Will Gant Wrote:
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> It received an engine for a class 90, I forget
> which one. However that engine suffered crankshaft
> problems, and was replaced by another 251 6B from
> an SP Alco S2. That work was done at the Loop.

Had that S2 itself been re-powered at some point?
Alco S-2's as built (1940-1950) and successor S-4's (1950-57) all had 539 engines.
Alco switcher built with 251 engines were the S-5 (1954), S-6 (1955-60) and T-6 (1958-69). Also in the C-415 (1966-68) "utility engine", which some might argue was a switcher.

Hank

ps and then there were the MLW engines...