The HF pumps are chinese knockoffs of other products, typically done in
fairly heavy cast iron but with low levels of quality control. The
result is high infant mortality (don't work at all or die quickly) then
they tend to run for a fairly long time. But don't expect to be able to
get parts after just a few years.

Look for a full cast iron unit, both crankcase, and cylinder. Skip the
aluminum crankcase units as well as the aluminu cylinder with cast liner.

Keep in mind that your "5 HP" unit was probably one of those that came
under the the lawsuit settlement, it is unlikely that it develops a real
5 hp unless you have it on a 240 volt 30 amp circuit. Match your
replacement compressor to the air output specs, not the HP.

Scott wrote:
My 5 HP Sanborn air compressor developed a rod knock and died.
Harbor freight has a replacement pump for $99. I goggled to see if
there were any opinions and did not find any. So has anyone had
experience with the above? Thanks. Scott