[ST] Shot-peen and others

["Bailey, Doug" <DBailey@xxxxxxxxxxxxxxxxxxxx>]

I haven't talked to Triumph or my dealer, but coming from a small aircraft
engine manufacturer...

As has been mentioned, you can bet your mortgage that all the crankshafts
are forged.  Nitriding the crankshaft to harden the surface is not
specialized to Triumph, Porsche, or whoever else was mentioned.  Lycoming
has been doing it for decades and I'm sure others have as well.  Forged
pistons for Daytona and cast for others makes sense as does the different
cylinder liners.  The different liners are more likely for wear rather than
weight savings with the minor weight savings being a secondary gain. 

As for shot peening, not very likely on the crankshaft, the connecting rods
perhaps, but since they are also likely forged they are not likely to be
peened.  Shot peening does harden, but by introducing compressive residual
stress, i.e., a tensile load has to overcome the compressive stress induced
by shot peening before the localized surface sees any tensile stress and
lowers the effective tensile load on the part.  Therefore, no, it does not
stress relieve.  Stress relieving is only accomplished by heat treating.
Shot peening is not done to smooth.  It slightly roughens a machined surface
and if you have a rough surface to begin with you are not likely to shot
peen to smooth it as there are better ways.       

Doug Bailey
Metallurgist/Failure Analyst
Lycoming Engines


>The difference is that the Daytona cranks have been shot-peened. This:
>- - hardens;
>- - stress-relieves;
>- - smoothes;
>the rod, making it less prone to fracture. 

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