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Oversized crank bearings...

Robertb

Well-known member
Joined
1 Sep 2003
Messages
8,406
I read the in a 911&Porsche World mag from a few weeks ago some comments in an interview with Westwood. They said they offer oversized crank bearings specifically so the crank can be reground and reused where it previously could not (presumably assumes there is not some crank bearing-related catastrophic failure which damages the crank beyond repair). This sounds like a good thing, unless I am misunderstanding.

Any thoughts?
 
I posed this very topic a while ago and Hartec replied. Regrinding is the easy bit. Re nitrocarbonising (hardening) the new surface can alter the journal dimensions and its a bit hit/miss.

I think thats what Baz said.

Ian
 
I was thinking about this very thing last night while watching Abom79 Metal Spraying shafts on Youtube, I was wondering if it was possible to induction harden the sprayed metal and then regrind the journals back to factory dimensions. I couldn`t find a definitive answer as to whether the newly sprayed metal can actually be hardened :?:
 
Thanks... exactly the article I'd seen.

It would be interesting to get Westwood's view too.
 
We started to sell these undersize bearings early last year. We took over the sales of the bearings from a company in Germany that had been doing them for many years. We spoke to a number of Porsche engine rebuilders who all agreed that with the sizes we were providing, the cranks did not need to be re-hardened. Of course, if you wanted to do this, it would not be a problem.

They are not a fast mover for us, but they compliment the other Porsche parts we sell.

Hope this helps.

Duncan
 
We started to sell these undersize bearings early last year. We took over the sales of the bearings from a company in Germany that had been doing them for many years. We spoke to a number of Porsche engine rebuilders who all agreed that with the sizes we were providing, the cranks did not need to be re-hardened. Of course, if you wanted to do this, it would not be a problem.

They are not a fast mover for us, but they compliment the other Porsche parts we sell.

Hope this helps.

Duncan
 
Thanks Duncan, that's useful to know.

BTW its always good to see the Westwood Cayman at Shelsley! :thumb:
 
Thank you. Not sure if we are doing Shelsley this year!!! Porsche Hill climb Championship are not going there, and i don't enjoy competing there, so will be at other venues. We will certainly be doing the Porsche Prescott event in June, because that was superb last year.
 
That's a shame... not realised that PCGB weren't attending in 2020. I've been spectating at Shelsley for years, but I had noticed that the field does seem a little thinner recently... are you able to share why don't you enjoy competing there? (apart from the rather binary outcome to any slight errors on the hill!). Feel free to go via PM if you want.

I'll try to get along to the Prescott meet in June.
 
Nothing to Hide about shelsley, just don't like the paddock, and the general way the club works. The club don't care about competitors, we are just a form of revenue!!
 
Grinding high performance engine crankshafts is always a risky process,

First off you should ideally know how deep the crankshaft has been hardened to. It is normally only just below the surface of the crank pin journal. If you grind to first undersized bearing assuming 0.010" then that's probably already gone through any hardening. If you are re-hardening then it can add thickness and slight distortion to the finish and would need re-sizing.

I have made enquires to have this type of work done and been advised by the companies of to bother due to inconsistency in the final product.

Does anyone know for certain what depth the 996/997 crankshafts are hardened to?

Personally I would advise against this on this particular crankshaft.

Mike
 
So I guess we need somebody with a hardness testing machine who can take a standard crank and test it at various undersizes to see how deep it is.

MC
 
For what its worth - we already did this about 10 years ago and the result was very shallow and from memory I think about 3 or 4 thou of full hardness (I will try and find the records from back then to confirm) but anyway not enough to regrind 0.25mm undersize and retain full hardness. We know of some that tried and they didn't last too long on track!

We found some oversized shells from other models that we modified in a jig and had some of our cranks re-hardened allowing a few tenths for the white layer (that grows during nitriding). We had to guess the resulting thickness of the white layer and found the right allowance which then had to be polished off.

The cranks are still running today but it is a bit hit and miss how thick the white layer turns out and at the time we found enough used cranks to continue without re-hardening.

Also several cranks would pick up a 4 thou bend during their life requiring all journals to be re-ground which runs out almost as expensive as a god used crank with a full set of new undersized shells.

Later Porsche moved the tag on the shells to the other side of the con-rods (previous early ones had a tag groove in both sides) so the non Porsche shells we had modified to use were no longer useful and we just continued to use good used or new cranks.

Now it is difficult to find a catalogue to find suitable shells from other models to seek out one you could modify to use and as with all our new computer technology you just input the model first to find its relevant shell and measurement details are often no longer available so it would take for ever.

We do presently happen to have some new racing special shells in production (for our own rebuilds) in standard size but if supply of cranks runs out we could order a quantity of undersized versions and consider setting up a service for others to suit but the quantities needed to make it viable are high, we would have to collect together a lot of cranks to make it all worthwhile and we would not accept the liability to pay for replacement cranks and a full engine rebuild that someone else undertook if it still failed..

It might be that a softer journal might last reasonably long in a road engine that is not driven too aggressively but as engine builders we have no control over how our customers drive their cars or look after them and would never take such a chance - anyway who pays for the new crank and second rebuild if such a process failed. We feel the risk is too great but others may not agree and that's up to them!


Baz
 
For background - decades ago cranks were 1st made with journals too big, then hardened for a long time in ovens, then quenched (which distorted them), then ground back to size - but could therefore be re-ground.

Once manufacturers have decided that rebuilding engines is generally unlikely these days (fashion replacing practicality), they didn't need the re-grind option - so (like Porsche) didn't make oversized shells as an option and then didn't need to harden the cranks any deeper than a thin surface depth (because if they did wear down that far they were scrap anyway).

Then they could Nitride harden the surfaces (which doesn't require quenching so didn't distort the crank) and then the more shallow the depth - the less growth in the white layer so there was a disadvantage in nitriding for any longer than needed for enough depth to stop wear and they would only need slight polishing to finish = quicker turnaround manufacturing time, a very much cheaper process, less time in ovens, less machining operations and the benefit is a lower price to us the consumer.

With pressure on manufacturers of sports cars to show willing in reducing emissions and increasing MPG - they all started reducing the width of big end shells (to minutely reduce drag and MPG) and Porsche were no exception. This cut down on crankshaft weight and often reduced the size of the engine but limited the life a little (which fitted in with the new philosophy of non rebuildable engines).

I have a picture I will try and post showing the difference between a 944 and 996 crank that makes this more obvious.

So nothing wrong with the whole plan to make a version of a 911 more affordable in the modern World - but creating a problem if the engines do eventually need or benefit from rebuilds and cranks wear!

Baz
 

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