go wrong with the treatment as well...
Not at all saying that Tenifer doesn't do any good - by all means. But my options to get the treatment so far will take time, and driving season is not that far away.... I am really tempted to just do a conventional grind, and polish. Assemble the engine - "have a ball all summer" and then remove the crank for treatment in the autum/winter when I've had the time to sort out where to do what. Hopefully the crank won't mind
A guy I spoke with at a Porsche dealer had done the treatment at a weapon factory. He had not been made aware of the issue with the bungs in the crank - which of course made their way out of the crank after assembly. Thus oil press went down the drains after driving a fiew miles. He had to rebuild the engine once more - not sure if he just replaced all the bungs, or if he got a replacement crank.

[tocobill]

From my understanding and this goes back to very large marine diesel engines ... my other job ... is that you can polish a hardened surface. We are talking about microns being removed. Similar to when you wax and compound the paint on your car. You are removing material but nothing that is very noticeable. Now over the course of the life of the car and the compounds used you can tell as so people have worn through paint but not in one, or two sessions but a ton of polishing. IIRC last time we sent had a tech come out and polish a cam lobe they measured the roughness of the surface with a special gauge. Afterwards the measurable difference was not really notable. A good polish should take about .0002” of material off which is about 5 microns.

I dont understand why one would just grind the crank and go with oversized bearings which are stupid expensive vs having the journal ground, rewelded, cut and polished back to standard. Depending on the crank this seems to be a much cheaper and better fix IMHO. My crank cost about $750 or so to have the #5 journal cut, welded, and polished. This also included x drilling and rehardning etc. It is now back to std/std. A new/used crank would have cost $1500-2K which then made GT3 crank a very nice option ... but I was able to get it fix for much less.

So my question .... why use oversized bearings with they are so damn expensive? I understand getting color coded bearings for specfic builds and oil clearance tolerances but not grinding a journal to then run non std bearing. Thanks.

[chris_seven]

The basic metallurgy of cranks is quite interesting and may be needs a little more consideration.

The hardness of cranks isn't that important and in the past many people trying to sell various different processes over-emphasised various properties to sell something that may not be needed, it is really about performance and they way an engine runs.

In the 1960's in the UK many engines had forged steel cranks made from very basic steels - commonly EN16T-. These cranks were durable and lasted well.

They were hardened and tempered - hence the T condition- but they were never surface treated.

It is clear that hydrodynamic bearings which have are separated by an oil film don't need hard journals - BUT - when the engine stops or starts there are landing forces and the potential for scuffing. Tenifer treatments help significantly in this area.

The main reason for crank failure is normally lack of oil or simply a worn out bearings with too much clearance and clearly this takes quite a time.

There is another issue and that concerns the fatigue life of the crank. If cranks are affected by fatigue it is normally due to torsional vibrations.

When we used to 'tune up' sixties engines and lighten flywheels we increased max revs for 6000 to 8000rpm crank failures became common.

This was due to an increased frequency and amplitude of the torsional vibration in the crank.

There were two 'solutions'.

The first was to buy a very expensive EN40B 'Steel Crank' (many Fords had cast cranks) which is the real solution.

The second was to Tuftride (Tenifer) the crank. This treatment was relatively cheap ($2-3 per Kg or crank weight) and did give some benefits.

The process worked on both basic medium carbon steels and the type of Cast Iron used in Ford cranks of the day.

The hardness of the surface is a result of the Carbon and Nitrogen atoms that diffuse into the surface of the steel during the process and this creates significant amounts of compressive residual stress.

In order for fatigue cracks to initiate and then grow the surface of the crank would need to see a tensile stress so these compressive stresses significantly increase the crank's fatigue life.

It is fair to say that Tenifer treatment is less effective in this respect than either gas nitriding but is does help.

As you shouldn't gas nitride these basic steels it is the only real option.

the original 66mm stroke cranks don't really suffer too much with torsional vibration as they are quite stiff and have a high harmonic frequency but there are some high order vibrations that can affect T cranks which have no counterweights.

