Repaired crankshaft

[sjf911]

I bought a 3.2 crank a few years ago that I didn't end up using right away. It was billed as std-std in great shape with no mention of damage and repair. I'm starting to plan to refresh the engine and am considering using this crank to go to 3.4L and lower compression pistons. I pulled the crank out today and noticed a couple of pits in one of the rod journals so I took it down to one of our local crank specialists who confirms that it appeared to have suffered a catastrophic rod failure and had been welded up and hard chromed. He is going to completely go over the crank for me and see if it is usable.
What are the implications in using this crank (if it checks out) especially in a higher horsepower boosted engine?

[Henry Schmidt]

If repair properly the crank could offer many years of service.
The key word being "properly". I am not a fan of chromed crank journals but it is a technique that has worked for many years.
If I had such a crank and wanted to build a high performance engine I would grind the rod journals to fit a stand 3.0 connecting rod and have it heat treated to bring back the surface hardening. At this stage you can use a custom rods or a standard 3.0 rod in you engine. The extra length of the 3.0 rod will require some dimensioning but the extra rod length is a bonus not a detriment.

[sjf911]

Thanks Henry. I'll know more when Tom Franks gets done checking it out. He's an old hot rodder from Costa Mesa that had a machine shop down there for years. Says he used to do a fair amount of stuff with Andial so we'll see.

[chris_seven]

Hard Chrome will significantly reduce the fatigue life of the crank and in a high performance engine will most likely cause a failure.

Hard chome to repair cranks is normally only applied to low rpm units such as marine diesels or industrial diesels.

If you try to heat treat a crank with chrome plated journals the chrome will diffuse into the steel substrate and cause a structural change of the material.

You will almost certainly end up sigma phase precipitates developing just below the surface. This phase is about 50/50 chrome and iron. It is very hard but extremely brittle and not much use other than to seriously damage the material's fatigue properties.

I would suggest you don't use this crank.

[sjf911]

So, in other words, I got royally screwed by a very popular member of this board.

[chris_seven]

Having re-read this I now realise that Henry was not suggesting heat treating a chrome plated crank but hard chroming on performance cranks is still not sensible.

The fact it is welded is not great either - the steel used for 911 cranks is CK45 or similar which is about 0.45% Carbon conent and would need heat treatment after welding.

If you look at this thread you will see a good photo of a crank which had been plated and subsequently failed.

Hard chrome crank back to std/std?

[Steve@Rennsport]

Originally Posted by sjf911

So, in other words, I got royally screwed by a very popular member of this board.

JMHO, but I would not use that crank in any engine. Its an ornament.

[Henry Schmidt]

I was not suggesting you heat treat the chromed crank.
I was suggesting that you grind the journal the a new smaller size.
The chromed crank was not necessarily welded.
It is common practice to grind a damaged journal undersize then chrome it to bring it back to size.
By resizing the journal (55mm to 53mm) you remove the chrome, exposing fresh material to be heat treated.
The new journal (if ground properly, large fillet) will actually make the crank stronger and the new 3.0 rod will greatly improve the rod length to stroke ratio.

Steve, if you have any of these damaged cranks (IE: one undersized rod journal) that you plan to trash, please give me a ring. I'll take them if there is no sign of excess heat.

[sjf911]

Thanks for your help. I'll chalk this one up to experience. Expensive mistake but nobody died.

[sjf911]

So, the 3.2 crank is trash. That leaves me with reusing my 3.0 crank or sourcing an alternative. I assume that a 964 crank is not reasonable without a harmonic balancer and I haven't seen any 993 cranks for sale. What is the going rate for a GT3 crank (one on ebay for $2400) and what is required to make it fit a 3.0 case with a 930 oil pump that has already been machined for a 3.6 crank? I have both 3.0 and 3.2 rods so I assume I can use my 3.0 rods with custom pistons on the GT3 crank.

