Straightening a 911 crank - hardening needed after heating?

[stownsen914]

I have a 2L counter weighted crank that that is bent. I believe the main journal diameters are fine, but the mains are out .005 - .010" due to the crank being bent. I do not know how it was bent - I bought it this way (full disclosure from the seller). I called a respected local crank grinder, who says no big deal to straighten, but noted that they will heat the crank to straighten it.

Any opinions on straightening cranks or how to have it done?

Also, will I have to have the crank tenifer hardened again (or some other method) after it's heated?

Thanks in advance.

Scott

[chris_seven]

How hot will it be when they bend the crank? and how will they heat it up?

If its only bent 5 thou I would just grind it straight.

I would crack detect it before straightening.

[stownsen914]

Excellent question, so I just called back to ask. Apparently they just hear it to 150 degrees F. Surpringly low ....

I was thinking to have it straightened rather than grind to avoid having to use undersized bearings on the mains (I realize there are options to use Chevy bearings on the rod journals, so potentially less of an issue there).

Unless of course straightening a crank is simply not advised.

Scott

[chris_seven]

Straightening a crank is always an issue as it will invariably produce some residual stress that will try to find a way to relax.

Depending on workshop ambient temperature I can't see 150 Deg F making any difference one way or another.

If the crank were very cold when you tried to straighten it may not be great but at around 60 degF there really shouldn't be a problem.

I would worry about trying to straighten a crank in Buffalo outdoors during winter but this seems unlikely.

I think you have two chances to straighten it well enough to avoid grinding - slim and none

If the shop that has measured the bending as between 0.005" and 0.010" I would worry about letting them straighten the crank.

This difference is just too great and I would think a maximum run out of around 0.001" is what I would be looking to achieve.

A 0.005" uncertainty is just too much.

[tharbert]

I should think with a little luck you could source a STD/STD 66mm crank. They're not unobtainable...yet. I got a STD/STD 70.4 for a reasonable price not too long ago but prices are certainly going up. Check the parts threads and ask. You might be surprised.

[stownsen914]

I've talked to 2 different crankshaft specialty shops who claim that they can straighten a 911 crank that's .005"-.010" out no problem to within .001", and have no concerns with residual stresses or racing usage. Do they not know what they are talking about?

Scott

[chris_seven]

If you can bend it straight to a thou that's great.

The reason for the bend in the crank may affect how much residual stress remains after straightening.

I have recently seen an article where a brass chisel is used to impact the fillet radius of the crank which is said to relax the residual stresses and allow the crank to straighten. I am not entirely happy with this explanation as I don't quite understand how hitting a crank with a club hammer - bronze chisel or not - will cause stresses to relax.

I am not sure that I like this method as any damage to the fillet radius is likely to help a fatigue crack to initiate so great care would be needed.

If a crank has deformed due to mechanical loads then stress relief may help.

If the crank has deformed due to thermal stresses then bending will increase residual stress.

As it is often difficult to predict when fatigue will occur parts such as cranks are normally designed in a conservative manner.

If we start increasing engine speeds and horsepower we will reduce the 'headroom' and any additional local stress or defect may cause a catastrophic failure.

To analyse the stresses would be costly and time consuming so there isn't a definitive answer.

I am not sure that I would run a straightened crank in a race engine but it really comes down to a risk assessment and cost benefit.