The first generation 70.4 mm stroke cranks do have torsional issues which is why they do have a habit of shedding flywheels and were known to have fatigue issues on RSRs used in 24 hour races.

The 3.0 SC seems quite good in this respect as the crank is stiffer for the same stoke and it probably has a higher resonant frequency than the 6 bolt crank.

I have no real experience with a 3.2.

Tenifer treatment is a cost effective and well established 'insurance' to help reduce the likelihood of fatigue failures on hard driven cars.

I don't think it would be essential on a gently driven road car but it is recommended in the 'Spec Books' for the 911.

I would tend to micropolish after the Tenifer which will slightly roughen the surface of the journal.

Many modern production cranks are manufactured from an Austenitic Spheroidal Graphite Cast Iron which was developed at BCIRA in the UK and I sold them some of their fatigue testing equipment more years ago than I am ever prepared to admit.

This material has great strength, toughness and excellent fatigue resistance. Ford used to make a very trick 'Darcast' crank for their Formula Fords using this material.

I believe BMW use salt bath nitirding for their current 6 cylinder cranks and that Toyota and Honda use the same basic process, which is a modern equivalent to Tenifer and quite effective.

I am surprised that people say that you shouldn't grind a 911 crank as I don't see why not other than the cost of the bearings.

The only risk you will run by grinding the crank and then not carrying out the Tenifer treatment is that you may reduce the fatigue life. If you don't do many miles and only drive the car in the summer this may not be an issue.

If it is a hard driven race car I would worry other wise I don't really think it is a big problem.

We would always treat the crank in one of our customer's cars if we were grinding the journals but I have 5-6 sub contract heat treatment companies that do this work within 20 miles.

Even if you don't heat treat I would still take out the bungs as you may not be able to clean all the grinding debris out of the oil passages with them in place.

I must say that I could never bring myself to weld a performance engine crankshaft. Submerged Arc Welding is an interesting process that is extremely valuable but not for this application.

[cmcfaul]

As was mentioned earlier, the cost of the over sized bearings can be more then a brand new crank. You are likely better off finding a used, standard crank and using standard bearings.

Chris

[Pellesin]

1.st
I`d like once more to give my thanx to chris_seven. I really enjoy reading up and learning from you and I`m really glad you take the time and shear youre knowleage with the rest of us.

Originally Posted by cmcfaul

As was mentioned earlier, the cost of the over sized bearings can be more then a brand new crank. You are likely better off finding a used, standard crank and using standard bearings.

Chris

Here in Norway at the Porsche dealer the price of the oversized bearing is 401 krone pr halve, thats 46,7 euros, or 53 us dollars - you need two of them.

If you can tell me where to get a new, or even used crank in good condition for that money - please give me a tip

Sure I will have to spend some money on grinding and hardening an all that, but as far as I can see I can spend quite a fiew before it balances getting a new crank.

A new carrera crank is 74`000 nkr ( 8633 euros, or 9883 us dollars) A turbo crank is approx half the price - beats me why. Anybody know why?

I know there is a fiew bucks to save buying a new crank from pelican parts - but for 4`400 dollars (turbo crank) - I can do some machining on the crank I`ve got.

Buying a used crank - well why does someone sell a crank??? chances are I`ll end up with another busted crank.

Well thats how I wiew it anyway.
Best regards from Norway

[Spenny_b]

Another fantastic education that I hadn't planned on, thanks Chris, invaluable write-ups as ever.

PS - hey mate - if you *do* decide to look at another crank (but I do agree with your thoughts about ending up in the same situation) then I'm sure Chris may well have a lead for you but if not then give me a shout, a pal of mine who's local has access to used 911 stuff.

[turbobrat930]

Or, you can offset grind your crank, then the bearings are Clevlite Nascar bearings... and you put the money into a good set of aftermarket rods (I used Pauter). Then you have a stroker engine, and the increased torque that comes with it...

