A steel gear assembly, probably from one of the four Pratt & Whitney R-2800 engines. The heat from the post-crash fire was so intense that engine parts made of light alloys melted or burned away, leaving only the steel parts behind.
On October 29, 1953, an Australian airliner crashed into a mountain south of San Francisco, killing all 19 people aboard. The aircraft, a Douglas DC-6 operated by British Commonwealth Pacific Airlines (BCPA), carried Australian registration VH-BPE and was named "Resolution."
Ironically, after two days of flying from Sydney, with stops in Fiji and Hawaii, "Resolution" was just minutes from landing at San Francisco International Airport.
I took these photos when I visited the site of the crash in 1993.
For more information about this accident, see:
en.wikipedia.org/wiki/BCPA_Flight_304 