Crank hardening

[davidl]

Hello all.

I'd appreciate opinion on rehardening cranks.

To date, I personally have never seen the need to regrind any 911 crank, as good used are available.

But I do have a stash of damaged cranks now, and good used will run out one day.

Even grinding them may be a non-trivial issue..special wheels etc...

But once ground, do we really need to harden them, with all the hassle of straightening and regrinding onto size..?

David

[Grady Clay]

David,

No, all Porsche cranks can be commercially reground and bearing sets are available from Porsche and others (Pelican). No hardening is required.

Carefully measure and document all the crank dimensions first. If necessary use an independent expert.

Of course exactly who regrinds your expensive Porsche crank is very important. Obviously, size and finish are most important. So is chamfering the oil ports and the journal side radius.

Unless you have someone who regularly (and long term successfully) regrinds 911 cranks, I would supply them with the exact dimensions and tolerances for them to confirm correct. Talk to them. Agree on exactly what is to be done.

Magnaflux is a worthwhile test. Find someone really good to interpret the test. I have never heard of a “cracked” 911 crank and the only broken crank I’ve seen was from where the crank grinder failed to grind the side radius. If the test catches only one problem in 10,000 tests, it is worth it for your engine.

Cleaning the oil passages is an absolute must, particularly the blind passages. I don’t think you need to pull the plugs to achieve good cleaning..

YOU need to be able to independently measure and confirm all the dimensions at least as well as the crank grinder. He needs to know that beforehand. You will get the best work if he knows there is post-grinding inspection by someone else.

A not-uncommon problem is you get a call saying that the first grind didn’t take care of all the problems. Take the crank to your independent inspector for analysis and then return it to the grinder.

Pay them something extra for good work. Make it clear that if they damage your crank, they are responsible. Impress them with the retail cost of a new crank.

There are also some acceptable repair techniques for badly damaged steel cranks. These can be appropriate for still great street engines. Supposedly there are also “trick” processes to apply to the crank. Tread carefully.

Best,
Grady

PS: Rod inspection and reconditioning are other important issues. What about the crankcase?
G.

[davidl]

What a helpful and obviously knowledgeable reply!

Thanks.
And yes, rods and especially cases are the usual suspects in these motors, I agree!

I'd really like to know how and where to drill the extra oiling holes for hi-rpm motors...

David

[Wayne 962]

Hmm, just about everyone else I know of doesn't use reground cranks that are not re-hardended.

-Wayne

[davidl]

Thanks , Wayne.

Do you think this is because of the superior fatigue performance, modulus, or the surface characteristics of the nitrided crank?

Thanks
David

[TimT]

Here is a thread where I showed my method of ensuring the 2&5rods get the oil they need for extended high RPM running (2&5 are prone to oil starvation)

Look here

another equally good method was shown also

[davidl]

Thanks very much, Tim.

I saw that thread.. very good.

What I'm looking to confirm is the drillings in the crank itself please.. I was planning to do more than one main .. but if one is enough I'll just do that.. whcih one?

Kind regards
David

[TimT]

the crank journal between the 2 and 5 rod throws is drilled to intersect the existing oil passage

[Damomma]

I thought that the cranks are hardened to a depth that will allow 1 regrind without a problem?

[adomakin]

a very respectable machine shop over here that has been regrinding porsche cranks for over 14 years told me that 70-90% of damaged cranks that have come to them for assesment and regrind are cracked from excess heat buildup at the journal. worth getting a proper mag test me thinks!


Andy, UK

[Grady Clay]

Wayne,

I agree; a standard crankshaft is most desirable. If it weren’t for the cost, many would install a new crank at rebuild. I actually would prefer a good used polished crank over a new one.

I disagree that a properly ground, polished, and inspected crank isn’t suitable for reinstallation in a 911 engine. Would I prefer a standard crank – yes. Would I use a properly reground crank – yes. Would I prefer it be properly rehardened - yes.

The issue of hardening is more difficult. Both you and Bruce address that issue in your books. It seems the most important difficulty is reliably reinstalling the oil plugs. I certainly don’t disagree that a properly re-hardened crank is desirable.

What do you think is the best journal hardening techniques? What is hardened, just surface of journals or the entire crank? How much control is there over the process, is there variable hardness? Will regrinding the crank again remove the hardened surface? Will the process make the crank more prone to cracking? Who can perform this task? I have never seen any specs for the aluminum plugs in the cranks. What is the proper alloy and interference fit? Why have there been failures?

Best,
Grady

[BURN-BROS]

Grady, the cranks that I have seen hardened have been spun while a flame makes contact on the journals. the journal is heated to the appropriate temperature for a certain duration. This duration dictates the depth the hardening will attain. So no, it is not just the surface. Keep in mind that once the crank is hardened, a finish grind will be done. Once the journal has been heated it is quenched and the process is started over untill a specific hardness is achieved.

It is entirely possible to machine past the heat treatment, but this is where everybody has a different opinion. Does Porsche recommend flame hardening the crank in order to regrind? If not then I would bet that the hardening is at a sufficient depth for all the regrinds.

Yes the crank is more prone to crack when you heat treat because it crates internal stresses. But it is required to provide a durable bearing surface.

As for the rest I cannot say, and as a disclaimer I am not a professional automotive machinist, but I did stay at a Holiday Inn Express last night.

[john walker's workshop]

it's nice that they treated the surface, but think how many other engines are out there running around with a stock or reground, untreated crank and doing just fine? stock and performance engines. multi-millions.

[BURN-BROS]

Most other engines have heat treated crank journals. Its a requirement for a durable bearing surface. The soft nitriding was the cool extra thing that Porsche used to do. If there is another engine running around without it I will gladly eat my shoe.(I should probably start untieing it). Aaron.


BTW John, I love type 2s. I even have a 64 double cab waiting for me to put the floors back in. If you have an ambulance fan switch in your pocket I sure would like one.

[camgrinder]

There is a chemical solution you drop on the crankshaft journal to see if it has been nitrided. Some Chevrolet steel cranks I have seen do not pass this test. They are usually worn past low limits enough to require grinding.
Nitriding really works on Porsche cranks.
If the journal is really bad, it can be built up by welding. Using two types of welding rod, one for the crank pin and the other , harder rod for the fillet radius. Then nitriding will harden the crank correctly and stress relieve.
This link has some info on heat treating.
http://www.nitrex.com/industries_automotive.ch2

[BURN-BROS]

John, are there any other methods (like visual) to see if the crank has been nitrided? My father has a 69 s crank and rods that should have been nitrided at the factory.

[camgrinder]

Porsche used a process called tennifer treating. Its a cyanide salt bath similar to
GM's tufftriding. It doesnt penetrate very deep (.0003" to .0007")
If the crank has been ground to the 1st undersize, the heat treat would be gone.
I wouldnt be able to tell visually between the untreated crank and a treated.

[911pcars]

I did a quick Google on Tuftriding. It seems most companies that do this are overseas. The process must be deadly enough to phase out the many companies that used to do it domestically.

I remember the advantage Tuftriding had over Nitriding was that the heat treatment didn't take as long and the temperature wasn't as high. Elevated treatment temperature tends to induce some bending in the crank. And yes, the treatment depth was only a few thousandths - enough for journal hardness but not enough to survive the next std. regrind.

Sherwood

[john walker's workshop]

burn, check www.thesamba.com in the bus parts classifieds for that fan switch. they come up occasionally.

[BURN-BROS]

Thanks John W. I will do that.



John D, do you know when or if Porsche has stopped the process? I know from books that most of the early stuff was done, but there is no mention of the later 70s to the present.