[cmcfaul]

I never considered only using one oversized bearing. Can you buy them individually? If yes than that could be a good way to go. For me, the machine shop doing all the machine work on my engine had several cranks on hand that were to spec. Think he charged me $250 which was a no brainer. Perfect crank with standard bearings....next

[KTL]

Agreed Chris always provides incredible technical knowledge. I am in awe when I read what he shares!

I like the thought process of Pellesin. I think often times people automatically assume all journals would be oversized? But I don't see why just one journal couldn't be done oversize as a repair solution. Just need to be sure to balance the crank after lightening that singular journal, correct?

An oversize rod bearing shell (one) is $27 here in the US. That is wholesale Porsche dealer pricing. Retail price is currently $40. Not cheap by any means but still not bad if doing only one journal (two bearing shells needed).

So if you grind one rod journal oversize, you're paying for the grinding/machining (you have to do the edge radii to do a proper job) and the hardening. I would think that the extra labor for one journal and the cost of two oversize rod bearing shells would be much less than the cost of welding and re-machining that one journal?

Oversize 3.2 crank bearing shells are part number:

930.103.147.50 for 0.25mm oversize
930.103.147.60 for 0.50mm oversize

If cmcfaul got a good used STD/STD crank for $250, that was a steal of a deal. Let me know where you got that deal. I'd like to get one for myself and maybe a few for friends!

[Pellesin]

[QUOTE Tacobill:
From my understanding and this goes back to very large marine diesel engines ... my other job ...

I dont understand why one would just grind the crank and go with oversized bearings which are stupid expensive vs having the journal ground, rewelded, cut and polished back to standard. Depending on the crank this seems to be a much cheaper and better fix IMHO. My crank cost about $750 or so to have the #5 journal cut, welded, and polished. This also included x drilling and rehardning etc. It is now back to std/std. A new/used crank would have cost $1500-2K which then made GT3 crank a very nice option ... but I was able to get it fix for much less.

A fiew colleagues of mine suggested the option of welding and regrinding the crank - but they were talking about VW Beetle engines.... And as you mention in the head of youre reply "..goes back to very large disel engines... No offence at all but my 1.st thought would be that this treatment belongs with engines like that. But when I say this , its just assemptions and not based on knowleage or experience.

Now if I understand you correctly you had this performed on youre Porsche crank? Thats interresting. Do you know if they care anything about tensions buildt into the crank after welding? Is there an annealing process after welding? When you weld things, they tend to bend - if you grind the weld, you can "upset" the tensions as well. Is this an issue?
What kind of welding rods are used? Is it the same steel as the crank?

You mention rehardeneing: This also included x drilling and rehardning etc. Is it the same process as Tenifer?

So my question .... why use oversized bearings with they are so damn expensive? I understand getting color coded bearings for specfic builds and oil clearance tolerances but not grinding a journal to then run non std bearing. Thanks.

Well, as we have seen oversized bearings are expensive compared to standard bearings - but it really isn`t much money...thats if you do the one damaged journal.


KTL

Just need to be sure to balance the crank after lightening that singular journal, correct?



Yes, If I head down this way I`ll have to rebalance. If I`m lucky I`ll get away with 2.nd oversize as I mentioned - so the crank has allready "grinded itself" out of balance. Thus a rebalance is allready needed.
One thing that bugs me though is that I cannot figure out why this has happened. When I opened the crank case - 1st of all there was no guidance sleeves installed. (Should be one on each side of the thrust bearing /#1 bearing) 2nd I was dissapointed/surprised that there was just these two.... would atleast have thought that there would be one diagonally - but no. Anyways - brainfart asside
I cannot find a blocked oil passage or any obvious reason for the rod bearing (#3) to shear



If cmcfaul got a good used STD/STD crank for $250, that was a steal of a deal. Let me know where you got that deal. I'd like to get one for myself and maybe a few for friends! [/QUOTE]

Count me in!