[Weissach911]

Originally Posted by sjf911

suffered a catastrophic rod failure and had been welded up and hard chromed

If it has been welded would you still re-grind?

[sjf911]

The machinist was pretty certain it had been welded and then chromed.

Will a 9 bolt 3.0 flywheel bolt to a 964/993/GT3 crank or is a custom clutch setup needed?

[Henry Schmidt]

Originally Posted by Weissach911

If it has been welded would you still re-grind?

I would send it to my crank guy (Dave @ Marine Crank) and get his analysis.
He would probably grind that single journal and make an assessment at that point.

[sjf911]

Originally Posted by Henry Schmidt

I would send it to my crank guy (Dave @ Marine Crank) and get his analysis.
He would probably grind that single journal and make an assessment at that point.

Thanks Henry, if the crank passes magnaflux, I may have that done. I'll hopefully get an opinion from the machinist tomorrow. I kind of like the idea of reusing the 3.0 rods, I'll just have to get custom pistons made.

[chris_seven]

All magnaflux will tell you is that there is no exising defect present either on or just below the surface.

It really won't tell you anything about the structure of the steel in either the weld or the heat affected zone and it is the probable microstructural change that could result in a rapid and catastrophic failure.

Heat treatment of any type is likely to result in distortion due the residual stresses that will be present because of the weld.

Even a low temperature treatment such as Tenifer (575 degC) will most likely cause movement to occur locally and could be problematic.

This temperature will only slightly stress relieve the weld area and I don't think it would fully recover the heat affected zone properties.

If you try to fully re-harden and temper you will suffer surface degradation - even in a vacuum furnace, andyou will have quite high levels of distortion that may not grind out.

I must say I would stop investing more money in this crank and use it as a door stop.

[sjf911]

Well, I'm already committed to having my local crank guy inspect it and give me his formal assessment of the damage done. This is a learning experience for me as I have never had a catastrophic rod or crank failure on any motor I have built or run, nor have I ever been scammed like this. I also do not have to use this crank as I have a perfectly serviceable 3.0 crank currently in the motor and a couple of offers for replacement of the 3.2.
I'll be dropping by his shop this afternoon to see if he has actually gotten around to it. So, if it has been welded as well as chromed, grinding the chrome off (and I assume the welded layer) and the journals down to 53mm to fit the SC rods is not a reasonable alternative because of the effect of the welding?

[chris_seven]

It is a difficult call without knowing how much weld was applied and how deeply the weld has penetrated the journal.

You are only taking about 1 mm off the OD and I am sure that the weld will have penetrated more than this and the heat affected zone will of course be much deeper.

The conduction of heat away from the weld region will, in a 0.45% Carbon Steel, have caused a reasonable amount of transformation of the microstructure. This means you will effectively have a hard 'nugget' of material present on the surface and down into the crank.

In itself this may not be catastropic and if it were nicely in the middle of the journal may not be too much of a concern but its size and location are not going to be easy to define.

The problem I have is that without doing further detailed tests it is difficult to quantify what has happened. If you tried to 'etch' the journal to make the hard 'island' visible on the ground surface you would just be doing more damage.

I would accept that I am being conservative but the cost of a crank failure in a high end race engine is worrying and metallurgically the treatment this crank has received is less than ideal.

Good Luck

[sjf911]

Well, the good news is it doesn't appear to have been chromed after all. The bad news is journals 2 and 5 have both been welded and both show significant cracks as well as a cracked third journal. So, it's an expensive ornament.

[Henry Schmidt]

Sorry to hear the news.
Good luck with the rest of the project.
It only gets better from here.

[billjam]

Originally Posted by sjf911

Well, the good news is it doesn't appear to have been chromed after all. The bad news is journals 2 and 5 have both been welded and both show significant cracks as well as a cracked third journal. So, it's an expensive ornament.

Steve,
Don't feel bad about it - it ain't so bad, my wife spends heaps on house ornaments all the time
This thread might give you some ideas ... What do you do with your old cranks?