Does anyone know what the differences are between ALCO 531, 538 and 539 engines?
Are they part interchangeable...ie heads, injectors,etc?
Just wondering how hard it might be to support an HH660 in regular service on a tourist operation...

"Win Cuisinier", Will Davis, and some others will provide distinctive-competence answers here, I trust.

My understanding is that the principal difference between the 538 and 539 is the way the engine is mounted in the frame: the latter being much lower down. So I would expect all the upper engine parts to be reasonably compatible.

I suspect that although the 531 and 538 are the same engine family, parts interchangeability without some machining or alteration might be unlikely. For purposes of restoring a 531 to practical operation, it might be relatively easy to alter parts from a later engine series, however.

I've seen reference to a testbed 531 that was given an Elliott BF 34 turbocharger and aluminum oil-cooled pistons in 1946, and successfully made 1200hp ("200hp per cylinder") at 740rpm, although longevity at that output might be an interesting discussion. One potential moral is that such changes on an engine run only at nominal rpm and hp through a stock generator might give considerable life in "museum" service -- although it's an interesting consideration, akin to discussions of hot rods in automobile preservation, as to whether making 1946-era modifications on a 1931 engine design counts as "preservation"...

The parts are interchangeable. The big difference is the way the block sits on or between the frame of the locomotive. The 531 and 538 sit ON TOP of the frame while the 539 sits BETWEEN the frame rails. I hope this helps.